Work And Energy

Question 1

A person holds a bag of groceries while standing still, talking to a friend. A car is stationary with its engine running. From the stand point of work, how are these two situations similar?

Answer 1

In both cases work done is zero.

As the person and the car both are at rest. So the displacement is zero.

Question 2

An object has 1J of potential energy. Explain what does it mean?

Answer 2

We know that energy is defined as the ability of a body to do work. So an object has one joule of potential energy means that the body has capacity to do a work of one joule.

Question 3

A ball of mass m is held at a height h1 above a table. The table top is at a height h2 above the floor. One student says that the ball has potential energy mgh1, but another says that it is mg(h1+h2). Who is correct?

Answer 3

Both of them are correct.
Since P.E. is always measured with respect to some reference point. Therefore, we can say that first student has measured P.E with respect to table top(mgh1)while the second student has measured P.E with respect to floor=mg(h1+h2)

Question 4

When a rocket re-enters the atmosphere, it’s nose cone becomes very hot. Where does this heat energy come from?

Answer 4

When the rocket re-enters the atmosphere, then some of its kinetic energy is used in doing work against friction with dust particles and air, which is converted into heat. Due to this heat energy, it’s nose cone becomes very hot.

Question 5

What sort of energy is in the following:

a) compressed spring
b) water in a high damn
c) A moving car

Answer 5

a) A compressed spring has elastic potential energy
b) Water in the high damn has gravitational potential energy
c) A moving car has kinetic energy

Question 6

A girl drops a cup from a certain height, which breaks into pieces. What energy changes are involved?

Answer 6

A cup thrown from certain height losses it’s gravitational potential energy and gain its K.E. When it strikes the ground then a part of this kinetic energy is used to break the cup and rest of the energy converts into;

a) Sound energy
b) K.E of moving pieces
c) Heat energy

Question 7

A boy uses a catapult to thrown a stone which accidentally smashes a green house window. list the possible energy changes?

Answer 7

The following energy changes occur:

a) when the boy throws the stone the elastic potential energy changes into kinetic energy
b) when stone hits the window, a part of its kinetic energy used to break the window into pieces.
c) Rest of the energy converted into heat and sound energy.