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Page No 136:

Question 1:

Which of the following is true about friction?
(a) Friction is a force that occurs only between solids.
(b) Friction is a force that only occurs on rough surfaces.
(c) Friction is a force that only occurs when an object moves.
(d) Friction can only slow things down.

Answer:

(a) Friction is a force that occurs only between solids. - False
    It can occur between liquids, solids or gases.
(b) Friction is a force that only occurs on rough surfaces. - False

   It occurs on smooth surfaces also, but is minimal on such surfaces.
(c) Friction is a force that only occurs when an object moves. - False

   It can occur even when an object is at rest.
(d) Friction can only slow things down.
- True

Page No 136:

Question 2:

A spring balance was used to measure the force needed to pull a wooden block across different floor surfaces. This table below shows the results. The force is expressed in Newton.

Floor surface Force needed to start
moving
carpet 8 N
glazed tiles 5 N
polished wood 4 N
doormat 10 N

A. If the block is pulled on the wooden surface with a force of 4.1 N
(a) the block just starts to move.
(b) the block does not move.
(c) the block starts moving smoothly.
(d) the block moves a bit and stops.
B. In which of the above surfaces is the friction greatest?
(a) carpet
(b) glazed tiles
(c) polished wood
(d) doormat

Answer:

A. (a) the block just starts to move

If the block is pulled on the wooden surface with a force of 4.1 N, it just starts to move.

B. (d) doormat
On the doormat, the friction will be greatest.

Page No 136:

Question 3:

The two common forces that are acting everywhere around us are
(a) gravity and friction.
(b) gravity and magnetic force.
(c) friction and magnetic force.
(d) electrostatic force and gravity.

Answer:

(b) gravity and magnetic force

The two common forces that are acting everywhere around us are gravity and magnetic force.

Page No 136:

Question 4:

One of these helps to reduce pressure that is applied on a surface. Which one?
Figure

Answer:

In figure (a), the area in contact with the surface is greater, so the pressure will be less on the surface.



Page No 137:

Question 5:

The balloon shown here is full of water. Dhruv wants to make a hole in the balloon so that water comes out with maximum pressure. At which point should he make the hole?
Figure

Answer:

Dhruv should make a hole at point (d) to make the water come out with the maximum pressure. As the depth at point (d) is the greatest, the pressure is the highest at this point.

Page No 137:

Question 6:

You want to lift a heavy box. The force of gravity pulls it downwards with a force of 500 N. Your father applies an upward force of 220 N from below. How much force will you have to apply to lift it upwards?
(a) 220 N
(b) 280 N
(c) 300 N
(d) less than 220 N

Answer:

(b) 280 N

Force of gravity acting in the downward direction = 500 N

Force applied in the upward direction = 220 N

So, net force required to apply to lift the box up = 500 - 220
                                                                         = 280 N

Page No 137:

Question 7:

Which of the following correctly shows the relation between force, pressure and area?
A. Pressure = Force/Area
B. Force = Pressure/Area
C. Area = Force/Pressure
D. Pressure × Area = Force

(a) A, B and D
(b) only A
(c) A, C and D
(d) A and D only

Answer:

 (c) A, C and D

Correct relations:
A. Pressure = Force / Area
C. Area = Force / Pressure
D. Pressure × Area = Force

Page No 137:

Question 8:

A box weighing 2 kg exerts a force of 20 N on the ground. The box covers an area of 2 m2 on the ground. The pressure exerted by the box on the ground is:
(a) 40 N/m2
(b) 80 N/m2
(c) 10 N/m2
(d) 200 N/m2

Answer:

(c) 10 N/m2

Force exerted by the box = 20 N
Area covered by the box on the ground = 2 m2
Pressure exerted by the box on the ground = Pressure = Force/Area

Pressure=20 N2 m2=10 N/m2

Page No 137:

Question 9:

To open a tightly closed bottle, we often wrap the lid in a thick, rough towel because
(a) sweat and grease on our hands makes them slip.
(b) this prevents the bottle from breaking.
(c) this does not hurt our hands.
(d) friction between the lid and towel prevents it from opening easily.

Answer:

(a) sweat and grease on our hands makes them slip

To open a tightly closed bottle, we often wrap the lid in a thick, rough towel because sweat and grease on our hands make them slippery.

Page No 137:

Question 10:

Which of the following statements are true?
A. Gravity attracts all objects towards the earth.
B. Magnetic force attracts all kinds of metals.
C. Friction always tries to prevent movement.
D. Lubricants are substances which increase friction.
E. Friction produces heat and causes wear and tear.
F. Friction can help in grinding substances.

(a) A, C, E, F are true.
(b) A, B, C, D are true.
(c) C, D, E, F are true.
(d) A, C, D, F are true.

Answer:

(a) A, C, E and F are true.
Magnetic force does not attract all kinds of metals. It attracts some metals, such as iron and cobalt.
Lubricants are substances that decrease friction.

Page No 137:

Question 11:

Two boys exert forces of 40 N and 60 N in opposite directions on an object.
The resultant force is
(a) a force of 100 N acting in the direction of the larger force.
(b) a force of 100 N acting in the direction of the smaller force.
(c) a force of 20 N acting in the direction of the smaller force.
(d) a force of 20 N acting in the direction of the larger force.

Answer:

(d) a force of 20 N acting in the direction of the larger force

Let Force FA be 40 N, and Force FB be 60 N.
The forces are acting in opposite directions. So, the net force will act in the direction of larger force, i.e., Force, FB.
Net force = FB - FA
                    
= 60 - 40
                = 20 N
Therefore, the resultant force is a force of 20 N acting in the direction of the larger force.

Page No 137:

Question 12:

Name the type of force acting in each of the following cases.
(a) attraction of iron pins to a magnet
(b) a man pulling a rickshaw
(c) a football in air
(d) falling raindrops
(e) your hair stands on end when you remove a woollen cap
(f) a ball rolling on the ground
(g) a carpenter driving a nail into wood with a hammer
(h) a boy pedalling a cycle

Answer:

(a) Attraction of iron pins to a magnet: Magnetic force.

(b) A man pulling a rickshaw: Muscular force

(c) A football in the air: Gravitational force

(d) Falling raindrops: Gravitational force

(e) Your hair stands on end when you remove a woollen cap: Electrostatic force

(f) A ball rolling on the ground: Frictional force

(g) A carpenter driving a nail into wood with a hammer: Muscular Force

(h) A boy pedalling a cycle: Muscular force



Page No 138:

Question 13:

In a game of tug of war, three girls of team A pull the rope with forces of 100 N, 120 N and 170 N. In team B, the three members pull the rope with forces of 130 N, 150 N and 155 N. Who will win the tug of war? what is the resultant force?

Answer:

Force applied by team A = FA = 100 + 120 + 170
                                                = 390 N

Force applied by team B = FB = 130 + 150 + 155
                                               = 435 N

As FB > FA, the resultant force is in the direction of FB. So, team B will win the tug of war.

Resultant force = FB - FA
                        = 435 - 390
                        = 45 N

Page No 138:

Question 14:

A horse pulls a cart with a force of 1500 N. The force of friction between the cart and the ground is 1500 N. The cart does not move. Why?

Answer:

When a horse pulls a cart with a force of 1500 N, and the force of friction between the cart and the ground is 1500 N, the cart does not move because both forces are equal but in opposite directions and the resultant force acting on the cart is zero.

Page No 138:

Question 15:

Match the terms in column I to those in column II.

Column I Column II
a. Sparks are produced when a pair of scissors is sharpened against a grinding wheel. i. It makes them rough and increases friction.
 
b. A piece of chalk wears out as it is used ii. Friction produces heat.
 
c. Trolleys have wheels. iii. Friction causes wear and tear.
d. The leather soles of shoes are rubbed on a rough surface. iv. Powder reduces friction.
e. A surgeon powders his hands before pulling on his plastic gloves. v. Rolling reduces friction.

Answer:

Column I Column II
a. Sparks are produced when a pair of scissors is sharpened against a grinding wheel. ii. Friction produces heat.
 
b. A piece of chalk wears out as it is used. iii. Friction causes wear and tear.
c. Trolleys have wheels. v. Rolling reduces friction.
d. The leather soles of shoes are rubbed on a rough surface. i. It makes them rough and increases friction.
e. A surgeon powders his hands before pulling on his plastic gloves. iv. Powder reduces friction.

Page No 138:

Question 16:

Define pressure. What are the units of pressure in this list:
N/cm2, N, N/m2, cm2, m2

Answer:

Pressure is the perpendicular force acting per unit area.

Units of pressure:

Pressure=ForceAreaUnitsofPressure=Nm2 andNcm2

Page No 138:

Question 17:

Give reasons for the following:
(a) Athletes wear specially designed shoes with spikes on the soles.
(b) If you walk on the snow, your feet will sink, but if you put on skis you can move over the snow easily.
(c) A woman walking across a lawn in high-heeled shoes would leave a deeper impression on the ground than an elephant.
(d) It is easier to sew with a pointed needle than with a blunt needle.
(e) It is necessary to keep the bathroom floor clean and free of oily substances.
(f) If you are hit with the edge of a ruler it is much more painful than if you are hit with the flat side.
(g) Heavy trucks and tractors have rear wheels that are extra wide.
(h) The size of the bubbles exhaled by a diver working underwater increases as the bubbles rise to the surface.
(i) Kabaddi players rub soil on their hands before a game.

Answer:

(a) Athletes wear specially designed shoes with spikes on the soles because the spikes make the soles rough and increase the friction with the ground, which enables the athletes to move easily without skidding.

(b) When we put on skis, they increase the area in contact with the surface.
We know that
Pressure = Force / Area
When the area increases, the pressure decreases. So, we can move over the snow easily.

(c) High-heeled shoes have a smaller area that comes in contact with the ground in comparison with an elephant's feet. 
      We know that
      Pressure = Force / Area
When the area decreases, the pressure increases. So, high-heeled shoes create a deeper impression on the ground than an elephant's feet.

(d) A pointed needle has a tip with a smaller area, and so it exerts more pressure for the same applied force than a blunt needle. This makes it easier to sew with a pointed need than with a blunt one.

(e) It is necessary to keep the bathroom floor clean and free of oily substances because oily substances reduces friction and can cause a person to slip on the smooth floor and fall.

(f) The edge of a ruler has a smaller area than its flat side.
We know that
Pressure = Force / Area
When the area decreases the pressure increases. So, if we are hit with the edge of a ruler, it is much more painful than if we are hit with the flat side.

(g) Heavy trucks and tractors have rear wheels that are extra wide to reduce the pressure on the ground
We know that
Pressure = Force / Area
When the area increases, the pressure decreases.

(h) The size of the bubbles exhaled by a diver working underwater increases as they rise to the surface because, near the surface, the pressure due to the water above the diver is less.

(i) Kabaddi players rub soil on their hands before a game to increase the friction as sweat makes their hands slippery. 

Page No 138:

Question 18:

Give three examples each where friction is a disadvantage and where it is helpful.

Answer:

Advantages of friction:

  1. Friction helps us to walk without skidding.
  2. Without friction, writing on a piece of paper or a black board would not be possible.
  3. Friction helps us to apply the brakes on a vehicle.

 Disadvantages of friction:

  1. Friction causes wear and tear. For example, shoe soles wear out after some time.
  2. It produces heat. For example, the engine of a car becomes hot after the vehicle has run for some time.
  3. It resists the motion of an object. For example, it slows down the speed of vehicles.

Page No 138:

Question 19:

Describe two ways of reducing friction.

Answer:

Two ways to reduce friction are as follows.

  1. Change sliding friction into rolling friction using ball bearings
  2. Use a lubricant between two surfaces

Page No 138:

Question 20:

Would it be difficult to swallow if you had no saliva in your mouth? Why?

Answer:

It would be difficult to swallow if we had no saliva in our mouth because saliva acts like a lubricant and reduces friction in the mouth.



Page No 139:

Question 21:

Complete the following table:

Pressure Force Area
a. _________ 50 N 5 cm2
b. 15 N/cm2 __________ 5 cm2
c. 5 N/m2 500 N _________
d. _________ 750 N 25 cm2
e. 80 N/m2 800 N _________
f. 45 N/m2 __________ 10 m2
g. _________ 200 N 10 cm2
h. 5 N/m2   20 m2
i. 20 N/m2 400 N _________

Answer:

The following table has been completed by using the formula 'Pressure = Force / Area'.
 

Pressure Force Area
a. 10 N/cm2 50 N 5 cm2
b. 15 N/cm2 75 N 5 cm2
c. 5 N/m2 500 N 100 cm2
d. 30 N/cm2 750 N 25 cm2
e. 80 N/m2 800 N 10 m2
f. 45 N/m2 450 N 10 m2
g. 20 N/cm2 200 N 10 cm2
h. 5 N/m2  100 N 20 m2
i. 20 N/m2 400 N 20 cm2
 

Page No 139:

Question 22:

A force of 500 N acts on a square piece of plywood, each of whose sides is 5 m long. Calculate the pressure acting on the piece of plywood.

Answer:

Force acting on the square piece of plywood = 500 N
Area of the square piece = Side × Side = 5 m × 5 m            
                                                             = 25 m2
Pressure acting on the piece of plywood = Force / Area
Pressure = 500 / 25
               = 20 N/m2
Thus, the pressure acting on the square piece of plywood is 20 N/m2.

Page No 139:

Question 23:

A boy stands on the ground. The area below his feet is 70 cm2. The pressure he exerts on the ground is 7 N/cm2. Calculate the total force acting on the ground.

Answer:

Area under the boy's feet = 70 cm2
Pressure exerted by the boy on the ground = 7 N/cm2
Total force acting on the ground = Pressure × Area
Force = 7 × 70
          = 490 N
Thus, the force exerted by the boy on the ground is 490 N.

 

Page No 139:

Question 24:

A force exerts a pressure of 45 N/m2 when it acts on an area of 10 m2. Calculate the total force.

Answer:

Pressure = 45 N/m2
Area = 10 m2
Force = Pressure × Area
Force = 45 × 10
          = 450 N

Page No 139:

Question 25:

A force of 400 N exerts pressure of 20 N/cm2. What is the area on which the force acts?

Answer:

Force       = 400 N
Pressure   = 20 N/cm2 

Area on which this force acts = Force / Pressure

Area = (400/20) cm2
        = 20 cm2

Thus, the force is acting on an area of 20 cm2.

 

Page No 139:

Question 26:

Indicate whether the following are true or false. Rewrite the false statements correctly.
(a) If we apply the brakes to a moving car, it comes to rest. This shows that force can make a moving object go slower.
(b) When a force of 1 N acts on an area of 1 m2 , the pressure exerted is equal to one Pascal.
(c) The foundations of high rise buildings are kept wide so that they exert less pressure on the ground and prevent the building from sinking into the ground.
(d) A boy throws a ball down from the second floor of a building. In this case, gravitational force is acting only on the ball.
(e) Friction helps us to walk more easily on a smooth, highly polished surface.
(f) An old truck tyre will cause more friction with the road than a new tyre.
(g) In the frozen northern regions of Canada, the Eskimos wear large snowshoes as that helps them to walk on freshly fallen snow.
(h) When a force acts on a large area, the pressure it exerts is more.
(i) Gravity and friction are both contact forces.

Answer:

(a) False

If we apply the brakes to a moving car, it comes to rest. This shows that friction can make a moving object go more slowly.


(b) True

(c) True

(d) True

(e) True

(f) False

An old truck tyre will cause less friction with the road than a new tyre.

(g) True

(h) False

When a force acts on a large area, the pressure it exerts is less than if it had acted on a small area.

(i) False

Gravity is a non-contact force, whereas friction is a contact force.



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