Physics Chapter 3|Short Question 9thclass||tutoronlines

Chapter 3

Dynamics

Table of content:-

Number of topics Name the topics
1 Dynamics
2 Chapter within short Questions
3 Exercise Short Questions
4  Answer the Question
Q.1 How much force need to prevent the books from falling?

Ans. Force equal to the weight of tiles book is needed to prevent the books from falling.

Q.2 What is action?

Ans. Weight of this book is action in this case.

Q.3 Is there reaction? If yes, then what is its direction?

Ans. Yes there is a reaction offered by hand. The direction of reaction is opposite to the weight.

Q.4 Which shoe offer less friction?

Ans. Shoe with flat sole will offer less friction.

Q.5 Which shoe is better for walking on dry track?

Ans. On dry track, shoe with flat sole is batter for walking.

Q.6 which shoe is better for jogging?

Ans. Shoe which has not flat sole is batter for jogging.

Q.7 which sole will wear out early?

Ans. Shoe with flat sole will wear out early.

Q.8 Why is it easy to roll a cylindrical eraser on a paper sheet than to slide it?

Ans. It is easy to roll a cylinder eraser on a paper sheet than to slide it because rolling friction less than sliding friction.

Q.9 Do we roll or slide the eraser to remove the pencil work from our notebook?

Ans. We slide the eraser to remove the pencil work from our notebook because we need more friction to remove the work.

Q.10 In which case do you need smaller force and why?

Ans. (i) Rolling (ii) Sliding

Rolling:

Incas of rolling friction we need smaller force beaches there is contact with earth on only a single point.

Sliding:

Incas of sliding friction we need greater force because all the body is in contact with the earth.

Q.11 In which case it is easy for the tyre to roll over?
Ans. Rough ground:

Incas of rough ground it is difficult to role over the tyre because rough surface offer more friction.

Smooth ground:

In case of smooth ground it is easier to role the tyre because smooth surface offer less friction.

Exercise short Questions

Q.1 Define the following terms. Inertia, momentum, force, force of friction and centripetal force?
Ans. Inertia:

Inertia of a body is its property due to which it resists any change in its state of rest or motion.

Momentum:

Momentum of a body is the quantity of motion it possesses due to its mass and velocity.

Force:

A force moves or tends to move, stops or tends to stop the motion of the body. The force can also change the direction of motion of a body.

Force of inertia:

 

Centripetal force:

Centripetal force is a force that Keeps a body moving in a circle.

Q.2 Differentiate between mass and weight?
Ans. Mass:
  1. Mass of a body is the quantity of matter possessed by the body.
  2. It is a scalar quantity.
  3. It is measured by physical balance.
  4. It remains same everywhere.
  5. It does not change with change of place.
  6. Unit of mass is kilogram (kg)
  7. It is a base quantity.
  8. It can be calculated by using the formula w=mg m=w/g
Weight:
  1. The weight of a body is the force of gravity acting on it.
  2. It is a vector Quantity and is towards the centre of the earth.
  3. It measured by spring balance.
  4. It does not remain same at all places because the value of ‘g’ does not remain same at all places.
  5. Unit of weight is newton (N)
  6. It derived quantity.
  7. It can be calculated by using the formula w=mg
Q.3 Differentiate between action and reaction?

Ans. Let a body B reacts against this force on another body B, the body B reacts against this force and exerts a force on body A. The force exerted by body A on B is the action force where as the force exerted by body B on A is called the reaction force. Note action and reaction act on two different bodies.

Q.4 Differentiate between sliding friction and rolling friction?
Ans. Sliding friction:

Friction force experienced by the body when a body slides over the other body. It is greater than rolling friction.

Rolling friction:

Friction force experienced by the body when a body rolls over the other body. It is less than sliding friction.

Q.5 What is law of inertia?

Ans. Newton’s first law of motion is deals with the inertial property of matter so it is also known as law of inertia, states that
A body continue its state of rest or of uniform motion in a straight line provide no net force act on it

Q.6 Why is it dangerous to travel on the roof of a bus?

Ans. The friction or drag force due to air acting on upper part of the body of a person standing on roof a running bus, tries to turn over which which is dangerous while the lower part of the body is at rest with respect to the roof of bus.

Q.7 Why does a passenger moves outward when a bus takes a turn?

Ans. Passenger standing in the bus fall forward when its driver applies brakes suddenly. It is because of the upper parts of their bodies tend to continue their motion, while lower parts of their bodies in contact with the bus stop with it. Hence, they fall forward.

Q.8 How can you relate a force with the change of momentum of a body?

Ans.

Q.9 What will be tension in a rope that is pulled from its end by two opposite force 100N each?

Ans. The tension in a rope that is pulled from its end by two opposite force 100N each will be 100 N. Because tension remain same Throughout the rope.

Q.10 Action and reaction are always equal and opposite then does a body move?

Ans. Action and reaction are equal in magnitude but opposite in direction. Reaction and do not act on the same body. Action is applied on one body due to which an equal and opposite reaction is acting on another body. Both of these do not neutralize each other due to which the body will move.

Q.11 A horse pushes the cart. If the action and reaction are equal and opposite then how does the cart move?

Ans. The horse apply action on the road by his feet, the reaction is given by the road on the horse, due to which horse move. The cart which is tied with the horse will also move.

Q.12 What is the law of conservation of momentum?

Ans. The momentum of an isolated system of two or more than two interacting bodies remains constant.
Note:-
Isolated system is a group of interacting bodies on which no external force is acting. If no unbalanced or net force act on a system them its momentum remains constant.

Q.13 Why is the law of conservation of momentum important?

Ans. With the help of law of conservation of momentum. It is possible to calculate force, velocity, acceleration of a body. Many elementary particles discovered by using law of conservation of momentum. Motion of rocket and jet engine are result of application of this law.

Q.14 When a gun is fired, it recoils. why?

Ans. Before firing the gun, both the gun and bullet are at rest, so the total momentum of the system is zero. As the gun fired, bullet shoot out of the gun and acquires momentum. To conserve momentum of the system, the gun recoils. According to the law of conservation of momentum, the total momentum of the gun and the bullet will also be zero after the gun is fired.

Q.15 Describe two situations in which force of friction is needed?

Ans. (i) We cannot write if there would be no friction between paper and the pencil.
(ii) Friction enables us to walk on the ground. We cannot run on a slippery ground. A slippery ground offers very little friction.

Q.16 How does oiling the moving parts of a machine lower friction?

Ans. As the friction of liquids is less than friction of solid. So oiling the moving parts of the machines lower the friction.

Q.17 Describe ways to reduce friction?

The friction can be reduced by:
• Making the sliding surface smooth
•  Fast moving objects a streamline shape (fish shape) such as car, aeroplane, etc this causes the smooth flow of air and thus minimizes air resistance at high speed
1. Lubricating the sliding surface.
2. Using ball bearing or roller bearing because the rolling friction is lesser than the sliding friction.

Q.18 Why rolling friction is less than sliding friction?

Ans. In case of rolling friction wheel rolls without rupturing the cold weds. When a wheel moves on a smooth surface, it has the contact with the surface only at a single point. So rolling friction is less than sliding friction.

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