Chapter 4: “Newton’s Laws of

Motion”

Created by G. Frega

Forces

• In order to make an object at rest move, you need to apply a push or a pull, otherwise known as a force.

• A force can make an object:– Speed up– Slow down– Change direction…AKA change velocity or accelerate

Force:Force: any influence that tends to accelerate any influence that tends to accelerate an object.an object.– Ex: pounds, Newtons– The SI unit is Newtons (N)

Newton’s First Law

The law of Inertia: – Every object continues in a state of rest, or Every object continues in a state of rest, or

of motion in a straight line at a constant of motion in a straight line at a constant speed, unless it is compelled to change speed, unless it is compelled to change that state by forces exerted upon it.that state by forces exerted upon it.

Inertia:Inertia: the property of a body to resist the property of a body to resist change.change.

• Things tend to keep doing what they’re already doing.

• Objects in a state of rest, stays at rest.– Ex: pulling the tablecloth out from under a

table full of plates and cups

• Objects in motion, stay in motion.– Only if moving at a constant velocity in a

straight line.– Ex: A car you are sitting in stops, but you

keep moving forward (this is why we wear seatbelts)

Mass and Inertia

• The amount of inertia depends on an object’s mass

Mass:Mass: the amount of matter present in the amount of matter present in an objectan object– SI units is kilograms (kg)**This means the SI units for inertia is kg as

well**

• The more mass, the greater the inertia…and the greater the force it takes to change the state of motion.– Directly proportion relationship

Check Your UnderstandingCheck Your Understanding

Why is it more difficult to stop a rolling car than a rolling coy car?

The car has more mass and therefore more inertia. The inertia an object has, the more is needed to change its state of motion (liking making something stop).

Mass and Weight• Mass and weight are used

interchangeably in everyday language, but they are NOT the same thing in physics!

Weight:Weight: the force of gravity on an the force of gravity on an objectobject– Because weight is a force, the SI unit is Because weight is a force, the SI unit is

Newtons (N)Newtons (N)

Weight varies with location, based on gravity.

• Mass is the same everywhere; weight is not.

Check Your UnderstandingCheck Your Understanding

A ball has a mass of 10 kg on Earth. Will A ball has a mass of 10 kg on Earth. Will its mass be more or less on the moon?its mass be more or less on the moon?

Neither, the mass will be the same in both locations because the mass of an object does not change.

What about the weight?What about the weight?

The ball will weigh more on the Earth than the moon because there is more gravity on Earth.

Weight

• Weight is a force

• It is measured in Newtons (N), just like forces are.

• A material that has a mass of 1 kg on Earth weighs 10 Newtons

W = mgW = mg• Weight is equal to mass times the

acceleration due to gravity.

Check Your UnderstandingCheck Your Understanding

A girl has a mass of 25 kg. What is her A girl has a mass of 25 kg. What is her weight on Earth?weight on Earth?

W = mg

m = 25 kg

g = 10 m/s2

W = (25)(10)

W = 250 N

Net Force

• A force is a vector– It takes into account direction

• It is the net force that changes an object’s state of motion.

Net force:Net force: the combination of all forces the combination of all forces acting on an object.acting on an object.

• If an object is resting on the table, the table is pushing on it with the same force that the book is pushing on the table, the object is in equilibrium.

Equilibrium:Equilibrium: when the net force is equal when the net force is equal to zero.to zero.

• An object in equilibrium has only inertia, no force acting on it and is:– Moving at a constant speed in a straight line– Not moving at all

Check Your UnderstandingCheck Your Understanding

A girl pushes a cart with a force of 10 N, A girl pushes a cart with a force of 10 N, however her mischievous friend however her mischievous friend pushes the same cart in the opposite pushes the same cart in the opposite direction with 5 N of force. What is the direction with 5 N of force. What is the net force?net force?

Since they are going in opposite directions, you subtract the forces to find the net force.

10 N – 5 N = 5N

Check Your UnderstandingCheck Your Understanding

A hockey player hits a hockey puck across the ice. 10 seconds after he hits it and it is still moving down the ice, is the puck in equilibrium?

Yes! Even though it is still moving, there is no net force being exerted on it, so it is moving at a constant velocity and only inertia is allowing it to keep moving.

Force and Acceleration

• If a force is applied to an object at rest, it starts to move– The object accelerates because it changed

motion (from rest to movement)

• Once it is moving at a CONSTANT VELOCITY (on a frictionless surface) no force is needed.– Because of INERTIA!

• To increase the acceleration of the object, more force needs to be applied

• Net force is directly proportional to acceleration– 2F = 2a– 3F = 3a

Mass and Acceleration

• As the mass increases, the acceleration decreases if the same amount of force is applied to the object

• Mass resists acceleration• Mass and acceleration are inversely

proportional– 2m, ½a– ½m, 2a– 10m, 1/10a– 1/10m, 10a

Newton’s Second Law of Motion

Newton’s 2Newton’s 2ndnd Law Law– The acceleration produced by a net force The acceleration produced by a net force

on an object is directly proportional to the on an object is directly proportional to the magnitude of the net force, is in the same magnitude of the net force, is in the same direction as the net force, and is inversely direction as the net force, and is inversely proportional to the mass of the object.proportional to the mass of the object.

F = maF = ma

Check Your UnderstandingCheck Your Understanding

A boy pushes on a 10 kg cart with a force of 50 A boy pushes on a 10 kg cart with a force of 50 N. What is the acceleration of the cart?N. What is the acceleration of the cart?

F = ma

a = ?

F = 50 N

m = 10 kg

50 = (10)a

a = 50/10

a = 5 m/s2

Check Your UnderstandingCheck Your Understanding

A car is set of cruise control and moving at a constant 50 m/s headed east. If the car’s mass is 1000 kg, what is its acceleration?

F = maF = ?

m = 100 kga = 0 m/s2

**Because the car is not CHANGING its velocity, the acceleration is zero**

F = 1000 (0)F = 0 N

Balance in Nature

• Everything in nature comes in pairs.

• A force is not just a push or pull, but a part of an interaction between one thing and another.

• In an interaction between objects, there are a pair of forces (one acting on each object).

Check Your UnderstandingCheck Your Understanding

Does a stick of dynamite contain force?Does a stick of dynamite contain force?

No. Force is not something an object has, like mass. Force is an interaction between two objects. An object may have the capability to exert a force on another object, but it cannot possess force as a thing in itself.

Newton’s Third Law

The Law of Action and ReactionThe Law of Action and Reaction– Whenever one object exerts a force on a Whenever one object exerts a force on a

second object, the second object exerts an second object, the second object exerts an equal and opposite force on the first equal and opposite force on the first object.object.

• One force is the action force and the other force is the reaction force.

• Neither coexists without the other.

• “To every action, there is an equal and opposite reaction.”

Check Your UnderstandingCheck Your Understanding

Earth and the moon are “connected” to Earth and the moon are “connected” to each other by a gravitational force. Is each other by a gravitational force. Is Earth pulling on the moon, or is the Earth pulling on the moon, or is the moon pulling on Earth?moon pulling on Earth?

They are part of the same interaction. They pull on each other in an action-reaction pair.

Identify the action and reaction pair of Identify the action and reaction pair of forces for the case of a bat interacting forces for the case of a bat interacting with a ball.with a ball.

The bat pushes on the ball and the ball pushes on the bat.

Check Your UnderstandingCheck Your Understanding

Interacting Forces

• Action: Object A exerts a force on object B

• Reaction: Object B exerts a force on object A

• The forces are equal and in opposite directions, even if the masses are very different.

• According to Newton’s 2nd law ( F = ma), if mass is not proportional to the pair of forces, neither is in acceleration…but the masses are still inversely proportional to the accelerations.

F = Fma = ma

ma = ma

Check Your UnderstandingCheck Your Understanding

Can you identify the action and reaction Can you identify the action and reaction forces of an object falling in a vacuum forces of an object falling in a vacuum of outer space?of outer space?

The interaction is the gravitational attraction between the falling object and another object in space, possibly a distant planet. So the planet pulls down the object (action) while the object pulls up on the planet (reaction).

According to Newton’s 3According to Newton’s 3rdrd law of motion, law of motion, the action and reaction forces are the action and reaction forces are equal. Does that mean that the planet equal. Does that mean that the planet is actually moving towards the object?is actually moving towards the object?

Yes. The same thing happens when you jump on Earth; the Earth moves towards you. But in both cases the mass of the planet is so large compared to the mass of the object/person, the acceleration would be infinitesimally small, almost zero. That’s why we don’t feel Earth move to one side every time a person jumps.