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Introduction to Conservation of Momentum with Demonstrations


– Good morning. Today we are going to use a skateboard to introduce the concept of
conservation of momentum. ♫ Flipping Physics ♫ Bo, what is the equation for momentum? – Momentum equals mass times velocity, and both momentum and
velocity are vectors. – Conservation of momentum is
true in all isolated systems. In physics, a system is
isolated when the net force on the system is equal to zero. In other words, if you
add up all the forces acting on all the objects in the system, they add up to zero. And we know net force
equals change in momentum over change in time, where both force and momentum are vectors. And multiplying through by
change and time gives us… the change in momentum
of the system equals zero and delta or change in always
equals final minus initial. And we can add the initial
momentum of the system to both sides to get… the initial momentum of the system equals the final momentum of the system. This concept is called
conservation of momentum. The equation is usually written the sum of the initial momenta of the system is equal to sum of the final momenta of the system, where momentum, of course, is a vector. And in an algebra based physics class like this one, we usually summarize that conservation of momentum is true during all collisions and explosions. I will now use a skateboard
and a medicine ball to demonstrate conservation of momentum. (chuckles) Now I know this is not
what you typically think of as an explosion, however,
it fits the physics definition of explosion, and therefore momentum is conserved. Billy, what is the initial
momentum of the system? In other words, what is
the momentum of the system right before the explosion. – Well your initial velocity is zero, so your initial momentum is zero. The initial velocity of
the ball is also zero, so the initial momentum
of the ball is zero. Therefore, the total momentum
of the system initial, or before the explosion, is zero. – [Mr.P] Correct. And Bobby, is the momentum of the ball after the explosion positive or negative? – After the explosion, the
ball is moving to the left, therefore the ball has negative momentum. – Therefore, because the
final momentum of the ball is negative, in order for
the total final momentum to add up to zero, my final momentum after the explosion has to be positive. In other words, I have
to move to the right. This is exactly how a rocket works. In the bottom of a rocket,
you have a bunch of people throwing medicine balls out the bottom. – That’s not funny.
– That’s not right, Mr. P. – [Mr. P.] Okay, you’re right. That’s now really how a rocket works. – Bo, what is that on your desk? – This is my rooster. – It’s stuffed animal day. (animal noises) – My rooster is stuffed. – I brought Bill the Cat. – Of course it’s a cat.
– Okay. – In a rocket, the rocket fuel
is ejected quickly downward, and therefore the rocket moves upward. We can illustrate this
using a hose spray nozzle. Again, the water is
ejected quickly downward. This means the water
has negative momentum. Therefore, because momentum is conserved, the spray nozzle gains
positive momentum upward. Thank you very much for
learning with me today. I enjoyed learning with you. (water spraying)
– [Geneve] Ahhh!

19 Comments

  1. Santiago
    Santiago September 29, 2016

    great work!

  2. Koregan
    Koregan September 29, 2016

    what a nice video 🙂

  3. Josh Young
    Josh Young October 4, 2016

    I found your channel because I was wondering. If an object (let's say a dirt bike) moving forward and jumps a ramp. Is it easier to front flip or back flip? My friend and I are divided I think its front because you are already moving in that direction. Can you help us out here?

  4. Ya Feel?
    Ya Feel? October 8, 2016

    great video as always mr.p keep it up!!

  5. s mishra
    s mishra October 24, 2016

    Great Video! Thank you for explaining everything thoroughly! My physics teacher uses a lot of your videos to help us learn!

  6. Bolivianitas Bolivia
    Bolivianitas Bolivia November 6, 2016

    love your videas they are very ilustrative and helpfull to understand the pbysics concepts and love the 3 guys,but a have a request SUBTITTLES cause i feel u speak to fast……u are a good teacher !!!! greeting from Bolivia

  7. Flipping Physics
    Flipping Physics December 26, 2016

    Could the person who submitted the Italian translation of the closed captions of this video please let me know who you are? I would enjoy thanking you and giving you credit for your work. [email protected]

  8. Skyler Andrews
    Skyler Andrews March 6, 2017

    i learned so much thank you god !!

  9. Skyler Andrews
    Skyler Andrews March 6, 2017

    does this mean that water stays in the toilet because of newton?

  10. alex mercer
    alex mercer March 11, 2017

    Thanks man you really saved me cause I need to make a story depicting conservation of momentum

  11. Gus Tetzloff
    Gus Tetzloff March 29, 2017

    That medicine ball rocket joke was hilarious, definitely needed some dry humor like that while I'm studying

  12. Eula Abucayon
    Eula Abucayon May 11, 2017

    Thank you!!! Excellent and so easy to understand. Your vids are helping me for my college entrance test review and its easier to understand than these reviewers.

  13. Dyllan Hunt
    Dyllan Hunt November 29, 2017

    Just wondering, but say the camera was behind you when you threw the medicine ball while on the skateboard. would the mean that sense the ball would be moving to the right due to you facing away from the camera that the ball now has positive momentum?

  14. Nasir Khalid
    Nasir Khalid May 4, 2018

    I thought these are all examples of Newton third law or is it just another way to explain it ? BTW Great video as always.

  15. Christine LaBeach
    Christine LaBeach September 3, 2018

    So the two systems cancel each other out?

  16. Fahad Alawadi
    Fahad Alawadi October 13, 2018

    I normally use Newton's third law to derive and demonstrate the idea of conservation of momentum of a system of bodies (such as collisions), because it is an interaction between two bodies. Newton's first and second laws are normally concerned with the forces acting on a single object rather than on a system of bodies.

  17. BRUCE JEANDUC
    BRUCE JEANDUC October 18, 2018

    Which has more momentum?sitting jet plane? Or crawling baby? and why?

  18. Kaylah B.
    Kaylah B. May 18, 2019

    His hair is beautiful!

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