Class 9 Science Chapter 10 Question Answers - Work and Energy

Q1: A spring which has been kept compressed by tying its ends together is allowed to be dissolved in an acid. What happens to the potential energy of the spring?
Ans: The PE of the spring gets converted into KE of acid molecules whose temperature rises.

Q2: The work done in lifting a box on to a platform does not depend upon how fast it is lifted up. Explain your answer giving proper reasoning.
Ans: The work done (W) in lifting a box through a distance (s) against the gravitational force (F) is given by W = Fs. Hence, it is obvious that it is independent of the rate at which the box is lifted.

Q3: A body moves along a circular path. How much work is done in doing so? Explain.
Ans: In case of a body moving along a circular path, the force (centripetal) is always along the radius while displacement is tangential. Hence, work done W = FS cos 90° = 0 as angle between F and S is 90°.

Q4: What type of energy is stored in the spring of a watch?
Ans: When we wind a watch, the configuration of its spring is changed. The energy stored in the spring is obviously potential in nature (elastic potential energy to be more accurate).

Q5: A spring which is kept compressed by tying its ends together is allowed to be dissolved in an acid. What happens to the potential energy of the spring?
Ans: The potential energy of the spring gets converted into heat energy (kinetic energy of acid molecules). Due to this heat, the temperature of the acid rises.

Q6: (a) How much work is done when a force of 1 N moves a body through a distance of 1 m in its direction?
(b) Is it possible that a force is acting on a body but still the work done is zero? Explain giving one example.
Ans:
(a) 1 J of work is done.
(b) Yes, it is possible when force acts at right angles to the direction of motion of the body.
Example : Gravitational force of Earth acts on a satellite at right angles to its direction of motion.

Q7: A man rowing a boat upstream is at rest with respect to the shore. Is he doing work?
Ans: The man is doing work relative to the stream because he is applying force to produce relative motion between the boat and the stream. But he does zero work relative to the shore as the displacement relative to the shore is zero.

Q8: Is it possible that a body be in accelerated motion under a force acting on the body, yet no work is being done by the force? Explain your answer giving a suitable example.
Ans: Yes, it is possible, when the force is perpendicular to the direction of motion. The Moon revolving round the Earth under the centripetal force of attraction of the Earth, but Earth does no work on the motion.

Q9: Justify giving proper reasoning whether the work done in the following case is positive or negative - Work done by resistive force of air on a vibrating pendulum in bringing it to rest.
Ans: Work done is negative because the resistive force of air always acts opposite of the direction of motion of the vibrating pendulum.

Q10: Name the common forms of the mechanical energy.
Ans: The common forms of the mechanical energy are :
(i) Kinetic e nergy
(ii) Potential energy

Q11: A spring is compressed, what change is expected in the potential energy of the spring
Ans: When a spring is compressed, its potential energy is used up to changing its shape.

Q12: What type of energy is possessed by a flying bird and a flying aeroplane?
Ans: Both potential energy and kinetic energy.

Q13: Give one example of potential energy due to position.
Ans: Water stored in the reservoir of a dam has potential energy.

Q14: Which type of energy is present in a battery?
Ans: Chemical energy.

Q15: At what rate is electrical energy consumed by a bulb of 60 watt?
Ans: A 60 watt bulb consume electrical energy at the ratio of 60 joule per second.

Q16: How much energy will an electric motor of 1 horsepower consume in one second?
Ans: An electric motor will consume 746 joule of energy per second.

Q17: What is meant by the term horsepower (hp)?
Ans: Horsepower is another commercial unit power 1 hp
= 746 W  
1W = 1 / 746 hp
= 0.0013 hp

Q18: A car and a truck are moving with the same velocity of 60 km/hr^–1, which one has more kinetic energy?
Ans: Truck has more kinetic energy as kinetic energy is directly proportional to the mass.

Q19: What kind of energy transformation takes place when a body is dropped from a certain height?
Ans: When a body falls, its potential energy gradually gets converted into kinetic energy. On reaching the ground, the whole of the potential energy of the body gets converted into kinetic energy.

Q20: Give one example each of potential energy (i) due to position (ii) due to shape.
Ans:
(i) Potential energy due to position : Water stored in dam has potential energy.
(ii) Potential energy due to shape : In a toy car, the wound spring possesses potential energy and as the spring is released, its potential energy changes into kinetic energy due to which the car moves.

Q21: Can any object have mechanical energy even if its momentum is zero? Explain.
Ans: Yes, mechanical energy comprises of both potential energy and kinetic energy. Zero momentum means that velocity is zero. Hence, there is no kinetic energy but the object may possess potential energy.

Q22: How much work is done by a force of 10 N in moving an object through a distance of 4 m in the direction of the force?
Ans:
 Work done = Force × Displacement
= F × s  
= (10 N) × (4 m)  
= 40 joule or 40 J.

Q23: When an arrow is shot from its bow, it has kinetic energy. From where does it get the kinetic energy?
Ans: A stretched bow possesses potential energy on account of a change in its shape. To shoot an arrow; the bow is released. The potential energy of the bow is converted into the kinetic energy of the arrow.

Q24: What do you mean by transformation of energy?
Ans: It is the change of energy from one form of energy into another form of energy.

Q25: A body performs no work. Does it imply that the body possesses no energy?
Ans: When a body does not perform any work, it never implies that the body has no energy. The body may have energy but still does not perform any work, e.g., a book placed on a table has potential energy but is not performing any work.