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

Q1. Define work.
 Ans. When a force acts on an object and the object moves, we say that the force has done work on the object.
W = F × s

Q2. What is the unit of work done?
 Ans. Joule.
 

Q3. Define 1 Joule of work.
 Ans. When a force of 1 Newton acts on an object and the object moves a distance of 1 metre in the direction of the force, the work done by the force is 1 Joule.
 

Q4. Name two conditions required to do work.
 Ans. Two conditions required to do work are force applied and displacement produced.
 

Q5. What is potential energy?
 Ans. The energy possessed by a body due to its height or position is called potential energy.
 

Q6. Name 2 types of potential energy.
 Ans. Gravitational potential energy and elastic potential energy

Q7. What is the formula of work and give the units of each symbol.
 Ans. Formula → W = F × s
∴ W = work done
Units → Joule = Newton × Metre
F = force s = displacement

Q8. If the work done is 20 J and displacement is 2 m then find the force applied.
 Ans. W = F × s
20 = F × 2
∴ F = 10 N
 

Q9. When we push a pebble lying on surface is work done?
 Ans. On pushing a pebble lying on a surface, it moves through a distance so work is done.
 

Q10. Give the formula of kinetic energy and potential energy.
 Ans. K.E. = (1/2) mv²
P.E. = mgh
m = mass
m = mass
v = velocity
g = acceleration due to gravity
h = height

Q11. Name the energy stored when a rubber band is stretched?
 Ans. On stretching a rubber band, potential energy is stored in it.
 

Q12. How many joules are there in 1 kJ?
 Ans. 1000 Joules = 1 kJ

Q13. Define power.
 Ans. Power is defined as the rate of doing work or the rate of transfer of energy.
P = (W/t)
 

Q14. What is the unit of power? Define it.
 Ans. The unit of power is watt. 1 watt is the power of an agent which does work at the rate of 1 joule per second.
 

Q15. What is the commercial unit of energy? Define it.
 Ans. The commercial unit of energy is kWh [kilowatt-hour].
1 kWh is the energy used in one hour at the rate of 1000J/s.
1 kWh = 1 kW × 1 h.
 

Q16. Convert 1 kWh into joules.
 Ans. 1 kWh = 1 kW × 1 h = 1000
W × 3600 s = 3600000 J
= 3.6 × 106 J
 

Q17. What is mechanical energy?
 Ans. The sum of kinetic energy and potential energy of an object is called its mechanical energy.
 

Q18. Give one example where work done on an object is zero.
 Ans. A person walking horizontally straight with load on its head is said to be doing no work on the load.
This is because the direction of force acting on the load and the displacement seen is perpendicular.

W = 0 
 

Q19. Give one example where work done on an object is negative.
 Ans. A person walking down the stairs with load on its head, the work done is negative as the force applied on the load is in opposite direction to its displacement.

W = − ve
 

Q20.

In the above figure a pendulum is shown oscillating from A to B, B to C and C to A. In its one oscillation state the position, where its kinetic energy is maximum.
 Ans. A pendulum when raised to certain height and is at B, on releasing it from B it attains motion and hence its potential energy is transformed to kinetic energy.
The maximum kinetic energy is at A when it is in motion.
 

Q21. A man does 60 J of work in 6 seconds. Calculate the power.

Ans.

∴ Power = 10 watts.

Q22. What would happen to the kinetic energy of an object if its velocity is doubled?
 Ans. Kinetic energy ∝ v².
∴ If the velocity is doubled, kinetic energy will become 4 times more.