Q.1. Fill in the blank
(i) When a ball is thrown upwards, kinetic energy is transformed into potential energy.
(ii) The sum of the potential and kinetic energies of a body is called mechanical energy.
(iii) Work is measured as a product of force and distance.
(iv) The electricity meter installed in our home measures electric energy in the units of kWh.
(v) The work done on a body moving in a circular path is zero.
Q.2. How are Joule (J) and ergs (erg) related?
(a) 1J = 107erg
(b) 1erg = 107J
(c) 1J = 10–7erg
(d) None
Ans: (a)
Explanation: 1 joule is equal to 107 erg. This follows from the definitions: 1 J = 1 N·m and 1 erg = 1 dyne·cm, and the conversion between SI and CGS units gives 1 J = 107 erg.
Q.3. If the force is applied at an angle θ then work done is
(a) W = FS Cos θ F = Force
(b) W = FS θ S = Distance
(c) W = FS Sin θ W = work
(d) None
Ans: (a)
Explanation: Only the component of the force along the direction of displacement does work. If the force F makes an angle θ with the displacement S, the component along S is F cos θ, so work W = (F cos θ) × S = FS cos θ.
Q.4. If a body is stored at a height ‘h’ then it will posses:
(a) Kinetic energy
(b) Potential energy
(c) Both
(d) None
Ans: (b)
Explanation: A body stored at a height h above a reference level has gravitational potential energy. Its potential energy is given by mgh (where m is mass, g is acceleration due to gravity and h is height). If the body is not moving, its kinetic energy is zero.
Q.5. If the body starts from rest, then change in its kinetic energy is
(a) Positive
(b) Negative
(c) Zero
(d) May be Positive or negative depending upon the mass of the body
Ans: (a)
Explanation: If a body starts from rest and later moves, its speed becomes greater than zero. Kinetic energy (½mv2) increases from zero to a positive value, so the change in kinetic energy is positive.
Q.6. When do we say that work is done?
Ans: Work is done when a force acts on an object and the object undergoes a displacement that has a component along the direction of the force.
Q.7. Define 1 J of work.
Ans: One joule is the work done when a force of 1 N moves an object through a distance of 1 m in the direction of the force.
Q.8. What is the kinetic energy of an object?
Ans: Kinetic energy is the mechanical energy possessed by an object because of its motion. For a mass m moving with speed v, its kinetic energy is ½mv2.
Q.9. What is power?
Ans: Power is the rate at which work is done or energy is transferred. It is denoted by P.
Explanation: P = W / t, where W is work done and t is the time taken. The SI unit of power is watt, where 1 watt = 1 J/s.
Q.10. An object thrown at a certain angle to the ground moves in a curved path and falls back to the ground. The initial and the final points of the path of the object lie on the same horizontal line. What is the work done by the force of gravity on the object?
Ans: The work done by the force of gravity on the object over the whole motion is zero.
Explanation: Gravity is a conservative force. The work done by gravity equals the change in gravitational potential energy. If the initial and final heights are the same, the change in potential energy is zero, so the net work done by gravity over the complete path is zero.
Q.11. Illustrate the law of conservation of energy by discussing the energy changes which occur when we draw a pendulum bob to one side and allow it to oscillate. Why does the bob eventually come to rest? What happens to its energy eventually? Is it a violation of the law of conservation of energy?
Ans: When the bob is pulled to one side, it gains maximum gravitational potential energy and negligible kinetic energy. As it is released and passes through the lowest point, potential energy is converted into kinetic energy, giving it maximum speed there. As it climbs to the other side, kinetic energy converts back into potential energy. This interchange of potential and kinetic energy continues during oscillation.
The bob eventually comes to rest because non-conservative forces, such as air resistance and friction at the support, remove mechanical energy from the system. The mechanical energy is transformed into thermal energy (heat) and slight sound, which spread into the surroundings. This is called dissipation of energy.
This is not a violation of the law of conservation of energy, because the total energy (mechanical plus thermal and other forms) remains constant; energy only changes from one form to another.
Q.12. Define average power.
Ans: Average power is the total work done or total energy consumed divided by the total time taken. Average power is defined as average amount of work done by a body per unit time. Mathematically, average power = total work / total time.
Q.13. In each of the following a force, F is acting on an object of mass, m. The direction of displacement is from west to east shown by the longer arrow. Observe the diagrams carefully and state whether the work done by the force is negative, positive or zero.
Ans:
- In the first diagram the force is perpendicular to the displacement, so the work done is zero.
- In the second diagram the force acts in the same direction as the displacement, so the work done is positive.
- In the third diagram the force acts opposite to the displacement, so the work done is negative.
Q.14. Define 1 watt of power.
Ans: One watt is the power when work is done at the rate of 1 J/s. In other words, 1 W = 1 J/s.
Q.15. A battery lights a bulb. Describe the energy changes involved in the process.
Ans: Chemical energy stored in the battery is converted into electrical energy in the circuit. When the current passes through the bulb, electrical energy is transformed into light energy and heat energy (thermal energy) in the bulb’s filament.