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Std 6 General Science · Chapter 11

11. Force and Pressure

Complete solutions to the Exercises, covering the types of force, friction in daily life and the link between pressure and the area of contact. Maharashtra State Board (Balbharati), Grade Six General Science.

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About the chapter

A push or a pull is called a force. This chapter tells us about different types of force. We learn about friction and its effects. We also learn how pressure depends on the area of contact.

Key points

  • Muscular, mechanical, gravitational, magnetic, frictional and electrostatic forces are types of force.
  • Friction opposes motion. It also makes things wear out.
  • Pressure is the force acting on a unit area.
  • When the area of contact decreases, pressure increases.
  • Air pressure decreases as we go higher above sea level.
  • The unit of pressure is the pascal.

Words to remember

  • force – a push or a pull
  • friction – the force that opposes motion between two surfaces
  • pressure – the force acting on a unit area
  • area of contact – the part of a surface that touches another surface
  • atmospheric pressure – the pressure of the air around us

Exercises

1Identify, which force am I?

(a) I am used in different machines like sewing machines, coolers and vehicles. – Mechanical force

(b) All the objects thrown in the upward direction are attracted towards earth because of me. – Gravitational force

(c) Every movement in different games is possible because of me. – Muscular force

(d) I am used in industries to lift iron scrap material. – Magnetic force

2Find the correlation.

(a) The coin struck on a carom board : frictional force :: The fruits falling from the tree : gravitational force

(b) Area of contact increases : pressure decreases :: Area of contact decreases : pressure increases

3Write the following statements true or false. Correct the false statement.

(a) Due to Earth’s gravitational force, other planets stay in their orbits and revolve around the Sun.

False. Due to the Sun’s gravitational force, planets revolve around the Sun.

(b) As we go higher above sea level, the air pressure increases.

False. As we go higher, the air pressure decreases.

(c) When frictional force decreases, the speed of the wheel of the machine increases.

True.

(d) The unit of pressure is pascal.

True.

4Carefully observe a sewing machine while stitching clothes on it. Make a list of different forces used in it and write the role of each.
  1. Muscular force – to press the pedal and move the cloth.
  2. Mechanical force – to move the needle up and down.
  3. Frictional force – to hold the belt on the wheel and push the cloth forward.
  4. Gravitational force – to keep the machine steady on the table.
5Write down the tasks that humans can perform by using the muscular force of animals.
  1. Bullocks – ploughing fields and pulling carts.
  2. Horses – riding and pulling tongas.
  3. Camels – carrying people and goods in deserts.
  4. Donkeys – carrying loads.
  5. Elephants – lifting heavy logs.
  6. Yaks – carrying loads in the mountains.
6Write down different tasks performed with the help of mechanical force in your surroundings.
  1. Grinding in a mixer.
  2. Washing clothes in a washing machine.
  3. Ploughing with a tractor.
  4. Grinding grain in a flour mill.
  5. Lifting heavy things with a crane.
  6. Travelling by bus, car or train.
7Fruit can be easily cut with the sharp side of a knife, but it cannot be easily cut with the blunt side. Explain, why does this happen.
  1. The sharp side has a very small area of contact.
  2. So it produces more pressure. The fruit is cut easily.
  3. The blunt side has a larger area of contact.
  4. So it produces less pressure. The fruit is not cut easily.
Sharp side of the knifeForceFruitArea of contact: very smallSo the pressure is very largeThe fruit is cut easilyBlunt side of the knifeSame forceFruitArea of contact: much largerSo the pressure is smallThe fruit is only pressed

Same force, different area, different pressure.

8What will happen if grease or lubricant oil is not applied to a bicycle chain?
  1. Friction will increase.
  2. Pedalling will become hard.
  3. The speed of the bicycle will decrease.
  4. The chain will wear out quickly.
  5. The chain will make noise and may rust.
9To prevent skidding on a smooth and wet floor, what precautionary measures can be taken?
  1. Wipe the floor dry at once.
  2. Use rough or anti-skid tiles.
  3. Put mats on the floor.
  4. Wear footwear with rough soles.
  5. Walk slowly and carefully.
  6. Put up a “wet floor” board.
10Car tyres and shoe soles get worn out on using for a long duration. What can be the reason behind this?

The reason is friction. Tyres and soles rub against the ground again and again. So they slowly wear out.

11What will happen in the following situations? Guess.

(a) Walking on ice wearing sandals with thin pointed heels.

The pointed heels have a small area of contact. They put more pressure on the ice. So they sink in and walking becomes difficult.

(b) Striking a pointed stone on a bat.

The pointed stone puts more pressure on the bat. So the bat gets damaged.

(c) A heavy bag with a broad strap on the shoulder.

The broad strap puts less pressure on the shoulder. So the bag feels comfortable.

(d) A heavy bag with a narrow strap on the shoulder.

The narrow strap puts more pressure on the shoulder. So the shoulder starts hurting.

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