Work & Energy numericals

In which of the following scenarios, work is not done?

• Suma is swimming in a pond.

• A green plant is carrying out photosynthesis.

• An engine is pulling a train.

• A donkey is carrying a load on its back.

When a battery lights a bulb, how is the energy conserved?

• Chemical Energy

  ---> Electrical Energy--->

  Nuclear Energy

• Light Energy

  --->Electrical Energy+Chemical Energy

• Chemical Energy → Electrical Energy → Light Energy + Heat Energy.

• Energy is not conserved

Certain force acting on a 20 kg mass changes its velocity from 5 m s^-1 to 2 m s^-1. Calculate the work done by the force.

• 210

• 250

• -210

• -250

Answer explanation

Initial velocity u = 5 m/s

Mass of the body = 20kg

Final velocity v = 2 m/s

The initial kinetic energy

E_i = (1/2) mu² = (1/2) × 20 × (5)² 

= 10 × 25

= 250 J

Final kinetic energy 

E_f = (1/2) mv² = (1/2) × 20 × (2)² 

= 10 × 4

 = 40 J

Therefore,

Work done = Change in kinetic energy

Work done = E_f – E_i

Work done = 40 J – 250 J

Work done = -210 J

An electric heater is rated 1500 W. How much work it does in 10 hours?

• 15kWh

• 20kWh

• 25kWh

• 30kWh

Answer explanation

Given

Power of the heater = 1500 W = 1.5 kW

Time taken = 10 hours

Energy consumed by an electric heater can be obtained with the help of the expression 

Power = Energy consumed / Time taken

Hence,

Energy consumed = Power x Time taken

Energy consumed = 1.5 x 10

Energy consumed = 15 kWh

Therefore, the energy consumed by the heater in 10 hours is 15 kWh.

In which position the potential energy of the pendulum maximum?

• Point A

• Point B

• Point P

• Point A & point B

Observe the diagram carefully and state whether the work done by the force is negative, positive or zero.

• Negative

• Positive

• zero

• none of these

Find the energy in kW h consumed in 10 hours by four devices of power 500 W each.

• 10kWh

• 20kWh

• 30kWh

• 40kWh

Answer explanation

Power rating of the device (P) = 500 W = 0.50 kW

Time for which the device runs (T) = 10 h

Energy consumed by an electric device can be obtained by the expression

Power = Energy consumed/Time taken

∴ Energy consumed = Power × Time

Energy consumed = 0.50 × 10

Energy consumed = 5 kWh

Thus, the energy consumed by four equal rating devices in 10 h will be

⇒ 4 × 5 kWh

= 20 kWh