Short Answers - Work and Energy, Science, Class 9 PDF Download

Q1. What is gravitational potential energy?
 Ans. The gravitational potential energy of an object at a point above the ground is defined as the work done in raising the object from the ground to that point against gravity.
G.P.E = mgh 

Q2. Differentiate between potential energy and kinetic energy.
 Ans.

Q3. Give two situations where energy is supplied but no work is done.
 Ans. (i) A person pushing a heavy rock is using all the energy but if the rock does not move no work is done.
(ii) A person standing with heavy load on his head, energy is used/spent in doing this but no work is done.
 

Q4. How is work and energy related to each other?
 Ans. An object having a capability to do work is said to possess energy.
The object which does the work loses energy and the object on which the work is done gains energy. The unit of both energy and work is Joule.
 

Q5.Give two examples in your daily life where you can see kinetic energy is doing work.
 Ans. (i) Blowing wind (ii) Rotating wheel

Q6. Name the form of energy present in the following conditions/situations:
 (a) stretched bow and arrow
 (b) coiled spring
 (c) a falling coconut
 (d) water stored in the dam
 Ans. (a) Potential energy (Elastic)
(b) Elastic potential energy
(c) Kinetic energy
(d) Gravitational potential energy

Q7. On what factor does the gravitational potential energy depends?
 Ans. Gravitational potential energy depends on the height from the ground level/zero level we choose.
Example, A ball tossed from II floor will have height h from its roof but from the I floor its height will be h′.
∴ h′ > h.
The potential energy of ball w.r.t. to I floor level is more as compared to the II floor.
 

Q8. Write the transformation of energy in the given cases.
 (a) winds moving
 (b) an athlete running
 (c) A coconut falling
 (d) playing of guitar
 Ans. (a) Potential energy of wind to kinetic energy when it comes in motion.
(b) Muscular energy of athlete into kinetic energy.
(c) Potential energy into kinetic energy
(d) Potential energy (when guitar is plucked) to kinetic energy (when it shows the vibration) 

Q9. Define work, give its unit. Give one example where work done is positive, zero and negative.
 Ans. Work: Work is said to be done when force is applied on a body and it shows displacement.
W = F × s Unit of work is Joule
Example:
(a) When we push a table in the direction of force. (Work done = positive)
(b) When the object is pushed/displaced in the direction opposite to the direction of force applied. (Work done = negative)
(c) When the displacement of the body is perpendicular to the direction of force applied. (Work done = ‘0’) Example, A person carrying load on its head and moving horizontally.
 

Q10. What is energy? Name different forms of energy. Give the unit of energy.
 Ans. Energy is the capacity of a body to do work.
Different forms of energy are: Potential energy, kinetic energy, heat energy, chemical energy, electrical energy, light energy, solar energy.
The unit of energy is Joules.
 

Q11. Two stones A and B of same mass fall from height h₁ and h₂ respectively on sand where h₂ > h₁. Which stone will exert more force on the sand and why? Name the energy present in it.
 Ans. Stone B with height h₂ has more energy, potential energy is present in it, when the stone falls on the sand it exerts more force than stone A due to more energy transformation. When the stone falls all the potential energy present in it, is transformed into kinetic energy.
 

Q12. Derive the formula for kinetic energy.
 Ans. Let an object of mass m, move with uniform velocity u.
Let us displace it by s, due to constant force F, acting on it.
Work done on the object of mass ‘m’ is W = F × s ...(i)
Due to the force, velocity changes to v and the acceleration produced is ‘a’.
Relationship between v, u, a and s can be given by formula v² − u² = 2as

∴                                 ...(ii)
F = ma                                      ...(iii)
Substituting (ii) and (iii) in (i) we get
W = F × s = ma x 

W = (1/2) m (v² − u²)

If u = 0, (object starts at rest) W = (1/2)mv²

Work done = Change in kinetic energy
∴
Ek = (1/2) mv².

Q13. Derive the formula for potential energy.
 Ans. Consider a body with mass ‘m’ raised through a height ‘h’ from the ground level.
Force required to raise the object = weight of object, mg.

Object gains energy equal to the work done on it.
∴ Work done on the object against gravity be W.
∴ W = force × displacement
= mg × h
W = mgh
∴ P.E. = mgh

Q14. Define power, give its unit and define it.
 Ans. Power is defined as the rate of doing work.

Unit → Watt Definition of watt. When 1 Joule work is said to be done in 1 second, the power of the body doing work is said to be 1 watt.
 

Q15. Explain, law of conservation of energy.
 Ans. Law of conservation of energy states that energy can neither be created nor destroyed but it can be transferred from one form to another. The total energy before and after transformation remains the same.
Let the body be at position A from the ground level. It has potential energy P_E. When the body falls down and reaches in the middle P.E. = K.E. and when the body is just about to touch the ground its K.E. > P.E. This shows that P.E. is getting transferred into kinetic energy.
 

Q16. Two boys A and B were given a task to carry 20 kg load from ground level to height 10 m. A completed the work in 40 s and B in 60 s. Calculate the power in both the cases. Who has greater power?
 Ans. Power =   Work = m × g × h
∴ mass = 20 kg
∴ W = mgh
h = 10 m,
t_A = 40 s
t_B = 60 s,
PB = ?
g = 10 m/s²

P_A > P_B
50 W > 33.34 W
∴ Power of A is more than B.
 

Q17. An electrical appliance of 100 W is used for 3 h per day. Calculate the units of energy consumed per day and for a month.
 Ans. Power of electrical appliance = 100 W = 0.1 kW

Time used = 3 h
Energy = power × time taken = 0.1 kW × 3h = 0.3 kWh = 0.3 units (Q 1 kWh = 1 unit)
The units of energy consumed per day = 0.3 units and for a month = 0.3 × 30 = 9 units.
 

Q18. A body possess potential energy of 460 J whose mass is 20 kg and is raised to a certain height. What is the height when g = 10 m/s²?
 Ans. P.E. = 460 J
m = 20 kg g = 10 m/s² h = ?
P.E. = mgh 460 = 20 × 10 × h

h = 2.3 m.
The height at which the object is raised from the ground is 2.3 m.
 

Q19. A mover pushes a 1700 N piano up a 4-metre ramp. How much work is done by the mover?
 Ans. Work = Force × displacement = 1700 × 4 = 6800 J
 

Q20. How much work is done when an engine generates 400 watts of power in 25 seconds?
 Ans. Power = work done / time taken

Work = Power × time = 400 × 25
= 10000 J
 

Q21. Why does an object float or sink when placed on the surface of the water?
 Ans. When the density of the object is more than the density of water then the object sinks. If the density of an object is less, then the object will float on water. To change the density of an object we can change its volume by giving it different shapes and the same object that was sinking can be made to float on water.
 

Q22. What do you mean by thrust or buoyant force? What are the factors the buoyant force depends on?
 Ans. Thrust: The upward force acting on an object is called thrust/buoyant force.
Buoyant force depends on the mass on an object and is independent of the density of the object. It also depends on the volume displaced by an object.
 

Q23. A container of mass 300kg is raised by an electric motor through a height of 60m in 45 secs. Calculate:
 (a) The weight of the container.
 (b) The work done by the motor.
 (c) The useful power of the motor. (Assume g = 10N/kg)
 Ans. (a) The weight of the container = m × g = 300 × 10 = 3000 N.
(b) Work done by the container = F × s, Hence W = 3000 N × 60 m = 180000J
(c) Power of the motor

P = Work done/ time = 180000/45 = 4000 Watt

Q24. The diagram shows a pendulum which was released from position A. What forms of energy did the pendulum have at?
 (i) A
 (ii) B
 (iii) C
 Eventually the pendulum would stop moving. Explain what has happened to the initial energy of the pendulum?

Ans. At A the pendulum is having gravitational potential energy, at C the pendulum has maximum kinetic energy as it is in the center of the oscillatory motion. At B the pendulum has equal potential and kinetic energy. The pendulum will eventually slow down and stop due to the air resistance.
 

Q25. An athlete of mass 54 kg is bouncing up and down on a trampoline.
 (a) State the form of energy stored due to the stretching surface of the trampoline.
 (b) The stretched surface of the trampoline begins to contract. The athlete is pushed vertically upwards and gets accelerates. At time t, when her upward velocity is 6.0 m/s, she loses contact with the surface. Calculate the kinetic energy at time t.
 Ans. (a) The stretching surface of trampoline stores potential energy.
(b) Kinetic energy of the athlete will be K.E.
 

Q26. In a power station, 2.05 × 10⁸ kg of water is pumped up through a vertical height of 400 m.
 (a) Calculate the weight of water.
 (b) Calculate the gravitational potential energy gained by the water.
 Ans. (a) Weight of water = mass × gravity = 2.05 × 10⁸ kg × 10 m/s² = 2.05 × 10⁹ N
(b) Gravitational potential energy = m × g × h = 2.05 × 10⁸ × 10 N × 400 m = 8.2 × 10¹¹ J.
 

Q27. A diver is at a depth of 40m beneath the surface of a sea. He carries a cylinder of high-pressure air on his back. Explain how the air molecules exert a pressure on the inside surface of the cylinder. The diver gradually uses up the air in the cylinder. Explain why the pressure falls.
 Ans. The air molecules are in random motion in the cylinder and will exert pressure on the walls of the container. As the gas from the cylinder is used up by the diver the pressure inside the cylinder will decrease because the volume of the gas is decreased.
 

Q28. The cylinder is air tight and the temperature of the air in the cylinder is increased.
 (a) State what happens to the volume of the air in the cylinder as a result of this temperature rise.
 (b) Does the pressure inside the cylinder changes, give reason for your answer?
 Ans. (a) The volume of air in the cylinder will increase as the molecules of the air gains heat energy and start moving away from each other.
(b) The pressure inside the cylinder will change as the molecules will hit more inside the cylinder and increase the pressure.