CBSE Class 9 Science Force and Laws of Motion Worksheet Set A

Note:- All the students have to write/ solve following questions in their separate register/A-4sheets/papers Momentum: It is the measure of motion in body or It is equal to the product of mass and velocity of a body
Formula : p=m x v
Units = Kg m/s
It depends upon velocity. As we know, velocity is a vector quantity so as momentum is a vector quantity

Have you noticed a karate player : He can break a pile of tiles with a single blow of his hand
Answer : The karate player strikes the pile of tiles with his hand very fast. In doing so the large momentum of his hand is reduced to zero in a very short duration.
Newton’s second law of motion
Let’s learn about Newton’s Second Law of Motion now:
Newton’s Second Law of Motion: According to it, the rate of change of momentum of a body is directly proportional to the force applied.
Let us try to understand it mathematically
Consider a ball of mass “m” moving with initial velocity “u”. Then suddenly a retarding force “F” is applied and it is brought to rest, final velocity say “v”. Retardation “a” is produced in a ball.
Consider a body of mass = m
Travelling with initial velocity =u
Suddenly brakes are applied so, force exerted =F
Final velocity = v
Initial Momentum =mu
Final Momentum =mv
Change in Momentum=final momentum –initial momentum =v-u
Rate of change in Momentum=mv-mu/t
Then , according to second law it was said by Newton that:
Rate of change in Momentum is directly proportional to force applied that is :
Applying second law we get,
mv- mu/t ∝F
or, F ∝m(v-u)/t
As we know, from Newton’s first equation that v-u/t=a
So, substituting the value of v-u/t as “a” , we get :
or,F ∝m.a
Removing proportionality sign, we get :
F=K m .a (where K is a constant of proportionality and its value is taken as 1 .
If we talk about the S.I unit of Force, please write it down, it is kept after the name of Newton that is: S.I. unit of force is Newton.
We can define it as:
As we know F= m x a
So, putting units of all involved in the equation, we get :
F =m x a
1N=1Kg x 1m/sec2
It is defined as:
If we consider mass of body as 1kg and acceleration to be 1 meter per second square then we have : 1 Newton as: Force is said to be one Newton, when it acts on a body of mass 1kg, producing an acceleration of 1 meter per second square.

Have you noticed that a cricket player lowers his hands while taking a catch
Answer : A fast moving ball has a large momentum. In stopping the cricket ball, the momentum of ball has to be reduced to zero. Now when the cricket player moves his hands back while taking a catch, he increases the time taken for rate of change of momentum. Therefore, less force is exerted on the hands of cricketer and hence, he doesn’t get hurt.

Have you noticed that in athletics mostly high jumping athlete is provided either with cushion or a heap of sand on the ground to fall upon.
Answer : This is to prevent injury to the athlete when he falls down after a jump. When high jumper falls on soft landing site; the jumper takes a long time to come to a stop. So, the rate of change of momentum is less. Therefore, the force exerted by the ground on athlete is also less and he doesn’t get hurt.

Have you noticed that when we push something harder, our hands also start paining.
Answer : Likewise, when we move on a road over a large distance, our feet also start paining.The reason is that: When we apply force on somebody, that body also exerts an equal and opposite force on us.

Newton’s third law of motion
Let us learn about Newton’s Third Law of Motion now.
According to it : For every action there is an equal and opposite reaction.
Suppose body A applies force on body B , then B also applies the force with the same magnitude but in opposite direction . So, mathematically we can write as : Fab=-Fba.
In this –ve sign shows the other force is in the opposite direction.

Have you noticed that the gun also Recoils after firing a bullet
Answer : When a bullet is fired from a gun, it moves with certain velocity in forward direction. As a result, it applies equal & opposite force on gun. Therefore, it recoils.

Another important application is how we walk.
Answer : When we walk on the ground, our foot pushes the ground backward and in return, the ground pushes our foot forward. The forward reaction is exerted by the ground, on our foot which makes us walk forward. However, if the ground is slippery it becomes difficult to walk because on slippery ground, friction is less. Due to this, we can’t exert backward action which would produce a forward reaction.
Law of Conservation of Momentum
As we have already discussed that all moving bodies possess momentum. So, it was seen that the momentum for a system is a constant quantity which is further explained by a law given below.
For a system, the sum total momentum is constant, provided no external force acts on the bodies. Or we can say that momentum is never created nor destroyed, it is always conserved. Let’s try to explain it mathematically:

Have you noticed it is difficult for a fireman to hold a hose which ejects a large amount of water at high velocity.
Answer : Fireman finds it difficult to hold a hose pipe because as water moves through it with certain action, with a large velocity, in turn it produces large reaction.

Have you seen a horse pulling a cart.
Answer : A horse pulls a cart by Action & Reaction because they are equal and opposite and are acting on different bodies & hence, do not cancel each other. Action is on ground and reaction is on horse and cart. Reaction is greater than friction & therefore, the horse & cart move forward due to an unbalanced force