UNIT 3:

DYNAMICS...

Q. Define the following terms.

i. Ineria                                                              ii. Momentum                               

iii. Force                                                            iv. Force of friction

v. centripetal force

Ans.

Inertia:

                The characteristic of the body to resist any change in its state of rest or state of uniform linier motion is called “inertia”.

Momentum:

The quantity of motion of a moving body is called “momentum”. It is  measured as a product of its mass and velocity (p = mv)

Force:

            The agency which stops or tends to stop the motion of the body, moves or tends to move and produces acceleration is called “force”.

Force of friction:

            The resistive force between two bodies is called “force of friction”.

OR…

The force that opposes the motion of an object is called “force of friction”.

Centripetal force:

            The force that keeps a body to move in a circular path is called “centripetal force”. Its formula is,

Q. what is difference between:

i. Mass and weight                      ii. Action and reaction

iii. Sliding friction and rolling friction

Ans.

Mass and weight:

Mass	Weight
Mass is the quantity of matter.	Weight is the force by which the earth attracts something towards its center.
Mass is scalar quantity.	Weight is vector quantity.
The unit of mass is kg.	The unit of weight is N.
Mass remains constant.	Weight changes due to change of gravitational acceleration.
Mass is denoted by m.	Weight is denoted by w.

 

Action and reaction:

                Action is applied force and applied by the first body on the second body. The reaction is the force exerted by the second body in response to the first body.

Sliding friction and rolling friction:

                The force of friction between two bodies when one slides over the other is called sliding friction.

                The force of friction between two bodies when one roles over the other is called rolling friction.

For some bodies rolling friction is less than the sliding friction.

Q. What is the law of inertia?

Ans. First law of Newton also known as law of inertia and it states that:

“Everybody  at rest will remain at rest and moving body continues

 it uniform motion in a straight line in the absence of net force.”

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

Ans. When the driver accelerates the bus or applies the brakes the passenger may fall due to inertia, that is why it is dangerous to travel on the roof of a bus.

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

Ans. When a bus takes a turn, the passengers sitting in the bus move outward due to centrifugal force. It may also be due to inertia that they want to move in their straight path.

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

Ans. According to the Newton’s second law, it can be written as:

Here...

By putting these values;

Force = time rate of change of momentum

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

Ans. when a rope is pulled from its end by two opposite forces 100 N each the tension in the rope will be 200 N, considering the magnitude of forces.

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Q. Action and reaction are always equal and opposite. Then how does a body moves?

Ans. Although action and reaction are always equal in magnitude and opposite in direction but they act upon two different bodies and do not cancel out each other’s effect. That is why body moves.

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

Ans. Although action and reaction are always equal in magnitude and opposite in direction but they act upon two different bodies. The horse applied action force on road and reaction force is exerted by the road due to which horse moves and cart tied with horse, also moves.

Q. What is the law of conservation of momentum?

Ans. This law states that,

“The momentum of an isolated system of two or more interacting bodies remains constant.”

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

Ans. This is an important law and has vast application. It is universally applicable all small bodies (electrons and protons) and large bodies (planets and stars). By using this law force, acceleration, velocity of a body can also be calculated.

Q. When a gun is fired, it recoils. Why?

Ans. Before firing the gun the total system is zero. When gun is fired, the fired bullet gains momentum. To conserve the momentum of the system, the gun recoils.

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

Ans.

o   To walk on the ground, friction is needed

o   To stop a moving vehicle, friction between tires and road is needed.

o   To fly in the sky, friction is needed by the birds.

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

Ans. No surface is smooth completely. By oiling the various parts of the machine, the surface of the parts becomes slippery due to which force of friction reduces.

Q. Describe way to reduce friction.

Ans. Friction can be reduced:

o   By making the contact surface smooth.

o   By making the streamline shape of moving objects.

o   By using the lubricants.

o   By reducing the ups and downs of two surfaces. 

Q. Why rolling friction is less then sliding friction?

Ans. In case of rolling, very small area is in contact and there is no relevant motion in rolling body and the surface. That is why rolling friction is less then sliding friction.

Q. What do you know about the following:

i.             Tension in a string                     ii.           Limiting force of friction 

iii.          Braking force                                                iv.          Skidding of vehicle

v.            Seatbelts                                          vi.          Banking of roads

vii.         Cream separation

Ans.

Tension in a string:

                The force acting along the string is called tension. The tension in the string is to neutralize the applied force on a string.

o   Its SI unit is N.

o   It is denoted by T.

Limiting force of friction:

                The maximum possible value of friction is called limiting force of friction. F_s

Braking force:

                The maximum force of friction between brake-bushes and wheel is known as braking force.

Skidding of vehicle:

                When force of friction between tyres and road surface is not sufficient and brakes are applied, the tyres slide over the road surface. It is known as skidding of vehicle.  

Seatbelts:

                The belts connected with seat prevent the upper part of the passengers from moving forward due to inertia, when brakes are applies suddenly.

Banking of road:

                On turning points, the outer edge of the road is kept higher than the inner edge to prevent the vehicle from slipping. It is known as banking of road or super elevation of the road.

Cream separator:

                It is a high-speed spinner used to separate the cream from milk. It works on the principle of centrifugal force

Q. What would happen if all friction suddenly disappears?

Ans. If all friction suddenly disappears, the bodies cannot change their states.  The bodies cannot be moved.

Q. Why the spinner of a washing machine is made to spin at a very high speed?

Ans. The spinner of a washing machine is made to spin at a very high speed to separate undesired particles from clothes.