Newton s Laws of Motion - PBworksmrdolanscience.pbworks.com/w/file/fetch/59055308/Newton's Laws of... · Newton’s Laws of Motion • 1st Law – An object at rest will ... F g,1

Newton’s Laws of Motion I. Law of Inertia II. F=ma III. Action-Reaction

Sir Issac Newton’s Contributions •  Calculus •  Light is composed of

rainbow colors •  Reflecting Telescope •  Laws of Motion •  Theory of Gravitation

While most people know what Newton's laws say, many people do not know what they mean (or simply do not believe what they mean).

Newton’s Laws of Motion

•  1st Law – An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force.

•  2nd Law – Force equals mass times acceleration.

•  3rd Law – For every action there is an equal and opposite reaction.

1stlaw: Homer is large and has much mass, therefore he has much inertia. Friction and gravity oppose his motion.

2nd law: Homer’s mass x 9.8 m/s/s equals his weight, which is a force.

3rd law: Homer pushes against the ground and it pushes back.

Newton’s Laws of Motion

Newton’s First Law (law of inertia)

An object at rest tends to stay at rest and an object in motion tends to stay in

motion unless acted upon by an unbalanced force.

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Newton’s First Law (Law of Inertia) An isolated object, free from external forces, will continue moving at constant velocity, or remain at rest.

Earlier, Aristotle said objects were “naturally” at rest, and needed a continuing push to keep moving.

Galileo realized that motion at constant velocity is “natural”, and only changes in velocity require external causes.

Objects in equilibrium (no net external force) also move at constant velocity.

1st Law •  Inertia is the

tendency of an object to resist changes in its velocity: whether in motion or motionless. These pumpkins will not move unless acted on

by an unbalanced force.

Balanced Force

Equal forces in opposite directions produce no motion

Unbalanced Forces

Unequal opposing forces produce an unbalanced force

causing motion

If objects in motion tend to stay in motion, why don’t moving objects keep moving forever?

Things don’t keep moving forever because there’s almost always an unbalanced force

acting upon them.

A book sliding across a table slows down and stops because of the force of friction.

If you throw a ball upwards it will eventually slow down and fall because of the force of gravity.

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Newton’s First Law of Mo0on

Object at rest (not moving)

No force on object

Uniform motion

Note: Object at rest is just uniform motion at zero speed.

then

then

1st Law • Once airborne,

unless acted on by an unbalanced force (gravity and air – fluid friction), it would never stop!

1st Law

• Unless acted upon by an unbalanced force, this golf ball would sit on the tee forever.

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Demo: Rolling, rolling, rolling W

ALL

All moving together

Bottom car stops. Other two continue With same speed.

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Demo: Tablecloth Pull

Tablecloth

Flower Vase

Yank quickly

Why then, do we observe every day objects in motion slowing down and becoming motionless seemingly without an outside force?

It’s a force we sometimes cannot see – friction.

Objects on earth, unlike the frictionless space the moon travels through, are under the influence of friction. The Earth’s atmosphere is a source of friction.

n There are four main types of fric0on: n Sliding fric0on: ice ska0ng n Rolling fric0on: bowling n Fluid fric0on (air or liquid): air or water resistance n Sta0c fric0on: ini0al fric0on when moving an object

What is this unbalanced force that acts on an object in motion?

Newtons’s 1st Law and You

Don’t let this be you. Wear seat belts.

Because of inertia, objects (including you) resist changes in their motion. When the car going 80 km/hour is stopped by the brick wall, your body keeps moving at 80 m/hour.

Newton’s Second Law

Force equals mass times acceleration. F = ma

2nd Law

The net force of an object is equal to the product of its mass and acceleration, or F=ma.

2nd Law

•  When mass is in kilograms and acceleration is

in m/s/s, the unit of force is in newtons (N). •  One newton is equal to the force required to

accelerate one kilogram of mass at one meter/second/second.

Physics 1D03 - Lecture 6 24

Forces

•  A force is a push or pull that tends to cause motion (more exactly, changes in motion)

•  From the Second Law, force should have units of

•  Force is a vector

•  In Newton’s dynamics, all influences on a particle from its surroundings are expressed as forces exerted on that particle

(N) newton 1 m/skg 1 2 =•


Newton’s Second Law

Fnet (or Ftotal) is the vector sum of all forces acting on the particle of mass m:

The acceleration a is parallel to the total force, and proportional

to it. The proportionality constant is the particle’s mass. Newton defines mass as a measure of an object’s inertia.

amFnet

=

∑ ∑==i i

iinet amFF

Newton’s Second Law •  Force = Mass x Acceleration •  Force is measured in Newtons ACCELERATION of GRAVITY(Earth) = 9.8 m/s2

•  Weight (force) = mass x gravity (Earth)

Moon’s gravity is 1/6 of the Earth’s If you weigh 420 Newtons on earth, what will you weigh on the Moon?

70 Newtons If your mass is 41.5Kg on Earth what is your mass on the Moon?


Contact Forces : direct contact is required examples - normal forces, friction, air resistance, buoyancy, ...

Non-Contact Forces :

gravity, electromagnetic, weak and strong forces

The gravitational force is also called weight and is measured in Newtons.

Weight is proportional to mass : Fw = mg, where g is the gravitational field (and is also the acceleration of an object in free fall).


Weight and Mass Weight is a force; it can be measured using a spring scale

On Earth, a baseball weighs 2.40 N

On the moon, a baseball weighs 0.40 N


•  Mass is a measure of inertia : on the earth or on the moon, a 24.5 N force applied to the baseball will give it an acceleration of 100 m/s2 (its mass is m = F/a = 0.245 kg)

•  We can compare masses with a balance, because of the remarkable property :

mass weight ∝

1,gF

2,gF

Weights are equal when masses are equal

2nd Law (F = m x a) n How much force is needed to accelerate a 1400 kilogram car 2 meters per second/per second?

n Write the formula n F = m x a n Fill in given numbers and units n F = 1400 kg x 2 meters per second/second n Solve for the unknown n 2800 kg-‐meters/second/second or 2800 N

If mass remains constant, doubling the acceleration, doubles the force. If force remains constant, doubling the mass, halves the acceleration.

Newton’s 2nd Law proves that different masses accelerate to the earth at the same rate, but with different forces.

•  We know that objects with different masses accelerate to the ground at the same rate.

•  However, because of the 2nd Law we know that they don’t hit the ground with the same force.

F = ma

98 N = 10 kg x 9.8 m/s/s

F = ma

9.8 N = 1 kg x 9.8 m/s/s

Check Your Understanding •  1. What acceleration will result when a 12 N net force applied to a 3 kg

object? A 6 kg object? •  2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s2.

Determine the mass.

•  3. How much force is needed to accelerate a 66 kg skier 1 m/sec/sec? •  4. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/sec/

sec?

Check Your Understanding •  1. What acceleration will result when a 12 N net force applied to a 3 kg object?

12 N = 3 kg x 4 m/s/s

•  2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s2. Determine the

mass. 16 N = 3.2 kg x 5 m/s/s

•  3. How much force is needed to accelerate a 66 kg skier 1 m/sec/sec?

66 kg-m/sec/sec or 66 N

•  4. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/sec/sec?

•  9800 kg-m/sec/sec or 9800 N

Newton’s Third Law

For every action there is an equal and opposite reaction.

Newton’s 3rd Law •  For every action there is an equal and

opposite reaction.

Book to earth

Table to book

Think about it . . .

What happens if you are standing on a skateboard or a slippery floor and push against a wall? You slide in the opposite direction (away from the wall), because you pushed on the wall but the wall pushed back on you with equal and opposite force.

Why does it hurt so much when you stub your toe? When your toe exerts a force on a rock, the rock exerts an equal force back on

your toe. The harder you hit your toe against it, the more force the rock exerts back on

your toe (and the more your toe hurts).

Newton’s Third Law •  A bug with a mass of

5 grams flies into the windshield of a moving 1000kg bus.

•  Which will have the most force?

•  The bug on the bus •  The bus on the bug


Newton’s Third Law (action and reaction)

If object A exerts a force on object B, object B exerts an equal, opposite force back on A.

gF

Block pushes down on table


Newton’s Third Law : examples

What is the “reaction” to the following forces?

gF

−Gravity (of block) pulls earth up

Balloon pushes on air outside

You push on a block


Contact Forces

Examples : A heavy block on a table

•  The table must push up on the block to prevent it from falling

•  The type of contact force is called a normal force if it is perpendicular (normal) to the surfaces in contact.

•  The normal force will be as large as necessary to hold the block (until the table breaks)

gF

Forces on Block

tableF


A 140-kg wrestler and a 90-kg wrestler try to push each other backwards out of the ring. At first they are motionless as they push; then the large wrestler moves the other one backwards. Compare the forces they exert on each other. Which statement is correct?

a)  The forces are always equal.

b)  The larger wrestler always exerts a larger force.

c)  When they are motionless, the forces are equal; they start to move when the large wrestler exerts a larger force on his opponent than his opponent exerts back on him.

Quiz

44

Free-Body Diagrams

•  Pick one object (the “body”).

•  Draw all external forces which act directly on that body (gravity, contact, electromagnetic). Imagine cutting around the body to separate it from its surroundings. Replace each external object with a force applied at the point of contact.

•  Indicate the direction of the acceleration of the object beside the diagram; but remember, ma is not a force on the diagram.

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Net Force

same as

When several forces act on an object, the forces add together.

Sum of forces called net force or total force

BRICK 3 Newtons

5 Newtons 8 Newtons

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Check Yourself

?

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Equilibrium Rule

same as

If an object is at rest then the net force must be zero. Similarly if in uniform mo0on.

BRICK 3 Newtons 3 Newtons

Zero Newtons (No Force)

When this happens we say that forces “balance.”

22-Sep-12 Physics 1 (Garcia) SJSU

Demo: Up+Up=Down+Down

STICK (2 Newtons)

10 Newton Iron mass

Scale reads 5 Newtons

Scale reads 7 Newtons

The two upward forces (by people holding the scales) must always be equal to the two downward forces (weight of stick and iron mass).

Stick is at rest.

5+7=10+2

22-Sep-12 Physics 1 (Garcia) SJSU

Support Force

Solid surfaces exert a force, called a support force, on objects pressed against them.

100 Newton Gold Brick

100 Newton Support force

Downward force (weight) balanced by upward force (support).

How much is the net force on the brick?

50

Ropes

•  Tension is uniform in a rope of negligible mass •  The tension is not changed if the rope passes

over an ideal pulley (assume frictionless and massless)

•  Tension has units of force (newtons)

•  A rope attached to something exerts a force parallel to the rope

•  The magnitude of the force is called the tension in the rope

rope

Force

51

-  normal force : is perpendicular to the surfaces in contact

-  friction : is parallel to the surface

n

f

friction has a more complex behaviour than the normal force (next lecture)

Moving a Block

n

AF

gF

f

!@%&*#$

Divide the contact force from the table into two components:


Friction

Friction is the force which resists sliding of two surfaces across each other.

We distinguish between static and kinetic friction:

Static Friction : - there is NO relative motion - fs prevents sliding Kinetic Friction : - the block is sliding - fk is opposite to v

sfFA

kf

∑ = 0F

v

53

A block of mass 10kg is resting on a surface with a coefficient of static friction µs=0.50. What minimum force F is needed to move the block?

Quiz

a)  10 N b)  49 N c)  98 N

10 kg F

54

A block of mass 10kg is resting on a surface with a coefficient of static friction µs=0.50. Once the block is moving, what force is needed to accelerate it?

Quiz

a)  less than 49 N b)  49 N c)  more than 49 N

10 kg F

55

Each block weighs 100 N, and the coefficient of static friction between each pair of surfaces is 0.50. What minimum force F is needed to pull the lower block out?

•100 N

100 N

Quiz

a)  50 N b)  100 N c)  150 N

F

•  Consider the flying motion of birds. A bird flies by use of its wings. The wings of a bird push air downwards. In turn, the air reacts by pushing the bird upwards.

•  The size of the force on the air equals the size of the force on the bird; the direction of the force on the air (downwards) is opposite the direction of the force on the bird (upwards).

•  Action-reaction force pairs make it possible for birds to fly.

Other examples of Newton’s Third Law

•  The baseball forces the bat to the left (an action); the bat forces the ball to the right (the reaction).

3rd Law •  Consider the motion of

a car on the way to school. A car is equipped with wheels which spin backwards. As the wheels spin backwards, they grip the road and push the road backwards.

3rd Law The reac0on of a rocket is an applica0on of the third law of mo0on. Various fuels are burned in the engine, producing hot gases.

The hot gases push against the inside tube of the rocket and escape out the bo^om of the tube. As the gases move downward, the rocket moves in the opposite direc0on.

Newton’s 3rd Law

• Suppose you are taking a space walk near the space shuttle, and your safety line breaks. How would you get back to the shuttle?

Newton’s 3rd Law

•  The thing to do would be to take one of the tools from your tool belt and throw it is hard as you can directly away from the shuttle. Then, with the help of Newton's second and third laws, you will accelerate back towards the shuttle. As you throw the tool, you push against it, causing it to accelerate. At the same time, by Newton's third law, the tool is pushing back against you in the opposite direction, which causes you to accelerate back towards the shuttle, as desired.

What Laws are represented?

Crash Test

Documents

Newton s Laws of Motion - PBworksmrdolanscience.pbworks.com/w/file/fetch/59055308/Newton's Laws of... · Newton’s Laws of Motion • 1st Law – An object at rest will ... F g,1