Newton’s Third Law and Momentum

Section 12.3

Review First and Second Laws

1. An object will remain at rest or in motion at constant velocity unless acted upon by a net force.

2. The acceleration of an object is directly proportional to force and inversely proportional to the mass

Contrasting Newton’s Laws

• The first and second laws of motion apply to single objects.

• In contrast, the third law of motion deals with pairs of objects.

When you push on the wall, the wall pushes on you.

Forces and Interactions

The interaction that drives the nail is the same as the one that halts the hammer.

Forces and Interactions

Newton’s Third Law

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

Newton’s third law describes the relationship between two forces in an interaction.

• One force is called the action force.

• The other force is called the reaction force.

• Neither force exists without the other.

• They are equal in strength and opposite in direction.

• They occur at the same time (simultaneously).

Newton’s Third Law

When this bumper car collides with another car, two forces are exerted. Each car in the collision exerts a force on the other.

Action and Reaction Forces

• The force your bumper car exerts on the other car is the action force.

• The force the other car exerts on your car is the reaction force.

• These two forces are equal in size and opposite in direction.

Newton’s Third Law

When the girl jumps to shore, the boat moves backward.

Newton’s Third Law

When action is A exerts force on B, the reaction is simply B exerts force on A.

Identifying Action and Reaction

When action is A exerts force on B, the reaction is simply B exerts force on A.

Identifying Action and Reaction

A given force exerted on a small mass produces a greater acceleration than the same force exerted on a large mass.

Action and Reaction on Different Masses

Earth is pulled up by the boulder with just as much force as the boulder is pulled down by Earth.

Because Earth has a huge mass, we don’t sense its infinitesimally small acceleration.

Action and Reaction on Different Masses

The balloon recoils from the escaping air and climbs upward.

A rocket accelerates in much the same way—it continually recoils from the exhaust gases ejected from its engine.

Action and Reaction on Different Masses

Using Newton’s third law, we can understand how a helicopter gets its lifting force.

• The whirling blades force air particles downward (action).

• The air forces the blades upward (reaction).

• This upward reaction force is called lift.• When lift equals the weight of the craft, the

helicopter hovers in midair. When lift is greater, the helicopter climbs upward.

Action and Reaction on Different Masses

Momentum• Momentum (p) measures a moving

object’s resistance to changes in its motion.

• Includes effects of speed and direction as well as mass.

•A moving truck is harder to stop than a car moving at the same speed because the truck has more mass.

The truck has more momentum than the car. By momentum, we mean inertia in motion.

•A fast car can have more momentum than a slow truck.

•A truck at rest has no momentum at all.

Momentum

Momentum

• Stopping a fast-moving object is harder than stopping a slow-moving one.

When you push with the same force for twice the time, you impart twice the impulse and produce twice the change in momentum.

Changing Momentum

Increasing Momentum

To increase the momentum of an object, apply the greatest force possible for as long as possible.

A golfer teeing off and a baseball player trying for a home run do both of these things when they swing as hard as possible and follow through with their swing.

Changing Momentum

Decreasing Momentum

If you were in a car that was out of control and had to choose between hitting a haystack or a concrete wall, you would choose the haystack.

Physics helps you to understand why hitting a soft object is entirely different from hitting a hard one.

Changing Momentum

If the change in momentum occurs over a long time, the force of impact is small.

Changing Momentum

If the change in momentum occurs over a short time, the force of impact is large.

Changing Momentum

Objects within a closed system can exert forces on one another.

According to the law of conservation of momentum, if no net force acts on a system, then the total momentum of the system does not change.

Conservation of Momentum

In each collision, the total momentum of the train cars does not change—momentum is conserved.

Conservation of Momentum

In each collision, the total momentum of the train cars does not change—momentum is conserved.

Conservation of Momentum

In each collision, the total momentum of the train cars does not change—momentum is conserved.

Conservation of Momentum