Class 9 Science
Question 1. A force of 7 N acts on an object. The displacement is, say 8 m, in the direction of the force. Let us take it that the force acts on the object through the displacement. What is the work done in this case?
Answer: Given; F = 7N and displacement s = 8 m
Work Done = Force x Displacement
`= 7 N xx 8 m = 56 Nm` or 56 J
Question 2. When do we say that work is done?
Answer: When a force is applied on a body and displacement happens in the body along the direction of the force applied, we say that work is done.
Question 3. Write an expression for the work done when a force is acting on an object in the direction of its displacement.
Answer: Work Done = Force x Displacement Or, `W = F xx s`
Question 4. Define 1 J of work.
Answer: When a force of 1 N causes displacement of 1 m in an object in the direction of force applied; work done is 1 J.
Question 5. A pair of bullocks exerts a force of 140 N on a plough. The field being ploughed is 15 m long. How much work is done in ploughing the length of the field?
Answer: Given; F = 140 N and s = 15 m
`W = F xx s`
`= 140 N xx 15 m = 2100 Nm`
Question 6. What is the kinetic energy of an object?
Answer: The energy possessed by a moving object because of its velocity is called kinetic energy.
Question 7. Write an expression for the kinetic energy of an object.
Answer: If m is the mass and v is the velocity of an object, kinetic energy is given by following equation.
`E_k = ½ mv^2`
Question 8. The kinetic energy of an object of mass, m moving with a velocity of 5 m s–1 is 25 J. What will be its kinetic energy when its velocity is doubled? What will be its kinetic energy when its velocity is increased three times?
Answer: We know that `E_k = ½ mv^2`
This means that kinetic energy varies directly as square of velocity and directly as mass.
When velocity becomes double, the kinetic energy will be 22 times, i.e. 4 times.
So, kinetic energy in this case `= 25 J xx 4 = 100 J`
When velocity becomes triple, the kinetic energy will be 32 times, i.e. 9 times.
So, kinetic energy in this case `= 25 J xx 9 = 225 J`
Question 9. What is power?
Answer: The rate of doing work or rate of transfer of energy is called power. If W is the work done in time t; then
`P = W/t`
Power is expressed in Watt (W).
Question 10. Define 1 watt of power.
Answer: When 1 J of work is done in 1 s then power is 1 W.
Or, `1 W = (1 J)/(1 s)`
Question 11. A lamp consumes 1000 J of electrical energy in 10 s. What is its power?
Answer: Given; W = 1000 J and t = 10 s
`P = W/t`
`= (1000 J)/(10 s) = 100 W`
Question 12. Define average power.
Answer: Total work done by an object in total time is called average power.
Average power = total work done/total time taken
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