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NIOS Class 10 Science and Technology Chapter 10 Force and Motion
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Force and Motion
Chapter: 10
INTEXT QUESTIONS 10.1
1. Is there any force applied when a cricket player changes the direction of ball by using his/her bat?
Ans: Yes.
2. Give an example from your daily life in which the shape of an object changes by applying a force.
Ans: Pressing a lump of dough with your hands.
INTEXT QUESTIONS 10.2
1. What are balanced forces?
Ans: When two or more forces acting on an object in opposite direction balances each other then the forces are known as balanced forces.
2. Can a balanced force produce any acceleration in a body?
Ans: No. Balanced forces do not change the state of motion of an object.
3. What type of change can be produced by an unbalanced force in a body?
Ans: Unbalanced forces acting on an object may change its state of rest or motion.
INTEXT QUESTIONS 10.3
1. Why does water comes out from a wet piece of cloth when you shake it?
Ans: Due to inertia of rest. When we shake the cloth, the water remains in its position and comes out.
2. Why do we fall forward, when a moving bus stops suddenly?
Ans: Lower part of our body comes to rest but due to inertia of motion our upper part tends to move in the forward direction and we fall in the forward direction.
3. Two similar trucks are moving on a road with the same velocity. One of them is empty while the other one is loaded. Which of the two has more momentum?
Ans: As momentum is equal to mass × velocity. So the momentum of loaded truck (more mass) has more momentum.
4. If a body of mass 5 kg moves with a velocity of 10 ms–1, then what is the momentum of the body?
Ans: Momentum = m × v = 5 kg × 10 ms–1 = 50 kg ms–1.
5. Why does a boxer move his head backward while taking an oncoming punch?
Ans: To decrease the rate of change of momentum boxer moves his head backward so that the impact of punch is reduced.
INTEXT QUESTIONS 10.4
1. Why does a fast moving car slow down when its engine is switched off?
Ans: Due to force of friction acting between wheel of car and ground.
2. Why do we slip when we step on a banana peel?
Ans: Because the friction between banana peel and ground is very small.
3. Why are tyres of automobiles threaded?
Ans: Treaded tyres provide better grip with the ground because in such tyres the friction between the tyres and ground is very large.
INTEXT QUESTIONS 10.5
1. Why does a porter carrying a heavy load place a round piece of cloth on his head?
Ans: Round piece of cloth increases the area of contact between the load and head of the porter, thereby decreasing the pressure on his head.
2. Why a nail has a pointed tip?
Ans: A nail has a pointed end so that the surface area is less, when the surface area is less the pressure increases which makes it easy for the nail to be driven into the wall or wherever it may be inserted.
3. Why are shoulder bags provided with broad straps? 4. State the SI unit of pressure.
Ans: To decrease the pressure.
4. State the SI unit of pressure.
Ans: Nm–2 or pascal (Pa).
| TERMINAL EXERCISE |
1. Why does a sprinter keep running for some time even after crossing the finish line?
Ans: A sprinter keeps running even after crossing the finishing line because the inertia of motion keeps him running. The inertia of motion is the tendency of mass to remain in constant motion.
2. Why is it advised to tie the luggage with a rope on the roof of buses?
Ans: When the bus stops suddenly, the luggage on the roof top will fall forward due to inertia of motion. Similarly when the bus starts the luggage will fall backwards due to inertia of rest. To avoid this, any luggage kept on the roof of a bus is tied with a rop.
3. Why do the dust particles from the hanging blanket fall off when it is beaten with a stick?
Ans: Beating the blanket with the stick makes the blanket move but the dust particles remain at rest due to inertia of rest, Thus the dust gets detached from the blanket.
4. State Newton’s first law of motion. Why do the passengers standing in a stationary bus fall in the backward direction when the bus begins to move suddenly.
Ans: Newton’s first law of motion states that an object will remain at rest or in uniform motion in a straight line unless acted upon by an external force.
When a bus begins to move suddenly, the passengers standing in the stationary bus fall backward due to inertia. Inertia is the tendency of an object to resist changes in its state of motion. When the bus accelerates forward, the passengers, initially at rest, want to remain at rest due to their inertia. As a result, they continue to stay in their original position momentarily while the bus moves forward. However, the bus floor exerts a force on the passengers, pushing them forward. Because of this force acting on them, they fall backward relative to the bus’s accelerating motion.
5. Define momentum. How the rate of change of momentum is related to force?
Ans: The rate of change in momentum of an item is proportional to the applied resultant force and is in the same direction as the applied resultant force. So, we can say that the rate of change of momentum is equal to the force applied. It is understood that, Momentum = mass × velocity.
Newton actually stated his second law of motion in terms of momentum: The net external force equals the change in momentum of a system divided by the time over which it changes. The change in momentum is the difference between the final and initial values of momentum.
6. If a body of mass 10 kg moves with a velocity of 7 ms–1, then what is the momentum of the body?
Ans: Momentum = mass × velocity
Or, momentum = 10 × 7= 70.
7. If a force of 50 N acts on a body of mass 10 kg then what is the acceleration produced in the body.
Ans: We know that force, F = m × a.
Here, a = acceleration = ?; mass=10kg, f = 50N.
So, 50 = a × 10;
Or, a = 5 m/s^2.
8. State Newton’s third law of motion. Why it is difficult for a fireman to hold a hose pipe which ejects larger amounts of water at a high speed?
Ans: Newton in his third law of motion stated a relation between action and reaction. According to this law, to every action there is an equal and opposite reaction. The action and reaction act on two different bodies if action and reaction are on the same body they will constitute a balanced force and body will not move.
When the water is ejected from the hose pipe at that time each amount of force is needed from the hand to hold it. As the hand doesn’t provide the same number of forces it is difficult to hold it.
9. “Action and reaction forces are equal in magnitude and opposite in direction”. Then, why do they not balance each other?
Ans: Action and reaction exist in pair and are always equal in magnitude and opposite in direction. But they act on different bodies, hence they cannot balance each other.
10. A motorcycle is moving with a velocity of 72 km/h and it takes 6 s to stop after the brakes are applied. Calculate the force exerted by the breaks on the motorcycle, if its mass along with the rider is 175 kg.
Ans: Initial velocity(u) = 72 km/h = 72×5/18 = 20 m/s.
Final velocity = 0; time = 6s.
So, v = u + a × t,
Or, a =- 20/6 =- 10/3.
Then force = mass× acceleration = 175× -10/3= 583.33N.
11. An object of mass 2 kg travelling in a straight line with a velocity of 10 ms–1 collides with and sticks to a stationary object of mass 6 kg. Then they both move off together in the same straight line. Calculate the total momentum just before the impact and just after the impact.
Ans: Momentum = mass × velocity;
Here, mass = (2 + 6) = 8kg. And velocity = 10m/s.
So, momentum = 8×10
= 80. Khm/s^2.
12. What is the force of friction? State two methods to reduce friction.
Ans: According to Newton’s first law of motion, a moving body continues to move along a straight line until unless an external force is applied on it. Is this external force slows down the motion of the ball or the car? Think! In Fact the ball or car is slowed down by a force called friction. Friction exists between the surfaces of all materials which are in contact with each other. The direction of the frictional force is always in a direction opposite to the motion.
Two methods to reduce friction are mentioned below:
(i) Using lubricating oil.
(ii) By polishing the surface.
13. What is the relation between thrust and pressure? State the SI units of thrust and pressure. Why can a camel run in a desert easily?
Ans: Observe some bodies around you like table, desk, bucket full of water, etc. They press the floor with a force equal to their own weight. You know that weight is the force acting vertically downwards. As the surface of the floor can be taken as horizontal, therefore, the force with which each of the above mentioned bodies presses the floor is directed perpendicular to the surface of the floor. The force acting upon the surface of a body perpendicular to it is called thrust. The SI unit of pressure is Nm–2. This unit has also been given a specific name pascal (Pa) in honour of the scientist named Blaise Pascal. Because of having wide feet and long legs the camel loses its weight in the sand and as a result of that it easily runs on the surface of the desert.
14. A block of wood kept on a table applies a thrust of 49 N on the table top. The dimensions of the wooden block are 40 cm × 20 cm × 10 cm. Calculate the pressure exerted by the wooden block if it is made to lie on the table top with its sides of dimensions
(a) 20 cm × 10 cm.
Ans: We know that, pressure = force/ area,
Here area = 20×10 cm^2 = 0.2× 0.1 m^2.
So, pressure = 49 / (0.2 × 0.1)
= 2450pa.
(b) 40 cm × 20 cm.
Here, area = 40× 20 cm^2 = 0.4 × 0.2 m^2.
So, pressure = 49 / (0.4 × 0.2)
= 612.5 pa.
