12.3 Newton’s Third Law of Motion and Momentum

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

12.3 Newton’s Third Law of Motion and Momentum

What is Newton’s third law of motion?

Newton’s Third Law

According to Newton’s third law of motion, whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object.

12.3 Newton’s Third Law of Motion and Momentum

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

12.3 Newton’s Third Law of Motion and Momentum

Suppose you press your hand against a wall. • Your hand exerts an action force on the wall. • The wall exerts an equal and opposite reaction force

against your hand.

Newton’s Third Law

12.3 Newton’s Third Law of Motion and Momentum

Action-Reaction Forces and Motion

A swimmer pushing against the water is an action force.

The reaction force acting on the swimmer causes motion through the water.

Newton’s Third Law

12.3 Newton’s Third Law of Motion and Momentum

Action-reaction forces propel the swimmer through the water. The swimmer pushes against the water, and the water pushes the swimmer.

Newton’s Third Law

12.3 Newton’s Third Law of Motion and Momentum

Action-Reaction Forces Do Not Cancel

For the swimmer, why do the action and reaction forces not cancel each other and produce a net force of zero?

Newton’s Third Law

12.3 Newton’s Third Law of Motion and Momentum

Action-Reaction Forces Do Not Cancel

For the swimmer, why do the action and reaction forces not cancel each other and produce a net force of zero?

The action and reaction forces do not act on the same object.

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

Force pairsWhich exerts more force: Earth pulling on the Moon, or the Moon pulling on Earth?

Action- Object A exerts a force on object B

Reaction- Object B exerts a force on object A

A given force exerted on a small mass produces a large acceleration, while the same force exerted on a large mass produces a small acceleration

A high-speed bus and an innocent bug have a head-on collision. The force of the bus on the bug splatters the poor bug all over the windshield. Is the corresponding force of the bug on the bus greater, less or the same? Is the resulting deceleration of the bus greater less than or the same as the bug?

Action reaction forces act on different objects so they don’t cancel each other out!

The person who exerts the greatest force against the ground not on the rope will win a game of tug of war!

QUIZ

• The product of an objects mass and its velocity is momentum.

• An object has a large momentum if the product of its mass and velocity is large.

• The momentum for an object at rest is zero.

Determine the momentum of …a. … an electron (m= 9.1 x10-31 kg) moving at 2.18 x 106 m/s (as if it were in a Bohr orbit in the H atom).b. … a 0.45 Caliber bullet (m = 0.162 kg) leaving the muzzle of a gun at 860 m/s.c. … a 110-kg professional fullback running across the line at 9.2 m/s.d. … a 360,000-kg passenger plane taxiing down a runway at 1.5 m/s

If no net force acts on a system, then the total momentum of the system does not change.

In a closed system, the loss of momentum of one object equals the gain in momentum of another object- momentum is conserved.

 A 120 kg lineman moving west at 2 m/s tackles an 80 kg football fullback moving east at 8 m/s. After the collision, both players move east at 2 m/s. Draw a vector diagram in which the before- and after-collision momenta of each player is represented by a momentum vector. Label the magnitude of each momentum vector.