P5d - Action and Reaction

Question 1

What did Newton’s third law state? What does this mean?

Answer 1

• To every action, there is an equal and opposite reaction.
• The action and reaction are equal and opposite forces which act on different objects.

Question 2

How do you calculate momentum?

Answer 2

Momentum (kgm/s) = mass x velocity

Question 3

What happens during a collision, in terms of forces and momentum?

Answer 3

• during a collision, the two bodies interact and exert equal and opposite forces on each other.
• during a collision, momentum is conserved.
• therefore, momentum before collision = momentum after collision.

Question 4

A 500kg car travels at 5m/s, and collides with a 400kg car travelling at 2m/s. After the collision, the first car is travelling at a speed of 3m/s, whilst the second is travelling at 4.5m/s.

Work out the total momentum before and after.

Answer 4

• total momentum before = mom. of car 1 + mom. of car 2 = (500 x 5) + (400 x 2) = 2500 + 800 = 3300 kgm/s
• total momentum after = mom. of car 1 + mom. of car 2 = (500 x 3) + (400 x 4.5) = 1500 + 1800 = 3300kgm/s

Question 5

What is an explosion? Give 3 examples.

Answer 5

• an explosion includes any situation where objects travel out in opposite directions.
• e.g. rifle being fired.
• person stepping out of a boat.
• rocket being launched.

Question 6

What happens during an explosion, in terms of momentum?

Answer 6

• momentum is conserved.
• nothing is moving at the start, so the momentum is 0.
• therefore, the momentum after must be 0.
• the objects travel in different directions with equal and opposite momentum.

Question 7

A rifle weighing 5kg fires a bullet weighing 20g at a speed of 40m/s. Calculate the recoil speed of the rifle.

Answer 7

• momentum before = momentum after = 0
• momentum of rifle and bullet are equal and opposite.
• momentum of bullet = mass of bullet x speed of bullet = 0.02 x 40 = 0.8kgm/s
• momentum of rifle = -0.8kgm/s = mass of rifle x speed of rifle = 5 x speed
• speed = -0.8/5 = -0.16m/s

Question 8

Why does increasing the temperature increase the pressure of a gas?

Answer 8

The particles have more energy and more momentum and hit the walls with more force.

Question 9

Why does decreasing the volume of gas increase the pressure?

Answer 9

The particles are closer togetther, so the collisions with the walls are more frequent.

Question 10

When gas particles hit walls, what happens? What equation can be concluded from this?

Answer 10

• they move off in the opposite direction, and experience and change in momentum.
• force x time = change in momentum.

Question 11

What happens when a rocket is launched, in terms of forces and particles?

Answer 11

• the force pushing the gas particles back is equal to the force pushing the rocket forwards.
• the particles collide with the walls of the rocket creating a force.

Question 12

What two things are needed in order for a rocket to lift a satellite into space?

Answer 12

• a large number of particles of exhaust gas must collide with the walls.
• the particles must move at high speeds.