Exam practice

The capacity to do work is called energy.

The energy a body has because of its motion or position is called mechanical energy.

Water stored in a reservoir has potential energy. (The word “forest” is not needed.)

A moving vehicle has kinetic energy.

The capacity (ability) to do work is called energy. Its SI unit is the joule (J).

The energy a body has because of its motion or its position is called mechanical energy. It is of two types: potential energy and kinetic energy.

Kinetic energy: the energy a body has because of its motion. Examples: a rolling ball, flowing water (also a flying bird, a moving bus).

Potential energy: the energy stored in a body because of its position or a change in its position or shape. Examples: water in a tank on a roof, a stretched catapult (also a squeezed spring, a raised brick).

The kinetic energy of a body depends on:

1. its mass: the heavier the body, the more kinetic energy at the same speed (a cricket ball has more than a tennis ball thrown at the same speed);
2. its speed (velocity): the faster it moves, the more kinetic energy it has (a fast cricket ball has more than a slow one).

If a formula is asked for: kinetic energy = ½ × mass × (velocity)², or KE = ½ m v², where m is the mass and v the velocity. The velocity is squared (multiplied by itself), so twice the speed gives four times the kinetic energy.

The change of energy from one form into another is called transformation of energy. For example, a bulb changes electrical energy into light energy.
If energy could not be transformed:

• the electricity from our power stations could not light bulbs, run fans or cook food;
• the energy in food could not move our muscles or keep us warm, so we could not live;
• fuel could not run vehicles, and solar panels and hydropower plants could not make electricity;
• we would have only the forms of energy that happen to be around us, never the form we need.

Green plants take in light energy from the sun and use it to make food in their leaves (photosynthesis). The light energy is changed into chemical energy and stored in the food: in grains, fruit, vegetables and leaves. We eat these plants, or animals that ate plants. So the chemical energy in all our food first came from sunlight that a plant transformed. The statement is true.

Our body stores some energy from the food we ate before, as chemical energy (in sugar in the blood and liver, and in fat). For a short time, the body uses this stored energy, so we can keep working without eating.
But the store is small. When it runs low, the body has no energy left for the muscles: we feel weak and tired and cannot work. To keep working, we must eat again to get new energy.

The SI unit of heat is the joule.

Heat is a form of energy.

A larger surface area absorbs more heat.

White objects absorb less heat. (“Negligible” is not needed: white things still absorb a little heat.)

True.

False. Heat flows from the skin into the colder object. That is why it feels cold.

False. A larger surface area absorbs more heat.

False. A black object absorbs more heat than objects of other colors.

Spread out, the quilts and mattresses have a large surface facing the sun, so they absorb more heat. They become warm and dry, and stay warm for a while at night. (The sunlight also helps kill some germs and dries out damp.)

White clothes reflect most of the sunlight and absorb little heat, so they stay cooler. Black clothes absorb a lot of heat and make us feel hotter.

• We need heat to cook our food and to boil water.
• Heat keeps our bodies and homes warm in winter.
• The sun’s heat dries clothes, grains, and wet things.
• Heat is used to iron clothes and to melt and shape metals (by blacksmiths and goldsmiths).
• Heat makes steam to run some engines, and it bakes bricks and pots.

The process in which a body takes in heat from hotter things around it is called heat absorption.
It depends mainly on:

1. the surface area of the object: a larger surface absorbs more heat;
2. the color of the object: a black object absorbs more heat than a white one.

Dark objects absorb more heat and light ones less: black absorbs the most and white the least.
Example: in the sun, a black umbrella, a black car or a black bag gets much hotter than a white one. Solar water heater panels are painted black so that they absorb more of the sun’s heat.

Washed clothes are spread out on a line, not left in a heap. Spread out, a large surface of the cloth faces the sun, so it absorbs more heat, and the water in it dries quickly. A heap of wet clothes, with only its top facing the sun, stays wet for a long time. (Grains spread on a biskun, and fingers spread out at a fire, are other examples.)

In summer, white or light-colored clothes keep you cool, because they reflect most of the sunlight and absorb little heat.
In winter, black or dark clothes keep you warmer when you sit in the sun, because they absorb more of its heat.

Light travels only in a straight line.

The smallest part of a beam of light is called a ray.

Light can’t pass through an opaque object, so its shadow is seen behind it.

In a pinhole camera, the tracing paper works as a screen. (The word “bending” is not needed.)

A shadow is the dark area formed behind an opaque object when it blocks the light from a source.
There are two types: the umbra (light blocked completely) and the penumbra (light blocked partly).
In the morning and evening the sun is low in the sky. Its light comes in at a slant, so an object blocks light over a long stretch of ground, and its shadow is long. At midday the sun is high. Its light comes down steeply, so the shadow is short. (Because the sun is in the east in the morning and the west in the evening, the shadow also points west in the morning and east in the evening.)

Stand three cards with small holes in the middle of each on a table, with a burning candle at one end (Figure 7.14). When the three holes are in one straight line, you can see the flame through them. When one card is moved a little to the side, you can’t: its solid part is in the way, and the light can’t bend round it to the next hole. So light travels in a straight line. (A straight pipe and a bent pipe show the same thing.)

Yes. A camera without a lens is called a pinhole camera. It is a closed box or tube with a tiny pinhole in one end and a screen of tracing paper at the other. Light from an object passes through the pinhole in straight lines and makes an inverted image on the screen.

It is upside down because light travels in straight lines. A ray from the top of the object travels in a straight line down through the pinhole and lands at the bottom of the screen. A ray from the bottom of the object travels straight up through the pinhole and lands at the top of the screen. The rays cross at the hole, so top and bottom swap, and the image is inverted (Figure 7.15). If light could bend, the rays would not cross neatly, and no clear image would form.

A bulb sends out light from every point of its glowing part, in every direction at once. Each ray goes out in a straight line, but the rays start out in all different directions: upwards, downwards, sideways, in front and behind. So the straight rays spread out like the spokes of a wheel and light up the whole room. Light spreading in all directions doesn’t mean it bends; it means straight rays leave in many directions.

1. How does a shadow form? When an opaque object blocks light, a dark shadow forms behind it.
2. What is it? Light is a form of energy.
3. Why do we need it? To see things around us.
4. In what kind of line does it travel? In a straight line.
5. Which activity proves it? Looking at a candle through a straight pipe and a bent pipe, or through three cards with holes in a line.

When a body vibrates, sound is produced.

Sound travels fastest in a solid medium.

Sound can’t travel through a vacuum.

An object that produces sound is called a source of sound. (The word “liquid” is not needed.)

Sound needs a medium (a solid, liquid or gas) to travel through. There is no air on the Moon, so the sound of a voice can’t travel from one person to another, even close by. A radio sends the message as radio waves, which can travel through a vacuum, so astronauts must use one.

In a solid, the molecules are very close together and tightly held. When one molecule is pushed, it passes the push on to the next one almost at once, so the sound wave travels very quickly from molecule to molecule.

In a gas, the molecules are far apart. A pushed molecule has to travel a long way before it bumps into the next, so the push is passed on slowly, and sound travels more slowly than in solids and liquids.

When an object vibrates, the process in which its energy is passed on through a medium is called a wave. A wave carries energy from one place to another without carrying the medium along.
A wave is produced by a vibrating (or disturbed) object. For example, a stone dropped in a pond makes ripples, a hand jerking a rope makes a wave run along it, and a vibrating plate makes sound waves in the air.

How it is produced: a sound wave is produced by a vibrating body, such as a plate struck with a spoon, a drum skin or a loudspeaker. As it vibrates, it pushes the air next to it forward and then pulls back, again and again.

How it travels: each push squeezes the air molecules together into a compression (C); each pull-back leaves them spread out in a rarefaction (R). Each molecule pushes the next one and moves back, so the compressions and rarefactions travel outwards through the medium to our ears (Figure 7.21). The molecules only vibrate to and fro, in the same direction as the sound travels: sound is a longitudinal wave. It needs a medium (a solid, liquid or gas) and can’t travel through a vacuum.

Similarities:

• In all three, sound travels as a longitudinal wave, as compressions and rarefactions.
• In all three, the molecules of the medium vibrate to and fro and pass the push on to their neighbors; the medium itself doesn’t travel.

Differences:

The lightning and the thunder happen at the same time, but light travels much faster than sound. Light travels about 3 lakh km in a second, so the flash reaches us almost at once. Sound travels only about 332–340 m in a second in air, so the thunder takes several seconds to reach us. That is why we hear it later.

You hear the scratch clearly with your ear on the wall, but hardly at all (or not at all) without it.
With your ear on the wall, the sound travels to you through the solid wall. In a solid, the molecules are close together, so sound travels fast and loses very little of its energy on the way.
Without your ear on the wall, the sound must travel through the air. In a gas, the molecules are far apart, and the faint sound spreads out in all directions and loses its energy, so very little reaches your ear.

1. What is it? A form of energy produced by vibrating bodies.
2. Sources: madal, guitar, bell, bansuri (flute), our vocal cords.
3. Media it travels through: solids, liquids and gases (not a vacuum).
4. How it travels: as a longitudinal wave, by compressions and rarefactions of the molecules of the medium.