Energy, Work, Power and Machines
14 Grade 10 Physics questions on Energy, Work, Power and Machines, with worked marking schemes. 14 questions are available on this topic in full.
Question 1
1 markAt a hydroelectric station on the Tana River, water held behind the dam flows down and turns turbines which drive generators. The main energy changes are:
A. kinetic energy to potential energy to electrical energy
B. potential energy to electrical energy to kinetic energy
C. potential energy to kinetic energy to electrical energy
D. chemical energy to kinetic energy to electrical energy
(1 mark)
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3 marksA farmer in Kitale lifts a 40 kg bag of maize from the ground onto a lorry platform 1.2 m high in 4 s. Taking g = 10 N/kg, determine (a) the work done on the bag, (b) the power developed by the farmer.
(3 marks)
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1 markA student carries a carton of exercise books at a steady height along a level corridor. The work done by the student against the weight of the carton is:
A. equal to the weight of the carton multiplied by its height above the floor
B. zero, because the carton does not move in the direction of its weight
C. equal to the weight of the carton multiplied by the distance walked
D. large, because the student becomes tired after a while
(1 mark)
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3 marksA ripe mango falls from a branch of a tree. Explain the energy changes that take place from the moment it begins to fall until it finally comes to rest on the ground.
(3 marks)
Marking scheme
- On the branch the mango has maximum gravitational potential energy and no kinetic energy (1) - As it falls, the potential energy is converted into kinetic energy so the mango speeds up; ignoring air resistance the total mechanical energy stays the same (1) - On striking the ground the kinetic energy is converted into sound and heat, and into work done in deforming the mango and the ground (1)
Question 5
3 marksA plank 6 m long is used as an inclined plane to roll a drum of weight 900 N onto a store platform 1.5 m high. Determine the velocity ratio of the inclined plane and the effort required if the plane is 60% efficient.
(3 marks)
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4 marksAt a building site in Eldoret, a wheelbarrow carries sand of weight 600 N whose weight acts 0.4 m from the wheel axle. The worker lifts at the handles, 1.2 m from the axle. Determine (a) the effort applied, (b) the mechanical advantage of the wheelbarrow.
(4 marks)
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1 markA matatu travelling along a straight, level stretch of the Nairobi–Nakuru highway doubles its speed. Its kinetic energy becomes:
A. half as large
B. eight times as large
C. four times as large
D. twice as large
(1 mark)
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2 marksDistinguish between the mechanical advantage and the velocity ratio of a machine.
(2 marks)
Marking scheme
- Mechanical advantage is the ratio of the load to the effort; its value depends on friction in the machine and on the weight of its moving parts (1) - Velocity ratio is the ratio of the distance moved by the effort to the distance moved by the load; it depends only on the arrangement (geometry) of the machine and not on friction. Both are ratios and so have no units (1)
Question 9
1 markIn raising a load with a machine, the effort does 500 J of work while the work done on the load is 400 J. The efficiency of the machine is:
A. 125%
B. 20%
C. 40%
D. 80%
(1 mark)
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1 markA learner of mass 50 kg runs up a staircase of 20 steps, each step 0.2 m high, in 10 s. Taking g = 10 N/kg, the power developed by the learner is:
A. 2 000 W
B. 200 W
C. 20 W
D. 100 W
(1 mark)
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1 markA labourer drags a crate through a distance of 8 m along a level floor using a horizontal force of 25 N. The work done by this force is:
A. 3.1 J
B. 0.32 J
C. 200 J
D. 33 J
(1 mark)
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5 marksA block and tackle of velocity ratio 5 is used to raise a load of 450 N through a height of 2 m using an effort of 120 N. Determine (a) the mechanical advantage, (b) the distance moved by the effort, (c) the work input, (d) the work output, (e) the efficiency of the machine.
(5 marks)
Marking scheme
- (a) MA = load ÷ effort = (1) - (b) Distance moved by effort = VR × distance moved by load = (1) - (c) Work input = effort × effort distance = J (1) - (d) Work output = load × load distance = (1) - (e) Efficiency = (work output ÷ work input) × % (1)
Question 13
3 marksDistinguish between work and power, stating the SI unit of each.
(3 marks)
Marking scheme
- Work is the product of the force and the distance moved in the direction of the force (1) - Power is the rate of doing work, that is the work done per unit time (1) - The SI unit of work is the joule (J) and that of power is the watt (W), where 1 W = 1 J/s (1)
Question 14
1 markThe efficiency of a real machine is always less than 100% because:
A. the velocity ratio of a machine is always smaller than its mechanical advantage
B. some of the energy supplied to the machine is destroyed inside it
C. the effort applied is always smaller than the load raised
D. part of the work input is used to overcome friction and to lift the moving parts of the machine
(1 mark)
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