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Forces & Newton's Laws
Forces
Force- A push or a pull
Long Range forces (four fundamental forces) “Jedi – Forces”GravityMagnetismWeak forceStrong force
Contact forcesForces that result from direct contact between objectsFrictionNormal forceAny force that’s not a Long-Range force
Past notions on forces
Objects needed a force in order to moveAnd object could not possibly move without
a force pushing itAristotle (~300 BC) first formulated these
laws and were not proven wrong until much later by Galileo
Galileoforce is not necessary to keep objects in motionGalileo argued, that only when friction is present
(which it almost always is) is a force necessary to keep an object in motion
Force - A push or a pull
Friction - a force that acts between materials that touch as they move past each other
o Acts opposite the direction of motion
He tested this with balls rolling up and own inclineshypothesized ….if no friction, a ball rolling on a level
surface would stay in motion forever w/ no force pushing it…. It just had a natural tendency to stay in motion
This natural tendency of objects to stay in motion and resist changes in its motion is called INERTIA
Newton’s 1st Law - The Law of InertiaIsaac Newton … was Galileo’s successor in
the world of Physics.
formulated Three Laws of Motionreplaced 2000 year old Aristotelian ideas
NEWTON’s 1st Law - The Law of Inertia
“An object in motion will stay at a constant speed in a straight line, an object at rest will stay at rest unless acted upon by outside forces”
Inertia -- property of matter that resists changes in its motion.
Examples of inertiaWhy seatbelts are necessaryAstronauts in space… get back to spaceship?The more mass something has the more inertia it hasLarge boats not being able to stop their motionA force is the only thing that can put an object in
motion OR stop a object from moving. Objects cannot change their own motion w/out an outside force
Nickel on a piece of paperRollercoaster1994 Nissan MaximaCar with cinder blocksCar going around a circle (stuff on dashboard?)Satellite
Example---Mr. Schober goes to the shooting range …
Which gun to choose??
Heavy gun, w/ small caliber bullets= means small kick back because gun has more inertia– Equals easier for smaller person to shoot Video
The moving EarthThrow a coin upward in a moving carBird catching a wormHelicopter hovering above Earth…. Does the
Earth move under it??
No, helicopters inertia keeps it moving with the Earth
Net Force (∑F) (Fnet) Net Force Total force acting on an object
∑F = F1 + F2 + F3…
force is a vector quantity Sign indicates direction Forces in opposite directions will cancel out
A 5 N force to the right would cancel out a 5 N force to the left
∑F = 6 N
Only the NET force acting on an object affects its motion
-7 N 13 N
Free Body
Diagrams. Simple drawings showing all force vectors
acting on an object
Skydiver (m=75 kg) falling towards Earth, if the force of air resistance is 100 N what is the Net Force on the skydiver?
Force of Gravity (aka Weight)
pulling skydiver towards Earth.
Fg=mg
Force from Air
Resistance directed upwards
75 kg x -9.8 m/s2= -735 N
100 N
∑F = -735N + 100N = -635 N
Newton’s 2nd Law
Force Causes Acceleration
We learned w/ the 1st Law that objects do not like changes in their motion… if a net force is applied to an object the
motion of an object will change…
‘changing motion’ basically means the same thing as Acceleration
So then we can say…. ‘force causes acceleration’
Mass resists acceleration
All forces do not accelerate all objects equally
Objects with more mass are more difficult to accelerate, and thus require more force
Newton’s 2nd Law ‘The acceleration of an object is directly
proportional to the net force on it, and inversely proportional to its mass’
This means…. The larger the force, the larger the acceleration
AND …. The larger the mass, the smaller the acceleration
This can be summed up w/ the equation ∑F=ma
a acceleration (m/s2); ∑FNet Force (N); mmass (kg)
Combining two concepts of Net Force
∑F=F1 + F2 + F3..AND
∑F=ma Soo we can say ∑F = ma = F1 + F2 + F3..
This also works in 2-D ∑Fx = max =F1x + F2x + F3x..
∑Fy = may = F1y + F2y + F3y..
Mass & Weight Mass (m)
How much matter is in an objectA measure of an object’s inertiaMass never changes depending on location… you have the same
mass in space as you do here on Earth
Weight (W)How much gravity is pulling on an objectAka… The FORCE of gravity acting on an objectIs a function of how strong gravity iscan change from place to place…. You have a much different
weight in space than you do here on Earth
B/c mass is a measure of inertia it is just as hard to move a large boulder in space as it is here on Earth
How to determine an object’s weight?Mass and weight are always directly proportionalWe find weight by using the equation
Weight = mass x acc. of gravity
Or.. W = mg g = -9.8 m/s2
So if an object has a mass of 1 kg, then its weight will be 9.8 N.
The “Newton” is the standard unit for force, and since Weight is a force, the Newton is used here.
1 N = 1 kg*m/s2
Why don’t heavier objects fall faster?
Newton’s 2nd says that the more force an object has on it the faster it should accelerate….. So then if heavier objects have a larger weight (force of gravity), why do they not accelerate faster? Ans. True, heavier objects have
more force, but they also have more mass, which means that it requires more force to accelerate as fast as lighter objects….
Essentially…. The larger force and larger mass cancel each other out
All falling objects accelerate downward at 9.8 m/s2
Example
You push a 10 kg cart with a force of 8 N. There is a frictional force of 3 N that acts between the cart and ground. What is the acceleration of the cart?
∑F = ma = FA + FF 10kg (a) = 8 N + -3 N
10kg (a) = 5 Na=.5 m/s2
NEWTON’S THIRD LAW
Equal & Opposite Forces
3rd Law
Forces are not a thing of themselves, but are part of a mutual interaction
Forces only exist in pairs Third Law says….
“For every force there is exists another force that is equal in magnitude but opposite in direction”
You cannot push on something without it pushing on you by the same amount in the opposite direction
Action & Reaction
Action forces produce the interaction, and the reaction force must be in existence at the same time, have the same size, and be opposite in direction
You apply a 10 N eastward force to wall, wall applies a 10 N westward force to you
How do objects move? What pushes them?
Car?? Rocket?? Person??
What is the reaction to the Force of Gravity?
Earth’s gravity pulls objects towards it
So if all forces have are part of a pair, what is the reaction force to this??
Object’s gravity pulling Earth up!
An object pulls upward on the Earth as much as the Earth pulls it downward Object’s mass is much less than Earths, so object
falls down rather than Earth rising up to object
ACTION- Earth’s Gravity pulls person down
REACTION-Person’s
gravity pulls Earth up
Examples
Draw all forces on the car/Earth system as it is driving down the road
Normal force of the Earth
pushing up
on the car
Contact force of the car pushing
down on the Earth
Earth’s Gravity pulling the car down
The car’s gravity
pulling the Earth up
The car’s tire
pushing on the road
The road pushing the car forward
The road’s friction on the car’s tires
The cars
friction on the road
**Action/Reaction pairs have matching colors in drawing
Astronauts & Reaction Forces
Stuck in Space?? An astronaut in space…not moving with just a
spacesuit on and he’s not tethered to the ship. If he is only a few meters away from his ship and there are no other people to help him how could you get back to his spaceship? Oh
No!!
Forces on different masses
Cannon and cannonball Do both the cannon and the cannonball
experience the same force the cannon is fired? Yes, the force on each are the
Action/Reaction pairs Does the cannon or the cannon ball have
a larger acceleration? The ball, b/c it has a small mass. It is
easier to accel. a small mass a = ∑F/m
5.2 - Applying Newton’s Laws
Different types of Forces Normal Force (FN) Weight (W) Apparent Weight (Wa) Force from a Spring Tension Friction (covered in section 5.3)
Normal Force (FN)
“Normal” means perpendicular Force of the Earth (ground) pushing up on an object when they
are in contact with each other
IF object is…. 1) on a horizontal surface and 2) no other vertical forces are applied and 3) no vertical acceleration.
THEN Normal Force is Equal to the weight (force of gravity) of the object.
Normal Force is NOT the reaction to the force of gravity acting on an object
Responsible for how heavy we “feel” AKA “Apparent Weight” --- Wa
Elevators & Normal Force
Elevator Accelerating Down
Elevator at rest ORMoving at constant velocity
Elevator accelerating Up
W=900 N
FN=890 N FN=900 N
W=900 NW=900 N
FN=910 N
FNET=+10 N
FNET=0 NFNET=-10 N
EquilibriumEquilibrium occurs when the Net Force
acting on an object is equal to zero (∑F = 0 N)
If an object is in equilibrium 1 of 2 things must be true The object is moving at a constant
speed in a straight line, OR The object is at rest
∑F= 0
13 N
13 N
Equilibrium in 2-DIf forces are acting in more than one
dimension then the equilibrium equation holds true separately for the x and the y
ΣFx= 0 = F1x + F2x …ΣFy= 0 = F1y + F2y…
A stretched or compressed spring exerts a force
Hooke’s Law
F = kxK – spring constantx- the stretch or compression distance
5.3 - More about Friction Always opposes motion
If direction of motion is positive, Force of Friction is negative
Two types of frictionForce of Static Friction (Fs)
Friction force that opposes the START of motion
Equation Fs,max = μsFN If no pushing force, no Fs. To start an object moving the applied force must be
larger than the maximum Fs possible.
Force of Kinetic Friction (Fk) Friction force that opposes the continuation of motion, exists only on objects that are moving
Equation Fk = μkFN Fk = force of kinetic friction (N) μk= coefficient of kinetic friction (no units)
FN = Normal Force (N)
Static Friction is always larger than kinetic friction Harder to start motion than it is to sustain it
More on Friction Equations
Fk = force of kinetic friction (N)
μk= coefficient of kinetic friction (no units)Constant that depends on the nature of the two surfaces in
contactNo units
Ratio of Friction force to Normal force (Fk/ FN)μs- coefficient of static friction is a different value for two
surfaces (will be larger) but same concept
FN = Normal Force (N)Force pushing two surfaces together“normal” means perpendicular If object is on horizontal surface AND no other vertical forces are
being applied Normal Force is equal to object’s Weight (mg)More on Normal Force Later
More difficult example Problem
A 40 kg cart is getting pulled by a 140 N Force, if there exists a coefficient of kinetic friction between the cart and road of .10 what will be the acceleration of the cart?
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