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Basic Physics lesson-7 : Momentum

Umesh is a freelance writer contributing his creative writings on varied subjects in various knowledge and educational sites in internet.



If we recall, in some earlier lessons in this series we read about mass (Basic Physics lesson-4 : Mass and weight) and velocity (Basic Physics lesson-2 : Speed and velocity). Now it is time to go ahead and learn what do we understand by momentum. We will now see in this lesson how mass and velocity are related to the momentum.


What do we understand by momentum

Momentum is related to the mass and velocity of a body. If a body is at rest then we say its velocity is zero. So it has no momentum which means that its momentum is zero. Suppose body starts moving with a velocity v then it has a momentum and that is defined as mass x velocity = mv. Momentum is represented by p and the formula for p goes as: p = mv.

So, momentum is nothing but the multiplication of the mass of a body with its velocity and it is apparent that these two entities together are responsible for momentum. A large mass body with little velocity can also have a good momentum while a small body with a large velocity can also have a sufficient momentum. At the same time a big mass body with a large velocity has a huge momentum.

Conservation of momentum

When bodies are physically interacting with each other then their momentum after the interaction remains same as before the interaction. Those who have seen the game of billiard must have noticed that the striking ball goes to the stationary ball and imparts some of its momentum to the latter and both the balls now move with certain velocities. Actually what happens is that a part of momentum is transferred from the first ball to second and first ball moves with remaining momentum. Sometimes it might also happen that all the momentum is transferred to the second ball and first ball stops. In any case, the total momentum after and before the collison in this system would remain same.

Example of momentum conservation

Let us take an example of a car having a mass of 1200 kg and moving with a velocity of 50 km/hour. It then accidentally bangs at the back of a pick-up van of mass 1800 kg which is standing there just in stationary position and then both are entangled with each other and start moving forward together. Can we find out the combined velocity of the system after the collision using conservation of momentum? Yes it can be done. Let us assume that the final velocity of the combined system is v. As per the conservation of the momentum the momentum before the collision should be equal to the momentum after the collision. So we can write -

(1200 x 50) + (1800 x 0) = (1200+1800) x v

Solving this we get v = 20 km/hour.

It is apparent that some momentum of the car has been shared by the pick-up van and that has finally decided the combined velocity after the collision.

Unit of momentum

Momentum is defined as mass x velocity so it has a unit like gram cm/second or kg meter/second or pound feet/second etc. In M.K.S. system it has got the units as kg meter/second. One can convert these units from one to another using the conversion tables available in internet.

Momentum is a vector quantity

Mass is a scaler quantity. Velocity is a vector quantity. When we multiply them then the result is a vector quantity only having a bigger magnitude as per the result of the multiplication. For examle if a body is having a mass of 30 kg and a velocity of 0.02 meter/second in the North-East direction then using the formula p=mv we can calculate its momentum as 0.6 kg meter/second and it would have the same direction as that of velocity. We can understand it from the diagram-1 below.





Momentum is defined by mass multiplied by velocity and has a M.K.S. unit of kg meter/second. Many problems in Physics can be solved using the principle of conservation of momentum.


1. https://www.physicsclassroom.com/Class/momentum/u4l1a.cfm

2. https://en.m.wikipedia.org/wiki/Momentum

3. https://www.unitconverters.net/

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2020 Umesh Chandra Bhatt


Umesh Chandra Bhatt (author) from Kharghar, Navi Mumbai, India on August 05, 2020:

SKMUNSHI, thanks a lot.

SKMUNSHI on August 05, 2020:

Nice and refreshing.

Introducing concept of momentum and principle of conservation of momentum is done thoughtfully.The applicability of conservation of momentum for isolated system or in the absence of external forces may also be talked about, may be in next lecture.

Please continue with this excellent nitiative.

Umesh Chandra Bhatt (author) from Kharghar, Navi Mumbai, India on August 05, 2020:

D R Rao, thanks for your comment. Highly appreciate.

DRRAO on August 04, 2020:

Momentum explained with easy to understand example.congrats

Sangita on August 04, 2020:


Explained. I love all these physics concepts, specially when they deal with conservation. Essentially, zero sum game

Umesh Chandra Bhatt (author) from Kharghar, Navi Mumbai, India on March 31, 2020:

Rajan, thanks for your visit. Appreciate much.

Rajan Singh Jolly from From Mumbai, presently in Jalandhar, INDIA. on March 30, 2020:

Felt like back in school only understanding it faster now. Thanks for the lesson.

Umesh Chandra Bhatt (author) from Kharghar, Navi Mumbai, India on March 20, 2020:

Dora, thanks a lot for your interest and appreciate your comment.

Dora Weithers from The Caribbean on March 20, 2020:

Thanks for the science lesson. I'm slow at this, but it makes some sense.

Umesh Chandra Bhatt (author) from Kharghar, Navi Mumbai, India on March 19, 2020:

Liz, thanks a lot for your visit. Appreciate.

Liz Westwood from UK on March 19, 2020:

This brings back memories of school days many years ago.

Umesh Chandra Bhatt (author) from Kharghar, Navi Mumbai, India on March 17, 2020:

RoadMonkey, absolutely right. Thanks a lot for your interest in article. Appreciate.

Umesh Chandra Bhatt (author) from Kharghar, Navi Mumbai, India on March 17, 2020:

Eric, thanks for your sincere and keen interest. The total momentum in the system before collision is only 1200 x 50 = 60000 as the momentum of pick up is zero. After the collision mass of system becomes 1200 + 1800 = 3000.

As the momentum after the collision should also be equal to 60000 so dividing it by 3000 we get 20 only.

Hope this makes it clear.

Thanks a lot for visiting.

RoadMonkey on March 17, 2020:

Eric Dierker. If a vehicle smashes into the back of a stationary vehicle, it will slow down but unless it slows to zero, then the crashing vehicle and the one it smashes into will both slide (keep moving) at a reduced speed.

Eric Dierker from Spring Valley, CA. U.S.A. on March 17, 2020:

I am not getting the 50 then being a velocity of 20. Seems like I am missing a step. Like the 0 velocity of the truck.

Umesh Chandra Bhatt (author) from Kharghar, Navi Mumbai, India on March 17, 2020:

RoadMonkey, thanks for your appreciation. It is a great encouragement for me. Highly appreciate.

RoadMonkey on March 17, 2020:

I found difficulty with momentum at school, not from the maths but understanding the "idea" or "concept" of momentum. Your article explains it clearly, thanks