# NCERT Class-9 Science Solutions: Chapter 11-Work And Energy Part 4

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**Question 19:**

A ball is dropped from a height of. If the energy of the ball reduces by 40% after striking the ground, how much high can the ball bounce back?

**Answer:**

When the ball is dropped from a certain height, it will have some stored gravitational energy. This energy can be shown by

The potential energy of gravity is directly affected by the height. When the ball is dropped from a 10m height, it loses 40% energy thus affecting the same percentage of height when it bounces back.

Now 40% of the energy lost means 40% of the height also lost on bouncing back. Given height is 10m and when you lose 40% of 10m, left will be 60% of 10m.

Energy is reduced by then the remaining energy is

Therefore,

The height of the bounce back is hence 60% of 10m which 6m

**Question 20:**

If an electric iron of is used for every day, find electric energy consumed in the month of April.

**Answer:**

We have Given:

Power of electric iron

Usage per day

Number of days in the month of April

Electrical energy consumed,

**Long Answer Questions**

**Question 21:**

A light and a heavy object have the same momentum. Find out the ratio of their kinetic energies. Which one has a larger kinetic energy?

**Answer:**

Linear momentum of first object

Linear momentum of second object

But,

**Question 22:**

An automobile engine propels a car (A) along a levelled road at a speed of . Find the power if the opposing frictional force is . Now, suppose after travelling a distance of , this car collides with another stationary car (B) of same mass and comes to rest. Let its engine also stop at the same time. Now car (B) starts moving on the same level road without getting its engine started. Find the speed of the car (B) just after the collision.

**Answer:**

Frictional force

Since, the car A moves with a uniform speed, it means that the engine of car applies a force equal to the frictional force

After collision