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WHAT DO OBJECTS DO WHEN NO FORCE IS ACTING ON THEM ??????. Aristotle (384 -322 B.C.) :. of CELESTIAL objects (Moon, planets, stars, Sun) was circular - without beginning or end. Natural Motion. of TERRESTRIAL bodies (apple, smoke, you) was - PowerPoint PPT Presentation
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WHAT DO OBJECTS DO WHEN NO FORCE IS ACTING ON THEM ??????Aristotle (384 -322 B.C.) :
objects would seek their natural resting places: apple on the ground and smoke high in the air like the clouds.
of CELESTIAL objects (Moon, planets, stars, Sun) was circular - without beginning or end.
no need for gravity to explain this motion – it is JUST NATURAL – what a life for physics
student!!!!
of TERRESTRIAL bodies (apple, smoke, you) was for light things to rise up and heavy things to
fall
was imposed motion – result of forces that pushed or pulled.
Important: violent motion had an external cause, it was not natural to the objects
THOUGHT FOR NEARLY 2000 YEARS: IF AN OBJECT WAS MOVING, IT IS AGAINST ITS NATURE AND THE FORCE OF SOME KIND WAS
RESPONSIBLE. NO FORCE – NO MOTION,
No wonder that most thinkers before the 16th century consider it obvious that the Earth must be in its natural resting place and
assumed that the force large enough to move it was unthinkable, it was clear that Earth did not move. – THE CENTER OF UNIVERSE
And in this intellectual climate of the 15th century Nicolaus Copernicus (1473-1543) formulated, in secret to escape persecution, his famous HELIOCENTRIC THEORY – idea that was extremely controversial at the time - the Earth is just a small planet and together with other planets
circle around Sun.
Only in the final days of his life he sent his ideas to the printer. The first copy of his work, De Revolutionibus, reached him on the day of
his death.
One of his most outspoken supporters was Galileo Galilei, the foremost scientist of late-Renaissance Italy. It took the genius of Galileo to claim that NO FORCE is needed to keep an object in the motion (straight-line, constant speed)Galileo argued (brainstorm – just pure thought – no experimental proof) that forces only CHANGE THE MOTION Left alone the things would travel in a straight line with constant speed forever. It is the force of friction that slows them down.
Aristotle: It is the nature of the ball to come to rest.Galileo: In the absence of friction the ball would keep on moving. No force needed to maintain the motion. The force changes the motion – velocity. Every object resists change to its state of motion/velocity. To change
it, the force must act on it. We call this resistance INERTIA.
Galileo’s findings about motion and his concept of inertia discredited Aristotle’s theory of motion.
So why in the world do we STILL sometimes think the same way???
On Christmas day in the year Galileo died
By the age of 24 he gave the world his famous three laws
of motion.
Isaac Newton (1642-1727) was born.
Before we talk about force, let us introduce inertia, mass and
weight.
Inertia is resistance an object has to a change of velocity. • sort of laziness (inerzia – laziness in Italian)
Mass is numerical measure of the inertia of a body • more mass – harder change of velocity is a measure of the amount of matter in the object • depends only on the number and kind of atoms in it. • doesn’t depend on the location of the object • If the object has mass of 1 kg here on earth it would have the mass of 1 kg on the moon, but it would weigh only one-sixth as much.Weight is the gravitational force acting on an object. • acting straight down toward the center of the earth (moon …) • depends on the location of the object. • depends on its mass and acceleration due to gravity: W = mg unit: 1 N
unit: 1 kg
All forces result from interactions between objects. To have a force, you have to have 2 objects - one object pushes, the other gets pushed . FORCE is an interaction between two objects involving a
push or a pull.“FORCE is an influence on an object that causes the object
to accelerate.
Forces are vector quantities, having both direction and magnitude.
unit: (F) = Newton (N),
1 N is the force that causes a 1-kg object to accelerate 1 m/s2.
The net force – resultant force is the vector sum of all forces acting on ONE
object.
Applied forces Net force
the object accelerates as if only one force – net force is applied
Fnet or ΣF
Galileo’s Law of inertia
so, if Fnet = 0, a = 0 no change in velocity, then
An object continues in motion in a straight line at constant speed or at rest unless acted upon by a net
external force."
"How many ways can you state Newton's First Law?"
Definition: If the net force acting on an object is zero, the speed and direction of the motion will not change (the object won’t accelerate). If it was at rest it will stay at rest, and if it was in motion it will continue the motion with constant velocity (in the straight line at constant speed) .
We say the object is in (TRANSLATIONAL) EQUILIBRIUM.
Translational equilibrium
Since velocity is constant, the body is in translational equilibrium:
2. object is moving at 3 m/s in a straight line. Two forces are acting on it. Find
- which means that the object’s acceleration is zero - therefore net force is zero
● F = 8N, 00
how to apply concept of translational equilibrium: 1. Two forces are acting on a body. Describe the motion of the body.
Since the net force on this body is zero, it is in equilibrium:
- which means that the object is not accelerating - the body is either at rest, or is moving with a constant
velocity 8 N F
F
8 N 8 N
if net F = 0 then a = 0, and velocity is constant or zero
equilibrium math:
if velocity is constant or zero, then a = 0, and net F = 0
Six force are acting on an object. What can you tell about the motion of that
object? Is it at rest? Is it moving? If it is moving, how?
The tendency of moving objects to continue in motion can have very unpleasant consequences.
Seat belts: Passenger and the vehicle share the same destiny.
Straps provide the force for accelerated and decelerated motion for passengers too.No seat belts: The passengers maintain their state of
motion assuming a negligible friction between the passengers and the seats. The passengers can become projectiles and continue in projectile-like motion.
In a car accident, the safest place to be is in the car; yet in a motorcycle accident, the worst place to be is on
the motorcycle.
Car: Wear your seat belt. Remember it's the law - the law of inertia.
Law of inertia would safe you from sharing the fate of the motorcycle itself .
No functioning straps: the ladder in motion would continue in motion. Assuming a
negligible friction between the truck and the ladder, the ladder would slide off the
top becoming a projectile.
You are driving at the same speed as a huge truck behind you. You apply the brakes. A huge truck behind you applies the brakes too, but has more inertia. Lazy
thing. And then Bang!!!
A car is turning left not changing the speed. But it is still changing velocity. Imagine a basket full of lazy strawberries in that car sitting on the seat. It tends to stay in the same state of the motion. If you don’t support that basket somehow, it will simply continue in the straight line. For the small speeds friction force is usually strong enough to keep the basket in place.When the car makes a turn, the passengers tend to continue in their straight line path. This straight line motion continues until the presence of a side door or another passenger pushes upon the passenger in order to accelerate him/her towards the center of the turn. The force experienced by the passenger is an inward force; without it, the passenger would slide out of the car.
greater mass – greater inertia (laziness) – smaller acceleration more force – greater acceleration
The acceleration of an object produced by a net force on that object is directly proportional to the
net force applied, and inversely proportional to the mass of the object.
Direction of the acceleration is in the direction of the net force,
netFam
If net force is zero, acceleration is zero, velocity is constant (or zero).
The object is in translational equilibrium.
In every interaction, the forces always occur only in pairs, BUT these forces act on two different
bodies.
Common definition: - to every action there is an equal and opposite reactionis very dangerous, so please do not use it. It is not defined what is action and what is reaction, so it looks as if we were talking about one body, but that’s not true.These forces act on different bodies.
Whenever object A exerts a force on object B, object B exerts an equal in magnitude and
opposite in direction force on object A.
YOU CAN’T TOUCH
WITHOUT BEING
TOUCHED
action: foot pushes the groundreaction: the ground pushes the foot that propels the turtle forward
You push the water backward, the water pushes you forward.
action: tire pushes roadreaction: road pushes tire
action: earth attracts ball a = F/m = 9.80 m/s2
reaction: ball attracts earth aE = F/ME ≈ 0
action: cannon pushes the cannonballreaction: cannonball pushes the cannon (recoil)The same force F (opposite direction), BUT
cannonball:
cannon:
a
=
Fm
a =
Fm
Koka, the clever horse, taught physics by Mrs. Radja says: You taught me Newton's third law: to every action there is an equal and opposite reaction.
Please help me! Why don’t action and reaction forces cancel? Should I find myself a less educated horse, or should I teach
better?
It says that if I pull on the wagon, the wagon pulls me back. If these two forces are equal and opposite, they will cancel, so that the net force is zero, right? So the wagon can never move! Since it is at rest, it must always remain at rest! Get over here and unhitch me, since I have just proven that Newton's law says that it is impossible for a horse to pull a wagon!
Only the forces that act on the same object can cancel. Koka: when the ground pushes forward on the horse harder than the cart pulls backward Koka accelerate forward. (Fnet = F1
’ – F2’ > 0)
Cart : accelerates forward when horse force is greater the frictional force
When we want to find acceleration of one body we have to find all forces acting on that body.
Forces between roller-skaters
If one skater pushes another, they both feel a force. The forces must be equal and opposite, but the acceleration will be different since they have different masses.
The person with a smaller mass will gain the greater velocity.
A roller-skater pushes off from a wall
The force on the girl causes her to accelerate
backwards.
The mass of the wall is so large compared to the girl’s mass that the force on it does
not effectively cause any acceleration.
It looks unbelievable but it is true.
when they clinch forces are equal – you would expect thatwhen they clinch forces are equal – would you expect that?
Sudden acceleration can kill
again, the same force but different acceleration
Our organs are not firmly attached to anything. When head is hit it gains acceleration. But the brain was not hit. It continues with the same velocity. Skull and brain crash!!!!!
again, the same force but different acceleration
physics
Tension: the force that the end of the rope exerts on whatever is attached to it. Direction of the force is along the rope.
T1T2
T2
Normal force (support force, normal reaction force) The force which is preventing an object from falling
through the surface of another body .
The normal force results from strong repulsive electromagnetic force between electrons of two bodies. The atoms in the
surface are compressed microscopically to create the normal force. The surface deforms slightly and produces a reaction
force equal to the force pressing the object into the surface.
That’s why normal force is always perpendicular (normal) to the surfaces in
contact.
Existence: by evidence – object is not accelerating in vertical direction,
therefore, the vertical net force must be zero
For an object sitting on a horizontal surface, the normal force is equal to
the weight of the object.
Fn = mg
Fn
mg
Fn
mg
F
Fn
mgF
If there is a force F trying to lift up the
object, it helps the normal force – the clever desk
doesn’t need to exert so much forceFnet = ma = 0
Fn + F = mg
Fn = mg - F
If there is push down force F
– the desk has to exert more force
Fnet = ma = 0 Fn = mg + F
If the desk can not exert enough force it will break
Friction force Ffr
N
Friction is a force that is created whenever two surfaces move or try to move across each other. Friction always opposes the motion or attempted motion of one surface across another surface. Friction is dependent on the texture/roughness of both surfaces. Friction acts parallel to surface in direction opposed to intended motion. Friction is also dependent on the force which presses the surfaces together, normal force.
Ffr = m Fn
coefficient of proportionality μ is called coefficient of friction
m has no units it is a measure of surface-to-surface roughness depends on characteristics of both surfaces
different values for static and kinetic coefficient of friction (tables). kinetic μ is smaller than static μ. You probably noticed that once you moved something from rest it becomes easier to push around.
Ffr
Fn
mg
pulling force
surface-on-surface μs μk
hook velcro-on-fuzzy velcro >6.0 >5.9avg tire-on-dry pavement 0.9 0.8
grooved tire-on-wet pavement 0.8 0.7
glass-on-glass 0.9 0.4metal-on-metal (dry) 0.6 0.4smooth tire-on-wet
pavement 0.5 0.4
metal-on-metal (lubricated) 0.1 0.05steel-on-ice 0.1 0.05
steel-on-Teflon 0.05 0.05You should keep in mind that it isn't possible to give accurate values for the coefficient of frictions due to changing surface smoothness. For example, not all pieces of metal have the same surface smoothness. Some that are highly polished may be more slippery than others that are pitted or scratched. These values are just meant to give you the approximate values.
At the points of direct molecular contact, electrons become confused.They forget which object they belong to, and wind up trying to orbit nuclei in molecules of both! The resulting bond is called molecular adhesion or a “cold-weld.”Each time they form a bond between uneven surfaces, force is required to break this bond
1. Mechanical interlocking of "rough" surfaces – teeth on the surfaces
Origin of friction :
On an atomic scale, few surfaces are very smooth. Bumps far smaller then we can see loom like mountains to an atom.
Thoughts of an electron with an identity crisis...
2. Microscopic level –
Air Drag and Terminal Velocity
If a raindrops start in a cloud at a height h = 1200m above the surface of the earth they would hit us at 340mi/h; serious damage
would result if they did. Luckily:When an object moves through air or any other fluid, the fluid exerts
drag force on the moving object. The force is called. Unlike the friction between surfaces, however, this force depends upon the
speed of the object, becoming larger as the speed increases. It also depends upon the size and the shape of the object and the density
and kind of fluid. A falling object accelerates due to the gravitational force, mg,
exerted on it by the earth. As the object accelerates, however, its speed increases and the drag on it becomes greater and greater until it is equal to the weight of the object. At this point, the net force on the falling object is zero, so it no longer accelerates. Its speed now
remains constant; it is traveling at its terminal speed. Terminal speed occurs when
the weight force (down) is equaled by the drag force (up).
Terminal velocity of table tennis ball is 9 m/s after approximately 10 m. A basketball has a terminal velocity of 20 m/s after approximately 47 m.; the terminal velocity of a baseball is 42 m/s after approximately 210 m. Skiers increase their terminal velocity by decreasing the drag force. They hold their bodies in egg shape and wear smooth clothing and streamlined helmets. How do skydivers control their velocity? By changing body orientation and shape, sky divers can both increase and decrease their terminal velocity. (60 m/s after approximately 430 m)
Parashoot – 5 m/s after approximately 3 m.
AND THE RAINDROP?How fast is a raindrop traveling when it hits the ground? It travels at 7m/s (17 mi/h) after falling approximately only 6 m. This is a much “kinder and gentler” speed and is far less damaging than the 340mi/h calculated without drag.
Draw all forces that act on a parachutist. Find Fnet and acceleration for a. parachutist that has just stepped out of the airplane.
b. parachutist is falling at increasing speed.
a = g
c. parachutist is traveling downward with constant velocity (terminal velocity)
=
=
mg
mg
mg
Fdrag
Fdrag
Fnet
Fnet = 0
Fnet = mg a = Fnet/m = mg/m
Fnet = 0 a = 0
Fnet = mg - Fdrag a = (mg - Fdrag) /m
a = Fnet/m
a < g
the speed is still increasing, and therefore air friction too until
Forces are usually divided into two types or classes.
1. Contact forces, arising because of physical contact between objects. For example when you push on a door to open it or throw or kick a ball, you exert a contact force on the door or ball.
2. Field forces – they act (push or pull) “on distance through space” - the presence of an object effects the space around it so, and that region is called a field (for example gravitational field of the earth).
Contact ForcesFrictional Force
Tension Force
Normal Force
Air Resistance Force – Drag Force
Applied Force
Spring Force
Field ForcesGravitational Force
attraction between objects due to their masses
Electromagnetic Forcebetween charges
Strong Nuclear Forcekeeps nucleus together
Weak Nuclear Forcearise in certain radioactive processes
At the atomic level – all contact forces are result of repulsive electromagnetic forces (at very small
distances)That means that objects have no actual contact, but their electric
fields (outer electrons repel each other)
Although there are many different contact forces, they are all some form of only four different fundamental field
forces existing in the nature.
One of the most significant intellectual achievements in the history of thought. It is universal – it applies to all objects regardless of their location anywhere in the Universe.
Every object in the universe attracts every other object. The force between two objects is proportional to their masses
and inversely proportional to the square of the distance between their centers. The force acts along the line joining
the two objects.
1 22
m mF = Gr
G = 6.67x10-11 Nm2/ kg2 – “Universal gravitational constant” the same value anywhere in the universe - very small value – no significant forces of attraction between ordinary sized objects.
r
rE – Earth’s radius mE – Earth’s mass. E E
2 2E E
m m mF = G = G m = gmr r
2E2E
mg = G = 9.80m/sr
The force between an object of mass m close to the Earth surface and the Earth
This force is commonly called weight W = mg. Now we can see that the gravitational acceleration g is a consequence of the gravitational force. Its magnitude depends on how far is the object from the center of the earth.
Double the distance from the centre, r = 2 rE , g is 4 times less, g = 2.45 m/s2 , and so is weight
STEPSThe most important one first
Draw free body diagram/force diagramDraw free body diagram/force diagram
Draw free body diagram/force diagramsketch of an object and all forces acting on
that objectNo velocity on that diagram, no acceleration on that diagram,
only object (circle or a box, and you can write mass in it)
and all forces acting on that object
Second step is to find net force
Third step is to apply second newton’s law
How to draw a force diagram
2. Make a simple sketch of the system – point system
1. Choose ONE body to be isolated dog or the cart?
F
fr
mg
F
dog
N
F
net
3. Identify forces that act on the system Label them on diagram
4. Find out the net force by adding the force vectors
decision: cart
5. Apply Newton’s second lawnetF = ma
VECTOR EQUATION ??????
So
1. Add all vectors to get net force
2. Apply newton's second law
netF = ma
Don’t worry, there is a way out.
Separate everything into vertical and horizontal motion
Howard, the soda jerk at Bea’s diner, slides a 0.60-kg root beer from the end of the counter to a thirsty customer. A force of friction of 1.2 N brings the drink to a stop right in front of the customer. a) What is the acceleration of root beer? b) What is the coefficient of kinetic friction between the glass and the
counter?c) If the glass encounters a sticky patch on the counter, will this spot have a
higher or lower coefficient of friction?
Ffr = Fn Ffr / = 1.2/6.0 = 0.20 (no units)
0.60 kg
Ffr
Fn
mg = 6N
Vertical acceleration = 0Vertical net force = 0Fn = mg = 6.0 N
Vertical direction:
Horizontal direction:Net force = friction force: Fnet = Ffr =1.2 NFnet = ma1.2 = 0.60 a a = 2.0 m/s2
c. higher
A boy exerts a 36-N horizontal force as he pulls a 52-N sled across a cement sidewalk at constant speed. What is the coefficient of kinetic friction between the sidewalk and the metal sled runners? Ignore air resistance.
W = mg = 52 N m = 5.2 kg
Vertical direction:Vertical acceleration = 0 Vertical Fnet = 0 Fn = mg = 52 N
Horizontal direction: v is constant, a = 0 and Fnet = 0
Ffr = F = 36 NFfr = μ Fn Ffr / = 36/52 = 0.69
A force of 40.0 N accelerates a 5.0-kg block at 6.0 m/s2 along a horizontal surface.a. How large is the frictional force?b. What is the coefficient of friction?
m = 5.0 kg F = 40.0 N a = 6.0 m/s2
Vertical direction: a = 0, so Fnet = 0 Fn = mg = 50 N → Ffr = μ Fn = 50 μ
horizontal direction: a = 6.0m/s2 Fnet = ma F – Ffr = ma 40.0 – Ffr = 30 Ffr = 10 N
Ffr / = 10/50 = μ = 0.2 Ffr = μ Fn
Luke Skywalker starts to pull a sled with Princess Leia across a large ice pond with the force of 100 N at an angle of 30.0° with the horizontal (with nails on his shoes). Find normal force and initial acceleration if the weight of sled and Princess Leia is 800 N and the friction force is 40 N.
free body diagram components
mg = 800 N m = 80 kg F = 100 N Ffr = 40 N
vertical direction : F sin θ + Fn = mg 50 + Fn = 800 Fn = 750 N
Horizontal direction: F cos θ – Ffr = ma 86.6 – 40 = 80 a a = 0.58 m/s2
INCLINE
An object is on a rough incline θ.
Components:
one parallel to the incline
one perpendicular to the incline.
Why? Simply because we know that acceleration perpendicular to the surface is zero, and acceleration is in the direction of the motion, parallel to incline.
Ffr
Fn
Ffr
Fn The only force that we have to
resolve into components is
weight mg
Resolve vector mg into two components. Now instead of three forces, we have four forces
direction perpendicular to the incline:
Fnet = ma = 0 Fn = mg cos θ
force pressing the object into the surface is not full weight mg, but only part of it,
So the normal force acting on the object is only part of full weight mg: Fn = mg cos θ
If the surface is horizontal: θ = 00 → Fn = mg If the object is in free fall not pressing the surface: θ = 900 → Fn = 0
Ffr
Fn
A cute bear, m = 60 kg, is sliding down an iced incline 300. The ice can support up to 550 N. Will bear fall through the ice? If the coefficient of the friction is 0.115, what is the acceleration of the bear?
m = 60 kg θ = 300
= 0.115 g = 10 m/s2
Perpendicular direction: Fnet = ma a = 0 Fn - mg cos θ = 0 Fn = 520 N < 550 Nice can support him, but he should not eat too much
Parallel direction: Fnet = ma mg sin θ – Ffr = ma Ffr = μ Fn = 60 N300 – 60 = 60 a a = 4 m/s2
cute bear is speeding up!!!!
Question:
How does the weight of a person in an elevator depend on the motion of that elevator?
What will the scale show if the elevator is
1. at rest or moving with constant speed
2. speeding up
3. slowing down
Newton’s 3. law: Force with which the person acts on the scale (reading of the scale) is equal to the normal force on the person.
So, if we find normal force we know the reading of the scale, so called APPARENT WEIGHT
Let’s assume that elevator is moving upward, and let this be positive direction. 1. draw free body diagram 2. apply Newton’s 2. law : Fnet = ma
mg Fn – mg = ma = 0 → Fn = mg
apparent weight = weight
+
1. elevator is at rest or moving with constant speed
2. elevator is speeding up: a is positive
mg Fn – mg = ma → Fn = mg + ma
apparent weight > weightthe scale would show more, and you would feel heavier
3. elevator is slowing down: a is negative
mg Fn – mg = - ma → Fn = mg - ma
apparent weight < weightthe scale would show less, and you would feel lighter
Fn
Fn
Fn
WHEN THERE ARE TWO BODIES YOU HAVE TO DRAW TWO BODY
DIAGRAMS !!!!!!
Two blocks are connected by a string and pulley as shown. Assuming that the string and pulley are massless, find a) the magnitude of the acceleration of each block b) Tension force on the blocks
the same string – the same tensionthe same acceleration, except that 110 g accelerate down, and 90 g accelerate up.
Fnet = ma a is upT – mg = ma T – 0.9 = 0.09a first equation
Fnet = ma a is upmg – T= ma 1.1 – T = 0.11a second equation
two equations with two unknownsT – 0.9 = 0.09a 1.1 – T = 0.11a
0.09a + 0.9 = 1.1 – 0.11a ⟹ 0.2a = 0.2 a = 1 m/s2
T = 1.1 – 0.11a = 1.1 – 0.11(1) = 0.99 N
A 10-kg block is connected to a 40-kg block as shown in the figure. The surface on which the blocks slide is frictionless. A force of 50 N pulls the blocks to the right.
a) What is the magnitude of the acceleration of the 40-kg block?b) What is the magnitude of the tension T in the rope that connects the two blocks? As these two objects are connected
with the same rope, tension is the same and acceleration is the same for two objects.
Fnet = maT = 10a
50 – T = 40a 50 – 10a = 40a a = 1 m/s2 T = 10a = 10 N