LIWA

Energy

ICSE Class 8 · Physics · Chapter 4

What this chapter covers

Key definitions

Work is said to be done when a force acting on an object changes the position of the object.

Energy is the ability or capacity of a body to do work.

Kinetic energy is the energy possessed by a body because of its motion.

Potential energy is the energy stored in a body because of its position or its shape.

Gravitational potential energy is the energy stored in a body because of its height above the ground, which is the work done in lifting it against the force of gravity.

Energy transformation is the change of energy from one form into another form.

The law of conservation of energy is the rule that energy can neither be created nor destroyed, but can only be changed from one form into another.

Power is the rate of doing work, which is the work done or energy used in one second.

The watt is the unit of power, equal to a rate of working of 1 joule per second.

Key points

Questions answered

A coolie stands still at a bus stand holding a heavy bag on his head. Is he doing work on the bag in the physics sense? Give a reason.
No. Work is done only when a force moves an object. The bag does not move, so no work is done on it, even though he feels tired.
Define the joule.
The joule is the SI unit of work and energy. It is the work done when a force of 1 N moves an object through 1 m in the direction of the force.
A boy pulls a toy cart with a force of 15 N through a distance of 6 m along the pull. Find the work done.
W = F × d = 15 × 6 = 90 J.
Name the two quantities on which the kinetic energy of a body depends. Which one has the bigger effect?
Its mass and its speed. Speed has the bigger effect, because doubling the speed makes the kinetic energy four times as large.
Find the kinetic energy of a ball of mass 0.5 kg moving at 6 m/s.
KE = (1)/(2) × 0.5 × 6 × 6 = 0.25 × 36 = 9 J.
What is gravitational potential energy? Give one example from your home.
Gravitational potential energy is the energy stored in a body because of its height above the ground. Water in an overhead tank is one example.
A flower pot of mass 3 kg rests on a balcony ledge 8 m above the ground. Find its gravitational potential energy. Take g = 10 m/s2.
PE = mgh = 3 × 10 × 8 = 240 J.
Describe the energy changes in a swinging pendulum.
At each extreme position the bob has maximum potential energy and no kinetic energy. As it swings down, potential energy changes into kinetic energy, which is greatest at the mean position. As it rises again, kinetic energy changes back into potential energy.