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### Course: UP Class 11 Physics>Unit 7

Lesson 4: Conservation of momentum

# Introduction to momentum

Momentum (P) is equal to mass (M) times velocity (v). But there are other ways to think about momentum! Force (F) is equal to the change in momentum (ΔP) over the change in time (Δt). And the change in momentum (ΔP) is also equal to the impulse (J). Impulse has the same units as momentum (kg*m/s or N*s). Created by Sal Khan.

## Want to join the conversation?

• I understand that the momentum is the same before and after the collision, but what happened to all the lost kinetic energy?
• In an elastic collision (http://en.wikipedia.org/wiki/Elastic_collision) no Energy is lost because the total initial kinetic energy of center of masses of both bodies before and after collision remains same.
But, in real life this Kinetic Energy is either converted to sound energy or heat and light energy because of friction generated during collision.
• why does initial momentum has to be equal to final momentum?
• If there is no external force acting on a system, there is no change in momentum which in turn means that the initial momentum is equal to the final momentum.
• When doing momentum problems to obtain the combined velocity, why is it that we do not use the concept of kinetic energy? We can use mv^2/2 to obtain this value, so why would it not work if when we assume v is a constant value after collision and that m is the sum of the two masses, we solve for a different value of v compared to solved with the momentum method?
• The main reason one cant apply "Work Energy Theorem (Kinetic Energy Conservation, as in this case)" on this problem, because some Kinetic Energy is stored as Potential Energy (Which is due to deformation caused by the collision at the surface of impact) thus the final Kinetic Energy is not equal to initial KE.
NOTE: If this question proclaimed an elastic collision between the objects.
Then KE(initial)=KE(final), because elastic collision means that the surfaces of the bodies AFTER (NOT DURING) the impact will reform fully to their original shapes, thus no extra PE is stored AFTER the collision.
• So do photons also have momentum?
• Yes. Their momentum (p) equals their energy (E) divided by the speed of light (c): p=E/c
You can also express that in terms of the photons wavelenth (l) and Planck's constant (h): p=h/l
• At , why do we have to calculate the COMBINED MOMENTUM before the collision?
• Momentum is conserved so the total momentum before the collision is the same as after the collision so it can give you information about what the end state is.
• What is elastic potential energy and how it is stored during collision ?
• What is difference between speed and momentum?
• Speed is how fast something is going, and only accounts for magnitude, (it does not take direction into account, only how fast). But momentum takes into account magnitude and direction. Furthermore, momentum is not only caused by velocity but also mass. An object can travel at a very high velocity but still can have a very small momentum, maybe because of its small weight. In addition, momentum can be negative (it takes direction into account) unlike speed.
• What is the unit for Momentum
• Does Sal ever go over elastic and inelastic collisions in any of his videos??
• Shortneym is wrong way wrong. Elastic is when the objects bounce.... NOT STICK. in elastic is when they do stick. if you look at the equations
Elastic's= M1(Vi)+M2(V2i)= M1(Vf)+ M2(V2f)
inelastic =M1(Vi)+M2(V2i)=(M1+M2)(VF)

It makes sense on why the the first equation is Elastic, when they bounce. you have two separate Final Velocities....in the second equation is inelastic because the objects are moving as one so just one Final Velocity
hope this helps!!!