Newton’s Laws:

Force and Motion

The First Law: Force and Inertia

The Second Law: Force, Mass and

Acceleration

The Third Law: Action and Reaction

The First Law: Force and Inertia

Investigation Key Question:

How does the first law apply to objects at rest

and in motion?

Force changes motion

A force is a push or pull, or any action that is able

to change motion.

The definition of force

The simplest concept of force is a push or a

pull.

On a deeper level, force is the action that has

the ability to create or change motion.

5.1 Force Force is an action that can change motion.

— A force is what we call a push or a pull, or any

action that has the ability to change an object’s

motion.

— Forces can be used to increase the speed of an

object, decrease the speed of an object, or

change the direction in which an object is

moving.

Newton’s first law

Newton’s first law says that objects continue the

motion they already have unless they are acted

on by a net force.

If the net force is zero, an object at rest will stay

at rest.

If an object is acted upon by unbalanced forces,

its motion will change.

Remember…Newton’s first law tells us that

motion cannot change without a net force.

Net force

Newton’s first law is often written in terms of the net force:

“An object at rest will stay at rest and an object in motion will continue in motion at constant velocity UNLESS there is a net force.” According to these vectors, in what

direction is the net force?

Force changes motion

Forces can be used to increase or decrease the

speed of an object, or to change the direction an

object is moving.

Key Question:

Why are heavier objects harder to start or stop moving?

Law of inertia

Force, mass, and inertia

Some objects resist changes in motion more than others.

Inertia is the property of an object that resists changes in its motion.

The greater an object’s inertia, the greater the force needed to change its motion.

A bowling ball has more inertia than a golf ball.

Force, mass, and inertia

Inertia comes from mass.

Objects with more mass have more inertia and are

more resistant to changes in their motion.

A 5-kilogram bowling ball is 100 times as massive as

a 50 gram golf ball, so it has 100 times the inertia.

Inertia

Inertia is a term used to measure the ability of an

object to resist a change in its state of motion.

An object with a lot of inertia takes a lot of force to

start or stop; an object with a small amount of inertia

requires a small amount of force to start or stop.

The word “inertia” comes from the Latin word

inertus, which can be translated to mean “lazy.”

The net force

The motion of objects changes in response to the

total force acting on the object, including gravity and

any other forces that are present.

The net force in the horizontal direction

The term net force is used to describe the total of

all forces acting on an object.

When used this way, the word net means “total.”

The net force in vertical direction

Gravity exerts a force downward on the box.

The floor exerts an equal and opposite force upward

on the box.

The net force in vertical direction

The net force on the box in

the “up-down” direction is

zero.

When equal forces applied

to the same object are in

opposite directions they

cancel.

Applications of Newton’s First Law

Two very important safety

features of automobiles are

designed with Newton’s first

law in mind: seat belts and air

bags.

Both supply a restraining force

to counteract your inertia and

to slow your body down.

Applications of Newton’s First Law

Prior to the invention of cup

holders, drink containers left

on the dash obeyed the first

law of motion and made quite

a mess.

Can you think of other

applications of Newton’s first

law?

The Second Law:

Force, Mass, and Acceleration

Key Question:

What is the relationship between

force, mass, and acceleration?

Newton’s second law

Newton’s first law tells us that motion cannot

change without a net force.

According to Newton’s second law, the

amount of acceleration depends on both the

force and the mass.

Newton's Second Law

If you apply more force to an object, it accelerates at a higher rate.

Newton's Second Law

If the same force

is applied to an

object with greater

mass, the object

accelerates at a

slower rate

because mass

adds inertia.

Acceleration and force

The second law says that

acceleration is

proportional to force.

If force is increased or

decreased, acceleration

will be increased or

decreased by the same

factor.

Applying the second law Keep the following important

ideas in mind:

1. The net force is what causes

acceleration.

2. If there is no acceleration, the

net force must be zero.

3. If there is acceleration, there

must also be a net force.

4. The force unit of newtons is

based on kilograms, meters,

and seconds.

Newton's Second Law

a = F

m

Force (newtons, N)

Mass (kg)

Acceleration (m/sec2)

Three forms of the second law

When using the second law, the force that appears is the net force.

Consider all the forces that are acting and add them up to find the net force before calculating any accelerations.

Using units in calculations

In terms of solving physics problems, use the

following units when using force in newtons:

— mass in kilograms (kg)

— distance or position in meters (m)

— time in seconds (s)

— velocity in meters per second (m/s)

— acceleration in meters per second per second

(m/s2)

A car has a mass of 1,000 kg. If a net

force of 2,000 N is exerted on the

car, what is its acceleration?

Using Newton’s second law

1. Looking for: … the car’s acceleration.

2. Given: …car’s mass (m= 1,000 kg) and the net force (Fnet = 2,000N).

3. Relationship: Use: a = F

m

4. Solution: a = 2,000N = 2 kg• m/s2 = 2 m/s2

1,000 kg kg

5.2 Finding force from acceleration

Wherever there is acceleration there must also be

force.

Any change in the motion of an object results

from acceleration.

Therefore, any change in motion must be caused

by force.

1. You asked for the force (F).

2. You are given the mass (m) and acceleration (a).

3. The second law applies: a = F ÷ m

4. Plug in the numbers. Remember: 1 N = 1 kg·m/s2.

Calculating force

An airplane needs to accelerate at 5 m/sec2 to reach take-off speed before reaching the end of the runway. The

mass of the airplane is 5,000 kilograms. How much force is needed

from the engine?

Newton's Third Law

Newton’s third law states that for every action force there has to be a reaction force that is equal in strength and opposite in direction.

Action and reaction forces act on different objects, not on the same object.

Newton's Third Law

Newton’s third law states that for every action force there has to be a reaction force that is equal in strength and opposite in direction.

Action and reaction forces act on different objects, not on the same object.

The forces cannot cancel because they act on different objects.

The Third Law: Action and Reaction

“For every action there is an

equal and opposite reaction.”

This statement is known as

Newton’s third law of motion.

Newton’s third law discusses

pairs of objects and the

interactions between them.

Forces occur in pairs

The two forces in a pair are called action and reaction.

Anytime you have one, you also have the other.

If you know the strength of one you also know the strength of the other since both forces are always equal.

Forces occur in pairs

The astronauts working on the space station have a

serious problem when they need to move around in

space: There is nothing to push on.

One solution is to throw something opposite the

direction you want to move.