**9**

^{th}physics: Force and laws of MotionAn object of mass 2 kg is sliding with a constant velocity of 4 m/ s on a frictionless horizontal table. The force required to keep the object moving with the same velocity is

(a) 32 N (b) 0 N (c) 2 N (d) 8 N (b]

Rocket works on the principle of conservation of

(a) mass (b) energy (c) momentum (d) velocity [c]

A car is moving at 45 km/hr.A constant force acts on the car for 10 sc .So that it's velocity becomes 63 km/hr .The distance travelled by car during this interval of 10 sec is :

a) 100 m b) 150 m c) 200 m d) 50 m [b]

What is the definition of balanced and unbalanced force?

When two equal forces acting on an object in opposite directions called balanced force.

*When something does not move the forces are balanced, e.g. Gravity pulls a table down but that table is pushed up by the floor or the ground it stands on so doesn’t’ move.*

An Unbalanced force is needed for something to change movement or change direction, e.g. aseesaw moving up and down because the forces are not equal.An Unbalanced force is needed for something to change movement or change direction, e.g. aseesaw moving up and down because the forces are not equal.

**Net force**: - The sum of all the forces acting on a body is known as net force.

Difference between Balanced force and unbalanced force.

**Balanced force**:- If net force on a body is zero it is called balanced force. Balanced forces do not cause a change in motion. They are equal in magnitude and opposite in direction. Therefore, the resultant of these forces will be zero.

Example: Two persons pushing a box with the same force in opposite directions.

**Unbalanced forces**:- If net force on a body is non-zero it is called unbalanced force.

Forces whose resultant is not equal to zero are called unbalanced forces. For example: An arm wrestling competition among a strong person and a weak one. The resultant force will be in the direction of the force applied by the strong person.

What Is Inertia

By Newton's law we know that ,a body at rest will remain at rest and a body in motion will continue in a straight line unless it is compelled by an external force.This tendency of a body is called inertia.So we can say that 'Inertia is that property of a body due to which it resists a change in its state of rest or of uniform motion.'

When the brakes are applied to the bike the back seater moves forward why?

When the bike accelerates suddenly the back seater who was initially at rest experiences a force of friction at the point of contact between him and the bike. His inertia of rest tries to keep him at rest which results in a backward push. Thus, the back seater moves backwards when the bike accelerates suddenly.

When the brakes are applied the inertia of motion of the back seater tries to keep him in motion so he moves forward.

There are three solids made up of aluminium, steel and wood, of the same shape and same volume. Which of them would have highest inertia?

Since steel has greatest density and greatest mass, therefore, it has highest inertia.

As the mass is a measure of inertia, the ball of same shape and size, having more mass than other balls will have highest inertia.

Two balls of the same size but of different materials, rubber and iron are kept on the smooth floor of a moving train. The brakes are applied suddenly to stop the train. Will the balls start rolling? If so, in which direction? Will they move with the same speed? Give reasons for your answer

Yes. the balls will start rolling in the direction in which the train was moving. Due to theapplication of the brakes, the train comes to rest but due to inertia the balls try to remain in motion, therefore, they begin to roll.

Since the masses of the balls are not the same, therefore, the inertial forces are also not same on both the balls. Thus, the balls will move with different speeds.

Horse need continues of force in order to move a cart with a constant speed. Why?

Horse need continues of force in order to move a cart with a constant speed to balances the force of friction.

Write a short note on third law of motion

**Third law of motion**: This law deals with the forces between bodies that appear in pairs.

*It says that “every action has an equal and opposite reaction”*

When you stand on the ground, your weight pushes the ground downwards; does this make the ground to move downward? No, because the ground pushes you upward with a force equal to your weight and hence these equal and opposite forces cancel out and you stand on the ground balanced.

Force by you on the ground and force on you by the ground are two equal and opposite forces acting in pairs at the surface that is common to both you and the ground.

Newton’s 3

^{rd }law of motion is applicable wherever there is action-reaction force pair. Like walking, shooting with a pistol, collision, rocket propelling, etc.How a karate player can break a pile of tiles with a single blow of his hand?

Karate player strikes the piles in a very short time in a very small area. Since, impulse, I = F × tÞ F = I/t. If ‘t’ will be very short the force becomes very large and act on smaller area .This produce enough pressure by his hand on break the brick

Define momentum?

Momentum (Linear momentum) for moving body is defined as the product of mass and velocity. If a body is moving with velocity, v and having mass, m then momentum of body is p = m x v.

Momentum is a vector quantity. Its direction is the same as that of velocity. SI unit of momentum (p) is kg-m/s

Explain why it is easier to stop a tennis ball than a cricket ball moving with the same speed?

Cricket ball has more momentum than the tennis ball due to greater mass. That’s why it is easier to stop a tennis ball than a cricket ball moving with the same speed.

Why a goalkeeper in a game of football pulls his hands backwards after holding the ball shot at the goal.

To reduce the force exerted by ball on hand by increasing time as F = I/t.

**A passenger in a moving train tosses a coin which falls behind him. Why?**

Because that motion of the train is accelerated.

State and explain the three laws of Newton?

**First law of motion**: Newton’s First law explains the “natural” motion of an object when it is left free. The law says that-

*Every object continues in its state of rest, or of uniform motion in a straight line with unchanging speed, unless compelled to do otherwise by forces acting upon it*.

So, naturally an object at rest will remain at rest and an object moving with a constant speed in a straight line will keep moving, unless they are disturbed by a force.

This property of matter to continue its state of rest or of uniform motion is called “Inertia”. Hence, this law is also called the Law of Inertia.

**Second law of motion**: Newton’s 2

^{nd}law of motion relates the external force (F) acting on a body with the mass (m) of the body; and is mathematically written as F = ma, ‘a’ is the acceleration of the body due to the force.

*It states that the acceleration of a body due to net external force acting on it is equal to the net force divided by the mass of the body*.

The difficulty you face while pushing or pulling a heavier box compared to a lighter box is explained by the 2

^{nd}law of motion.**Third law of motion**: This law deals with the forces between bodies that appear in pairs.

*It says that every action has an equal and opposite reaction*. When you stand on the ground, your weight pushes the ground downwards; does this make the ground to move downward? No, because the ground pushes you upward with a force equal to your weight and hence these equal and opposite forces cancel out and you stand on the ground balanced. Force by you on the ground and force on you by the ground are two equal and opposite forces acting in pairs at the surface that is common to both you and the ground.