Section 4: Action and Reaction

• Objectives:state Newton’s third law of

motiondefine and calculate

momentum and state the law of conservation of momentum

I. Newton’s 3rd Law of Motion

A. For every action, there is an equal and opposite reaction.i. Forces act in pairs

A hammer exerts a force on anail, pushing it into a piece ofwood. At the same time, thenail exerts a force back on thehammer, causing its motionto come to a sudden stop.

FYI: Equal but opposite

FYI:

FYI:

FYI: Action-Reaction Example

When a squid pushes water out, the expelled water pushes back and forces the squid to move ahead. The force the squid exertson the water is the action force.

FYI: If a toy fire engine and a real fire engine were rolling towards a concrete wall at 10 mps, which one would cause more damage to the wall? Why?

II. Momentum

Of course the real fire engine would cause more damage because it has more mass, therefore it has more momentum.

A. Momentum = Mass X Velocity

B. The more momentum an object has, the harder it is to stop or to change its direction.

FYI: Think about an empty shopping cart and a shopping cart full of canned goods. Which one is harder to stop or turn? Why?

FYI: Momentum is Conserved

FYI: In the absence of friction, momentum is conserved when two train cars collide. Look at the mass and speed of each car before colliding. What is the momentum of each car and the total momentum? What happens to the mass and speed of each car after colliding? What about the momentum?

III. Conservation of Momentum

A. The total amount of momentum of a group of objects does notchange unless outside forces act on the objects.i. Momentum lost by one object is picked up or gained by the

other object.

The amount of momentum before the train cars collision is thesame as the amount of momentum after the train cars collide.

What is the mass and speed of each car before colliding?What is the total momentum before colliding?What happens to the mass, speed and momentum of the cars after the cars collide?

Example: Red Train is moving, blue is not moving

What is the mass and speed of each car before colliding?What is the total momentum before colliding?What happens to the mass, speed and momentum of the cars after the cars collide?

Example: Same thing, but trains get connected

Problems

Which has more momentum, a 250-kg dolphin swimming at 6 m/s, or a 450-kg manatee swimming at 2 m/s?

Why? (Explain using the formula.)

Which has more momentum: a 3-kg sledgehammer swung at 1.5 m/sor a 4-kg sledgehammer swung at 0.9 m/s?

Explain using your formula.

A golf ball travels at 16 m/s, while a baseball moves at 7 m/s.The mass of the golf ball is 0.045 kg and the mass of the baseball is 0.14 kg. Which has greater momentum?

(Explain completely.)

Answer

momentum = mass x velocity

0.045 kg x 16 m/s = 0.72 kg•m/s

0.14 kg x 7 m/s = 0.98 kg•m/s

The baseball has greater momentum.

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

Answer

0.018 Kg X 15 m/s = 0.27 kg•m/s