AS-LEVEL: Force, Energy and Momentum QP (PMT)

Question 1

Figure 2 shows the direction of the thrust exerted by the ship’s propeller as the propeller rotates. The ship’s engine makes the propeller rotate. When more water is accelerated, more work is done by the engine.

Explain, using Newton’s Laws of Motion, how the thrust of the propellor on the water enables the ship to maintain a constant momentum.

(4 marks)

Answer 1

-propellors cause change in the waters momentum
-NII: F=Δp/Δt ∴ the propellors exerted a force on the water

-NIII: the water exerts an equal & opposite force on the propellors (thrust)

-NI: thrust must = the drag force experienced by the ship, so no resultant force, so no acceleration, ∴ constant momentum

Question 2

What is true for an Inelastic Collision between two isolated objects? (1)

A) Both total Momentum and total Kinetic Energy are Conserved

B) Neither total Momentum nor Kinetic Energy is Conserved

C) Only total Kinetic Energy is Conserved

D) Only total Momentum is Conserved

Answer 2

D) only total momentum is conserved

Question 3

Figure 3 shows the bottom of the hull with a drag reduction system in operation.
Air bubbles are introduced into the water below the hull. This reduces the work done per second against the drag on the hull at any given speed. However, when the air bubbles reach the propeller they decrease the mass of water being accelerated by the propeller every second. This decreases the thrust produced by the propeller at a given speed of rotation.

The system enables the ship to save fuel while maintaining the same momentum.
Explain why the system delivers this fuel saving. In your answer, consider the effects of the introduction of the system on
*   the thrust
*   the drag on the hull

(3 marks)

Answer 3

-the air bubbles reduce the work done per second against drag

-the air bubbles also reduce the mass of water accelerated by the propellor per second, ∴ decrease in thrust

-decrease in work done per second against drag > decrease in thrust

-for constant momentum, drag must = force

-∴ the engine can do less work, & save fuel

Question 4

explain in terms of the forces what happens as soon as a Skydiver jumps out of a plane to when he lands.

Answer 4

-as soon as he jumps out the plane, the primary force acting on him is weight, so a = 9.81ms-2

-as he falls, he collides with more air particles, so air resistance increases. not yet large enough, so he still accelerates downwards

-when he deploys his parachute, surface area increases

-∴ collides with more air particles

-∴ air resistance increases, is now greater than the downwards force of weight

-skydiver decelerates

-as he decelerates, he collides with less air particles, so resistive force of air resistance decreases

-his speed decreases until air resistance = weight

-he is now in equilibrium, and is given a new, lower, terminal velocity. it is now safe for him to land on the ground