Exam-style Questions: Power and Internal Energy

  1. The front of a lorry can be considered a flat vertical surface of area 8.0m2. The average pressure due to air resistance is 150 Pa when the lorry travels at a constant speed of 16 m s-1. You may assume the air resistance force acts perpendicular to the front of the lorry. When the lorry travels at 16 m s-1.

    a) calculate the force due to the air resistance

    force due to air resistance = ................ N

    (2 Marks)

    b) calculate the power dissipated in overcoming the air resistance.

    power = ..................... W

    (3 Marks)

    c) State and explain how the values of the quantities calculated above would change if the lorry increased its speed.

    (3 Marks)

    (Marks available: 8)

  2. An electric motor drives a pulley which lifts a mass of 6kg at a constant speed of 3 m s-1 (g = 9.81 m s-2)

    a) What force is needed to lift the mass?

    (1 Mark)

    b) What is the power output of the motor?

    (2 Marks)

    c) If the motor is 60% efficient, what is the power input from the electricty supply?

    (2 Marks)

    (Marks available: 5)

Answer

Answer outline and marking scheme for question:

  1. a) force = pressure x area / 150 x 8

    = 1200 (N)

    (2 Marks)

    b)power = F x v

    = 1200 x 16 ecf

    = 19200 (W)

    (3 Marks)

    c) (force greater as) air resistance is greater explanation of why: correct quantification, air resistance proportional to v or v2 or in terms of molecules harder collisions or increased rate of collision.

    power greater as force is greater

    power greater as velocity is greater

    (3 Marks)

    (Marks available: 8)

  2. a) F = mg = 6 x 9.81 = 58.86 N

    (1 Mark)

    b)

    Power = Force x distance = Force x speed (1 Mark)

    time

    = 58.86 x 3 = 176.58 W (1 Mark)

    (2 Marks)

    c)

    60 = Power output


    100 Power input

    So power input = Power output x 100/60 (1 Mark)

    = 176.58 x 100/60

    = 294.3 W (1 Mark)

    (2 Marks)

    (Marks available: 5)