♦ A few sources of energy are listed above. There are many other sources of energy. List them.
♦ Discuss in small groups how certain sources of energy are due to the Sun
♦ Are there sources of energy which are not due to the Sun?
♦ Take a heavy ball. Drop it on a thick bed of sand. A wet bed of sand would be better. Drop the ball on the sand bed from height of about 25 cm. The ball creates a depression.
♦ Repeat this activity from heights of 50 cm, 1 m and 1.5 m.
♦ Ensure that all the depressions are distinctly visible
♦ Mark the depressions to indicate the height from which the ball was dropped.
♦ Compare their depths.
♦ Which one of them is deepest?
♦ Which one is shallowest? Why?
♦ What has caused the ball to make a deeper dent?
♦ Discuss and analyse.
♦ Set up the apparatus as shown in Fig. 11.5.
♦ Place a wooden block of known mass in front of the trolley at a convenient fixed distance
♦ Place a known mass on the pan so that the trolley starts moving.
♦ The trolley moves forward and hits the wooden block.
♦ Fix a stop on the table in such a manner that the trolley stops after hitting the block. The block gets displaced.
♦Note down the displacement of the block. This means work is done on the block by the trolley as the block has gained energy.
♦ From where does this energy come?
♦ Repeat this activity by increasing the mass on the pan. In which case is the displacement more?
♦ In which case is the work done more?
♦ In this activity, the moving trolley does work and hence it possesses energy.
♦ Take a rubber band.
♦ Hold it at one end and pull from the other. The band stretches.
♦ Release the band at one of the ends.
♦ What happens?
♦ The band will tend to regain its original length. Obviously the band had acquired energy in its stretched position.
♦ How did it acquire energy when stretched?
♦ Take a slinky as shown below.
♦ Ask a friend to hold one of its ends. You hold the other end and move away from your friend. Now you release the slinky.
♦ What happened?
♦ How did the slinky acquire energy when stretched?
♦ Would the slinky acquire energy when it is compressed?
♦ Take a toy car. Wind it using its key.
♦ Place the car on the ground.
♦ Did it move?
♦ From where did it acquire energy?
♦ Does the energy acquired depend on the number of windings?
♦ How can you test this?
♦Lift an object through a certain height. The object can now do work. It begins to fall when released.
♦ This implies that it has acquired some energy.
If raised to a greater height it can do more work and hence possesses more energy.
♦ From where did it get the energy? Think and discuss.
♦ Take a bamboo stick and make a bow as shown in Fig. 11.6.
♦ Place an arrow made of a light stick on it with one end supported by the stretched string.
♦ Now stretch the string and release the arrow.
♦ Notice the arrow flying off the bow. Notice the change in the shape of the bow.
♦ The potential energy stored in the bow due to the change of shape is thus used in the form of kinetic energy in throwing off the
♦ Sit in small groups.
♦ Discuss the various ways of energy conversion in nature.
♦ Discuss following questions in your group :
(a) How do green plants produce food?
(b) Where do they get their energy from?
(c) Why does the air move from place to place?
(d) How are fuels, such as coal and petroleum formed?
(e) What kinds of energy conversions sustain the water cycle?
♦ Many of the human activities and the gadgets we use involve conversion of energy from one form to another.
♦ Make a list of such activities and gadgets.
♦ Identify in each activity/gadget the kind of energy conversion that takes place.
♦ An object of mass `20 kg` is dropped from a height of `4 m`. Fill in the blanks in the following table by computing the potential energy and kinetic energy in each case.
♦ For simplifying the calculations, take the value of `g` as `10 m s^(–2)`.