Q1. Why do we beat a blanket with a stick to remove dust particles?
Solution
When we beat blanket with stick it comes into motion. But, the dust particles remain at rest due to inertia and get detached from the blanket.
Q2. Which force provides the necessary centripetal force to the planets to move in circular orbit around the sun?
Solution
Gravitational force between sun and the planets provides the necessary centripetal force for the planets to move in circular orbit.
Q3. Give any four examples of Newton's third law of motion.
Solution
Following are the examples of Newton's third law of motion:-
1. Recoil of a gun. When a gun is fired, bullet from the gun moves in the forward direction (action) and the gun recoils in the backward direction (reaction).
2. Swimming. A swimmer pushes water backwards (action) with his hands and the water pushes the swimmer forward (reaction) with the same force.
3. Motion of jet planes or rockets. When burnt fuel in the form of gases comes out of the nozzle (action) in the downward/backward direction, the rocket or jet plane moves in the upward/forward direction (reaction).
4. Walking on a road. When a person walks on a road, he presses against the ground in the backward direction (action) by his feet. The force on the person is in the forward direction (reaction), which makes the person move on the road.
Q4. Newton's third law says that action and reaction are equal and opposite, then how does motion ever take place?
Solution
The motion always take place because, action and reaction do not act on the same body. Hence, they do not cancel out for a body and motion is always possible.
Q5. What is the magnitude and direction of acceleration for a body moving in a circular motion?
Solution
Acceleration of a body moving in a circle of radius R with uniform speed v is v2/R and it is directed towards the centre.
Q6. Which has more inertia, a car or a bus? Explain.
Solution
Inertia is the property of a body on account of its mass. A bus has more inertia, because it has more mass. So, greater force is needed to change the state of the body of greater mass.
Q7. What is the need of free body diagram in mechanics?
Solution
The drawing of a free-body diagram is an important step in solving mechanics problems, since it helps to visualize all the forces acting on a single object.
Q8. Explain the term inertia. What are different types of inertia? Give one example of each type?
Solution
Inertia: The inherent property of a body by virtue of which it cannot change its state of rest or of uniform motion until unless a force acts on it.
Different types of inertia:
Inertia of rest: The tendency of a body to remain in its position of rest is called inertia of rest.
Example: While standing in a bus, we fall backward if the bus starts suddenly.
Inertia of motion: The tendency of a body to remain in its state of uniform motion in a straight line is called inertia of motion.
Example: When a running horse stops suddenly, the person sitting in it falls forward.
Inertia of direction: The inability of a body to change its direction of motion by itself is called inertia of direction.
Example: When car takes a sharp turn, a person sitting in the car experience force, acting away from the centre of the curved path. This happens, due to the tendency of a body to hold its original direction.
Q9. Why does gun recoil while firing?
Solution
When a gun is fired, the force of the gas produced by burning gunpowder hurls out of the bullet. Hence, in accordance with the Newton's third law of motion, the gun itself recoils backwards, i.e., in the opposite direction of the bullet.
Q10. Smoothening of surface beyond the limit increases the friction. Why?
Solution
Smoothening the surface beyond a certain limit increases the friction because the area of actual contact increases. Increase in the area of actual contact increases the cohesion between the surfaces.
Q11. What is the need of banking a circular road?
Solution
When a circular road is banked, the horizontal component of the normal reaction of the road provides the necessary centripetal force for the vehicle to move it along the curved path.
Q12. When a ball is thrown up in a moving train, it comes back to the person's hands instead of falling backwards. Why?
Solution
During its upward and downward motion, the ball continues to move with the same horizontal velocity as the train. In this period, the ball covers the same horizontal distance as the train and so it comes back to the hands of the person throwing it.
Q13. What were the views of Greek philosopher Aristotle about the state of motion?
Solution
According to Aristotle's law of motion, an external force is necessary to keep a body in motion. However, Aristotle was proved to be wrong by Galileo. It was observed that the external forces were necessary to counter the opposing forces of friction to keep bodies in uniform motion. If there were no friction, no external force would be needed to maintain the state of uniform motion.
Q14. A spring balance is attached to the ceiling of a lift. A man hangs his bag on the spring and the spring reads 30 N, when the lift is stationary. What will be the reading of the spring balance, if the lift moves downward with an acceleration of 5 ms-2.
Solution
When the lift is stationary:
The reaction is equal to the weight of the man, that is
R = Mg = 30 N
Therefore, M = 30/g = 30/9.8 = 3.06
When the lift is moving:
Let R1be the reaction, when the lift is moving with an acceleration a = 5 ms-2 in the downward direction.
Then, we have
R1= M (g-a) = 3.06 × (9.8 - 5) = 14.7 ≈ 15 N
Q15. Write any three applications of concept of impulse.
Solution
Following are the applications of impulse:-
1. Shock absorbers are used in vehicles such as scooters, motorcycles, car, etc. While moving on an uneven road, the passengers inside, receive jerks due to impulsive forces exerted by the road. The shock absorbers increase the time of impulse thereby reducing the impact of jerk experienced by the passengers.
2. Buffers are provided between the bogies of a train. The buffers avoid severe jerks on the passengers during shunting of the train. The buffers in between the bogies increase the time of impact thereby reducing the force of jerks.
3. A cricket player lowers his hands while catching a fast moving cricket ball to avoid injury. By lowering his hands, he increases the time of catch, and hence time of impact increases, that reduces the chance of injury.
Q16. State the methods to reduce friction.
Solution
Polishing
Polishing the surfaces reduce the irregularities thereby reducing friction.
Lubrication
We reduce friction by using a lubricant, such as oil or grease which is placed between the two surfaces. This often dramatically lessens the coefficient of friction. Basically the lubricant forms a layer between the two surfaces thereby reducing the friction.
Bearings
Using "ball bearings" or "roller bearings", where balls or rollers allow the surface to move easily without actually touching each other reduces friction to a great extent.
Q17. A gardener is mowing a lawn by pushing and pulling the machine. In which condition is the normal force highest?
Solution
Normal force depends on the weight. Higher the weight, higher is the normal force. So, while pushing, the gardener will put more effort and hence increasing the weight leading to higher normal reaction.
Q18. Why chinaware and glassware are wrapped in paper or straw pieces before packing?
Solution
Chinaware or glassware are wrapped in paper or straw, because in the event of a fall, impact will take a longer time to reach the glass/chinaware through paper/straw. As a result, the average force exerted on the chinaware or glassware is small thereby reducing their chance to break.
Q19. Why are the passengers thrown forward when a speeding bus stops suddenly?
Solution
This is due to inertia. When the speeding bus stops suddenly, lower part of the body comes to rest while the upper part of the body tends to maintain uniform motion. Hence, the passenger’s are thrown forward.
Q20. Give two examples in which force can be exerted on the body without making any contact with them.
Solution
(1) A ball released from the top of a building accelerates downward due to the gravitational pull of the earth.
(2) A bar magnet can attract an iron nail from a distance.
Q21. What are contact forces? Give some examples of contact and non contact forces.
Solution
In contact force, the force on a body arises due to contact with some other body.
Examples of contact and non-contact forces:
1. Contact Forces - Friction, Normal Reaction, Tension in string, Force due to Collision, etc.
2. Non Contact Forces - Gravitational Force, Force between the Like and unlike charges, etc.
Q22. A box of 20 kg is resting on a table. What will be the magnitude of normal force acting on the box?
Solution
Action of box on the table = 20 x 9.8 = 196 N
Normal reaction of table on box = Action of box on table = 196 N.
Q23. Explain why a stone thrown on a glass window smashes the window pane into pieces, but a bullet from the gun passes through making a clean hole.
Solution
The velocity of stone is much less than that of the bullet fired from a gun. Due to its low speed the stone remains in contact with the window pane for a longer time and hence motion is shared by whole of the window pane.
On the other hand, the bullet fired from gun, remains in contact with the window pane for such a small time that it shares its motion only with a portion of the window pane. That's why it makes a clean hole in the window pane.
Q24. What do you mean by momentum?
Solution
Momentum of an object is a vector and is defined to be the product of its mass m and velocity v, and is denoted by p, that is, p = m v.
Q25. What are free body diagrams?
Solution
Free body diagrams are the sketches of the forces which act on an object. It is the very first step to solve problems in mechanics.
Q26. What is the difference between the working of a jet plane and a rocket?
Solution
A jet plane sucks oxygen necessary for combustion of fuel from the atmosphere (as it flies within the atmosphere), whereas a rocket has to carry liquid oxygen cylinders with it, along with the fuel cylinder because it generally flies out of the atmosphere.
Q27. State the types of friction.
Solution
Types of friction are
1. Static Friction
2. Kinetic Friction
3. Rolling Friction
Q28. Can a body undergo change in speed or direction due to internal forces?
Solution
No, it is not possible. In order to change the state of motion of a body, some net external force must act on the body.
Q29. What do you understand by weight or gravitational force.
Solution
The force with which the earth attracts a body towards its centre is called its weight or gravitational force.
If we have a body of mass m then, weight or gravitational force is given as
W=F= mg
Q30. State Newton's second law of motion.
Solution
The rate of change of momentum of an object is directly proportional to the applied force and takes place in the direction in which the net force acts.
Or
The net force on an object is equal to its mass times the acceleration.
Q31. What is SI unit of force? State it.
Solution
The SI unit of force is Newton (N).
Force is said to be 1 N, if it produces an acceleration of 1m/s2 in a body of mass 1 kg.
Q32. State Newton's third law of motion.
Solution
Newton's third law of motion states that "To every action, there is an equal and opposite reaction".
Q33. Why is a hammer used to drive a nail into wood?
Solution
A heavy hammer imparts large momentum, due to its large mass. Hence it is preferred to use a hammer while driving nail into wood.
Q34. The terms 'action' and 'reaction' in the third law gives an impression that action comes before reaction. Is there is any cause-effect relationship?
Solution
It is a misconception that in the Newton's third law of motion, action comes before reaction. There is no cause and effect relationship. In fact here we have two forces, say the force on A by B and the force on B by A that act at the same instant. As both the forces act simultaneously, any one of them may be called action and reaction.
Q35. Explain briefly, how a car is able to go round a level curved path?
Solution
When a car moves round a level curved path, the frictional force between the tyres and the road provides the necessary centripetal force to move in that curved path.
While taking a turn, the tyres of the car tend to leave the road and skid away from the centre of the curve. The force of friction thus acts in the inward direction and provides the needed centripetal force.
Q36. Ice skating is based on the phenomena of reduced friction. Explain briefly.
Solution
Ice skaters actually move on a layer of water, and don't skate on ice at all. When ice is subjected to high pressure it melts. The narrow blades of the skates create a very high pressure and thus the skaters glide along on a layer of water that has just melted. The water refreezes as soon as they've moved on.
Q37. Give some examples from daily life to illustrate the first law motion and the law of inertia.
Solution
(1) When we shake the branch of a tree, its fruits and dry leaves fall down. This is because, on shaking, the branch comes into motion while the fruits and leaves tend to remain at rest and so they get detached from the branch.
(2) An athlete runs some distance before taking a long jump. This is because the inertia of motion gained by the athlete at the time of jumping increases its muscular effort and helps him take a longer jump.
(3) When the horse suddenly starts running, the rider falls backward. This is because the lower part of the rider, which is in contact with the horse, tends to remain in rest due to inertia.
Q38. What do you mean by banking of a curved road and banking angle?
Solution
Raising the outer edge of a curved road above its inner edge is called banking of a curved road.
The angle through which the outer edge of the curved road is raised above the inner edge is called angle of banking.
Q39. What does Galileo conclude about the motion of an object on a frictionless horizontal plane?
Solution
Galileo concluded that an object moving on a frictionless horizontal plane must neither have acceleration nor retardation, i.e., it should move with constant velocity.
Q40. Why is the Newton's first law of motion also called the law of inertia?
Solution
According to Newton's first law of motion, every object tends to remain in state of uniform motion or rest unless an external force acts upon it. This shows that a body, by itself, cannot change its state of rest or of uniform motion. This inability of body to change its state of rest or of uniform motion along the straight line is called inertia. Thus, the Newton's first law defines inertia, and hence it is also called the law of inertia.
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