EX 2

Physics Ex 2

C. W. Prescott Primary School, Room 15

2^nd December 2010 5:30 – 6:30 Time 1hr.

Instructions: Answer all questions on your own paper. Staple the question paper at the back of your answer sheet

1. A falling metal hammer is used to drive a hollow steel post into the ground, as shown. The hammer is lifted by an electric motor and then falls freely to hit the baseplate.

(a) (i) State the law of conservation of energy.

(ii) State the energy conversions that take place as the hammer falls, starting from the initial potential energy of the hammer before it is dropped. [4]

(b) The metal hammer has a mass of 1500 kg and it hits the baseplate with a speed of 8.0 m/s.

(i) Calculate the kinetic energy of the hammer as it hits the baseplate.

(ii) Assuming that there is no loss of energy, state the initial potential energy of the hammer.

(iii) Calculate the height above the baseplate from which the hammer is dropped. Take g as

10 N/kg. [6]

(c) (i) In raising the hammer, the electric motor uses more energy than that stated in (b)(ii). State and explain two reasons why the motor uses more energy.

(ii) State one change to the equipment that would cause the baseplate to move further into the ground each time the hammer falls. [5]

2. A student compares the efficiency of two electric water heaters in the kitchen. Each heaterconsists of a tank to contain the water and an electrical heating element to heat the water. The two tanks are different, but each tank contains the same amount of water and is heated by the same type of electrical heating element.

Fig. 10.1 shows part of the tank from the heater that is less efficient, and Fig. 10.2 shows the heater that is more efficient.

(a) Energy is lost by conduction, convection and radiation from each water heater.

(i) Describe what happens to the molecules of the plastic case in Fig. 10.1 during conduction.

(ii) Describe what happens to the molecules of the air outside the heater in Fig. 10.1 during

convection. [2]

(b) (i) Explain how the more efficient heater in Fig. 10.2 reduces the energy lost by conduction,

convection and radiation.

(ii) State two advantages of using an efficient water heater in a kitchen. [5]

(c) When one heater is switched on, 15 kg of cold water at 20 °C is heated to 56 °C in ten minutes.

The electrical heater is rated 5000W. The specific heat capacity of water is 4200 J/(kg °C).

(i) Calculate the increase in the internal energy (heat) of the water in ten minutes.

(ii) Calculate the electrical energy input to the heater in ten minutes.

(iii) Calculate the efficiency of the heater. [8]

Questions taken from past GCE O Level physics exam papers