Unit 3 - Test Yourself
Question 01: State what is meant by (i) energy, (ii) power. Give the unit in which each is measured.
Question 02: A car of mass 1000 kg, starting from rest, increases its velocity to 15 m s-1 over a time of 5 seconds. Calculate: (i) The kinetic energy of the car at the end of the 5 seconds; (ii) The average power generated in giving the car this velocity over this time.
Question 03: When a car of mass 1000 kg is accelerated from rest to a speed of 20 m s–1 in 8 seconds, what is the average power developed by the engine?
Question 04: A 30 kW motor in a crane lifts a block of 1000 kg through a height of 10 m in 5 seconds. Calculate the efficiency of the motor if it is working at full power. (g = 9.8 m s–2)
Question 05: A 0.4 kW lift hoist on a building site is used to raise concrete blocks to a height of 6.0 m. If the elevator raises 10 blocks per minute and the weight of each block is 200 N, calculate, ignoring friction, the average power output of the elevator. Also, calculate the efficiency of the lift.
Question 06: Calculate (a) the energy used, and (b) the average power developed, by a man whose mass is 70 kg and who runs upstairs from the ground floor to the first floor of a building in 8 seconds, if there is a vertical height of 3 m between the floors. (Take g = 9.8 m s-2)
Question 07: What are the units of the quantity that is obtained when the area under this graph is calculated?
Question 08: A motor of efficiency 85% is used to haul a lift up its shaft in 16 seconds. If the total length of the shaft is 100 m and the mass of the lift is 1400 kg, what will the electrical power supplied to the motor be? (Take g = 9.8 m s-2)
Question 09:[Higher Level] This is a difficult one! What is the maximum power that you could achieve from a wind turbine that has a blade radius of 7 m in a wind of velocity 5 m s-1? Take the density of air as 1 kg m-3)
Question 10:[Higher Level] A wheel is connected to a motor and turned clockwise at a constant speed of 10 revolutions per second. Two weights are hung from a string passing over the wheel as shown. With the wheel turning, the system is in equilibrium. The length of string in contact with the wheel is 0.5 m. What power must the motor produce to turn the wheel?
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