Sri pratyangira Institute
1. A child drops a stone of 1 kg from the top of a tower. Find its kinetic energy, 5 s after its
starts falling.
Answer: 1250 J
2. An electric bulb marked with 100 V-40 W is used for 5 h in a week. What is its the cost of
electricity, if the rate is Rs 3 per unit?
Answer: Rs 6.3
3. When a force retards the motion of a body, what is the nature of work done by force state
reason. List two examples of such a situation.
4. A body of mass 5 kg is thrown vertically upwards with an initial velocity of 50ms-1. What will
be its potential energy at the end of 5 s?
Answer:6250 J
5. A body of mass 1.5 kg is thrown vertically upwards with an initial velocity 15ms-1. What will
be its potential energy at the end of 2 s?
Answer: 750 J
6. Mohan lifts his cell phone at a height of 1m. He takes 1s to do this. After lowering the phone,
he then lifts its 2 m in 2 s. Has he generated more power in doing the second task? Give
reason to justify your answer.
7. 300 J of work is to be done in lifting a bag of 5 kg in weight up to height of 4m from the
ground. What will be the acceleration with which the bag was revised?
Answer:5ms-2
8. (i) give one situation where force is applied but no work is done. Explain, why?
(ii)a pump is used to raise water to height of 20m. It transfers 2000 kg of water in 15min.
Calculate power of the pump.
Answer: 444.44 Js-1
9. A labourer whose own mass is 50kg carries a load of an additional 60kg on his head to the
top of a building 15m high. Find the total work done by him. Also, find the total work done
by him, if he carries another additional block of mass 10kg to the same height. Answer: w=0
10. A force applied on a body of mass 4kg for 5s changes its velocity from 10m/s to 20m/s. Find
the power required.
Answer:120 J/s
11. Calculate the K.E of a car of mass 750kg moving with a velocity of 54km/h. Find the new
kinetic energy of the car, if a passenger of mass 50kg sits in the car.
Answer: 90000 J
12. A machine gun takes 10s to fire 30 bullets with a velocity of 500m/s. Find the power
developed by the gun when each bullet has a mass of 100g.
Answer:37500W
13. A boy of mass 50kg runs up a staircase of 45 steps in 9s. If the height of each step of the
staircase is 15cm, find the power of the boy.
Answer: 375W
14. Shyam drops a ball of 100g from a building of height 10m. What will be its K.E at the height
of 4m? What will happen to its total mechanical energy? Give reason to justify your answer.
Answer: 10 J
15. A mass of 10 kg is dropped from a height of 50cm. Find its (i) potential energy just before
droping.(ii)kinetic energy just on touching the ground. (iii)velocity with which it hits the
ground
Answer: (i)50J (ii)50J (iii) 3.16m/s
16. What is the power output in watt of a 60kg sprinter who acceleration from 0 to 10m/s in 3s?
Answer: 1000W
17. Two boys A and B weighing 60 kg and 40kg respectively, climb on a staircase each carrying a
load 20kg on their head. The staircase has 10 steps, each of height 50cm. If A takes 20s to
climb and B takes 10s to climb, then (i) who possesses greater power? (ii) Find the ratio of
their powers.
Answer:
(i) b possesses greater power than A
(ii)1000W
18. Power supplied to change the velocity of a car from 10m/s to 5m/s in 4s is 100W. Find the
work done. If the KE before and after the power is supplied to it.
Answer:12500 J
19. Two women shanti and kamla each of mass 50 kg and 60kg respectively, climb up through a
height 10m. Shanty takes 20s while kamla takes 40s to reach. Calculate the difference in the
power expended by shanty and Kamla.
Answer: 100 J/s
20. On a level road, a scooterist applies brakes to slow down from a speed of 54km/h to
36km/h. What amount of work is done by the brakes?
Answer: -9375J
21. Find the ratio of gravitational potential energy, if height of an object is doubled and mass is
tripled. Also, find the ratio of work done by gravity in bringing the object to zero height in
both cases.
Answer: 6:1
22. A car weighing 1200kg is uniformly accelerated from rest covers a distance of 40m in 5s. cost
per unit is Rs1.80
Answer:205.20
23. (i)Name two forms of mechanical energy. Define the SI units of energy.
(ii)A man of mass 50kg jumps from height of 0.5m. If g=10m/s2, what will be his energy at
the highest points?
Answer: 250J
(iii) Calculate the energy of a body of mass 20kg moving with velocity of 0.1m/s.
Answer: 0.1J
24. A vehicle of 1 tone travelling with speed of 60m/s notices a cow on the road 9m ahead
applies brakes. It stops just in front of the caw.
(i)
Find the KE of the vehicle before applying brakes.
Answer:1800000J
(ii)
Calculate the retarding force provided by the brakes.
Answer:-200000N
(iii)
How much time did it take to stop after by brakes were applied. Answer:0.3 s
(iv)
What is the work done by the braking force?
Answer:-1800000J
25. A car is moving with uniform velocities; 18km/h, 36km/h, 54km/h and 72km/h at some
intervals. Find the KE of the boy of 40kg sitting in the car at these velocities. Draw a graph
between the KE and the velocities. Also, find the nature of the curve.
Answer: 8000J
26. A man weighing 40kg climbs a vertical height of 200m. Calculate the amount of work done
by him. How much potential energy does he gain?
Answer:7.84 X 104J
27. Suppose two bodies A and B having equal masses are kept at heights of h and 3h
respectively. Find the ratio of their potential energies.
Answer:1:3
28. A rocket moving is moving up with a velocity v. If the velocity of this rocket is suddenly
tripled, what will be the ratio of two kinetic energies?
Answer:1:9
29. An object of mass m is moving with a constant velocity v. How much work should be done
on the object in order to bring the object to rest?
1
Answer:2 𝑚𝑣 2
30. A bag of wheat 20kg. What will be the height to which it should be raised so that its
potential energy is 1000J?
Answer:5m
31. Four men left a 250kg box to a height of 1m and hold it without raising or lowering it. (i)how
much work is done by the man in lifted the box ?(ii)how much work do they do in just
holding it ? (iii) why do the get tired while holding it?
Answer:(i)2500J(ii)zero
32. A motorbike is initially at rest driven by a force of 1000N. If mass of the bike is 50kg, then
calculate KE acquired by the bike at the end of 5s.
Answer:250KJ
33. The weight of a person on a planet A is about half that on the earth. He can jump up to 0.4m
height on the surface of the earth. How high he can jump on the planet A?
Answer:0.8m
34. What is the work to be done to increase the velocity of a car from 30Km/h to 60km/h, if the
mass of the car is 1500kg?
Answer:15637J
35. Calculate the work required to be done to stop a car of 1500kg moving at a velocity to
60km/h.
Answer:208333.3
36. A machine makes a man to move with high velocity of 40m/s. Determine the total
mechanical energy of the man weighing 80 kg, if he is on height of 100m at this speed.
Answer: 1.4 X 105J
37. An object of 50kgraised to a height of 5m above the ground. What is the potential energy? If
the object is allowed to fall, find its kinetic when it is half way down.
Answer:2500J, 1250J
38. An object of mass 10kg is thrown upward with a speed of 5m/s. What is its potential energy
when its reaches its maximum height? What is its maximum height?
Answer: 125J,1.28m
39. A boy does 400j of work in 20s in the first time and during the second time, same boy does
100J of work in 2s. Find the ratio of the power delivered by the boy.
Answer: 2:5
40. A man weighing 60kg climbs up a staircase carrying a load of 20kg on his head. The staircase
has 20 steps each of height 0.2m. if he takes 10s to climb, find his power.
Answer:313.6W
41. A machine gun fires 60 bullets per minute with a velocity of 700m/s . If each bullet has a
mass 50g, find the power developed by the man.
Answer: 12250W
42. A lift is designed to carry a load of 4000kg through 10 floors of a building overaging 6m per
floor in 10s. Calculate the horse power of a lift.
Answer: 315.28HP
43. If a house has two 60W bulbs lighted for 5H, a100W lamp lighted for 2h, a 50W TV running
for 8h and two electric fans working for 10h of power of 50W. Then, how many electric units
are consumed each day?
Answer: 2.2 units
44. Compare the power at which each of the following is moving upwards against the force of
gravity? (i) a butterfly of mass 1 kg that flies upward at a rate of 0.5m/s (ii)a 250g squirrel
climbing up on a tree at a rate of 0.5m/s.
Answer: (i)1/200W(ii) 250/200W
45. Two boys A and B asked to take four chairs from ground floor to at the second room. A takes
30 min to do this work while B takes 60 min for the same. (i) name the physical quantity that
is same for both boys. (ii) name the physical quantity that is different and find its ratio for
two boys.
Answer: 2:1