Force And Pressure Book Back Questions 8th Science Lesson 2

8th Science Lesson 2

2] Force And Pressure

Book Back Questions with Answer and Do You Know Box Content

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Problem 1: The average weight of an elephant is 4000 N. The surface area of the sole of its foot is 0.1 m². Calculate the pressure exerted by one foot of an elephant.

Solution: Average weight of the elephant = 4000 N

Weight of one leg = Force exerted by one leg = 4000 / 4 = 1000 N

Area of the sole of one foot = 0.1 m²

Pressure = Force/Area = 1000/0.1 = 10000

N/m² = 10⁴Nm^-2.

It is very difficult for us to walk on sand. But, camels can walk easily on it because they have large padded feet, which increase the area of contact with the sandy ground. This reduces the pressure and enables them to walk easily on the sand.

Cooking in a place located at a higher altitude is difficult. Why? At a higher altitude, due to lack of atmospheric pressure the boiling point of a substance reduces. So, water boils even at 80^oC. The thermal energy that is produced at this temperature is not sufficient enough for baking or cooking. So, cooking is difficult at higher altitude.

Why dams are made stronger and broader at the bottom than at the top? Why do scuba divers wear a special suit while they go into deep sea levels?

Choose the correct answer:

1. If we apply force against the direction of motion of the body, then the body will

(a) Stop moving

(b) move with an increased speed

(c) Move with a decreased speed

(d) move in a different direction

2. Pressure exerted by a liquid is increased by

(a) The density of the liquid

(b) The height of the liquid column

(c) Both a and b

(d) None of the above

3. Unit of pressure is

(a) Pascal

(b) Nm^-2

(c) Poise

(d) Both a and b

4. The value of the atmospheric pressure at sea level is

(a) 76 cm of mercury column

(b) 760 cm of mercury column

(c) 176 cm of mercury column

(d) 7.6 cm of mercury column

5. Pascal’s law is used in

(a) Hydraulic lift

(b) Brake system

(c) Pressing heavy bundles

(d) All the above

6. Which of the following liquids has more viscosity?

(a) Grease

(b) Water

(c) Coconut oil

(d) Ghee

7. The unit of viscosity is

(a) Nm²

(b) Poise

(c) kgms^-1

(d) No unit

Fill in the blanks:

1. The pressure of a liquid column _______________ with the depth of the column.

2. Hydraulic lift works under the principle of _____________.

3. The property of ____________ of a liquid surface enables the water droplets to move upward in plants.

4. A simple barometer was first constructed by ________________.

State true or false. If false, correct the statement:

1. Force acting on a given area is called pressure.

2. A moving body comes to rest due to friction alone.

3. A body will sink if the weight of the body is greater than the buoyant force.

4. One atmosphere is equivalent to 1,00,000 Newton force acting on one square metre.

5. Rolling friction is slightly greater than the sliding friction.

6. Friction is the only reason for the loss of energy.

7. Liquid pressure decreases with the decrease of depth.

8. Viscosity depends on the pressure of a liquid.

Match the following:

(a)

1. Static friction – Viscosity

2. Kinetic friction – Least friction

3. Rolling friction – Objects are in motion

4. Friction between the liquid layers – Objects are sliding

5. Sliding friction – Objects are at rest

(b)

1. Barometer – reduce friction

2. Increasing area of contact – Atmospheric pressure

3. Decreasing are of contact – cause of friction

4. Lubricants – increases friction

5. Irregular surface – decreases friction

Complete the analogy:

1. Knot in a thread: _____________ friction :: Ball bearing: _____________ friction.

2. Downward force: Weight :: Upward force offered by liquid: ____________.

Numerical problem:

1. A stone weighs 500 N. calculate the pressure exerted by it, if it makes contact with a surface of area 25 cm².

Consider the statements given below and choose the correct option:

1. Assertion: Sharp knives are used to cut the vegetables.

Reason: Sharp edges exert more pressure.

(a) Both assertion and reason are true and reason is the correct explanation of the assertion.

(b) Both assertion and reason are true but reason is not the correct explanation of the assertion.

(c) Assertion is true, but reason is false.

(d) Both assertion and reason are false.

2. Assertion: Broad straps are used in bags.

Reason: Broad straps last for long.

(a) Both assertion and reason are true and reason is the correct explanation of the assertion.

(b) Both assertion and reason are true but reason is not the correct explanation of the assertion.

(c) Assertion is true, but reason is false.

(d) Both assertion and reason are false.

3. Assertion: Water strider slides easily on the surface of water.

Reason: Water strider experiences less buoyant force.

(a) Both assertion and reason are true and reason is the correct explanation of the assertion.

(b) Both assertion and reason are true but reason is not the correct explanation of the assertion.

(c) Assertion is true, but reason is false.

(d) Both assertion and reason are false.

Answers:

Choose the correct answer:

1. Stop moving, 2. Both a &b 3. Both a &b 4. 76 cm of mercury column 5. All the above 6. Grease 7. Poise

Fill in the blanks:

1. Increases 2. Pascal’s law 3. Surface tension 4. Torricelli

State true or false. If false, correct the statement:

1. True

2. True

3. True

4. True

5. False

Correct answer: Rolling friction is slightly lesser than the sliding friction

6. True

7. True

8. True

Match the following:

A.

1.Static friction – Objects are at rest

2.Kinetic friction – Objects are in motion

3.Rolling friction – Least friction

4.Friction between the liquid layers – Viscosity

5.Sliding friction – Objects are sliding

B.

1.Barometer – Atmospheric pressure

2.Increasing area of contact – increases friction

3.Decreasing area of contact – decreases friction

4.Lubricants – reduce friction

5.Irregular surface – cause of friction

Complete the analogy:

1. Rolling, Static

2. Buoyant force

Numerical problem:

1.Pressure, P = F/A

= 500/25*10^-4

=20 * 10⁴ Pa

Consider the statements given below and choose the correct option:

1.Both assertion and reason are true and reason is the correct explanation of assertion.

2.Both assertion and reason are true, but reason is not the correct explanation of assertion.

Correct Reason: Broad straps produce large area of contact with shoulder which reduce pressure

3.Both assertion and reason are true, but reason is not the correct explanation of assertion.

Correct Reason: It is due to surface tension of water