Newton’s Laws of Motion

Newton’s First Law

• An object at rest remains at rest and an object in motion remains in motion at constant speed and in a straight line unless acted upon by an unbalanced force.– An object at rest means it is not moving– An unbalanced force is a push or pull.

Inertia

• Newton’s first law is sometimes called the law of inertia.

• Inertia is the tendency of all objects to resist any change in motion.– More mass= more inertia

• The acceleration of an object depends on the mass of the object and the amount of force applied.

• Force= mass x acceleration

Newton’s Second Law

F

M A

Units

• Force measured in Newtons (N)

• Acceleration measured in m/s2

• Mass is measured in kg

• A Newton can be described as the amount of force required to give a 1 kg mass an acceleration of 1 m/s2

Try it out…

• Find the force needed to accelerate a 800-kg car at a rate of 5m/s2

– F=MA– F=(800kg) X (5 m/s/s)– F= 4000 N

Newton’s Third Law

• If one object exerts a force on another object, then the second object exerts a force of an equal strength in the opposite direction.

“For every action there is an equal and opposite reaction.”

“Action…reaction”• Forces always occur in pairs. Single, isolated

forces never happen!– However, you cannot always detect the motion.

• You cannot see the Earth’s equal and opposite reaction when the Earth’s gravity pulls on something.Ex: When you drop your pencil gravity pulls it downward. At the same time, the pencil pulls the earth upward. You do not see the earth accelerate.

• These given names are confusing for two reasons: – Either force in an interaction can be the ‘action’ force

or the ‘reaction’ force. – Unfortunately we associate ‘action’ and ‘reaction’ with

‘first an action, then a reaction’.• This is NOT what occurs in the third law. The action force

and the reaction force exist at the SAME time.

              

“Equal”

• Equal means two things:

1. Both forces are exactly the same size. They are equal in magnitude.

2. Both forces exist exactly at the same time. They are equal in time.

– So why don’t they always cancel out?• They are not always acting on

different objects.– Volleyball- setting vs. blocking

“Opposite”

• Opposite means that the two forces always act in opposite direction.

• Exactly 180 degrees apart

Momentum• The momentum an object has depends on mass and

velocity.– Formula:

Momentum= Mass x Velocity– Unit:

kgm/s (“kilogram-meters per second”)

– The more momentum an object has the harder it is to stop.

– If a car and a Mack truck are

traveling at the same speed the

truck has more mass, therefore

it has more momentum and is

harder to stop.

Try it out….

• What is the momentum of a bird with the mass of 0.018 kg flying at 15 m/s?

Momentum= M x V

Momentum= 0.018 kg x 15 m/s

Momentum= .27 kgm/s

Try it out…

• Which has more momentum?

A 3 kg sledge hammer swung at 1.5m/s

or a 4kg sledgehammer swung at .9m/s.Momentum= M x V

Momentum= 3 kg x 1.5 m/s

Momentum= 4.5 kgm/s

Momentum= M x V

Momentum= 4 kg x 0.9 m/s

Momentum= 3.6 kgm/s

The Law of Conservation of Momentum• The total momentum of any group of

objects remains the same, or is conserved unless outside forces act on the object.– The total momentum of objects that interact does not

change, in the absence of outside forces. – Momentum may be transferred from one to another, but

none is lost.

Collisions With Two Moving Objects• Some of the momentum is transferred.

• The momentum of one object decreases while the momentum of the other increases.

Collisions With One Moving Object• All of the momentum is transferred

Collision With Connected Objects• The momentum is transferred so that

both objects are moving at the same momentum.