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Laymens definition of momentum

  1. Nov 21, 2003 #1
    Hi everyone I am new to the board and I am glad I found it.

    I am having a hard time defining momentum in laymens terms to someone that has no clue about physics. I have tried and tried and can't. I can give examples but not a definition. Anyone have any suggestions or answers.

  2. jcsd
  3. Nov 21, 2003 #2
    Maybe I should shed some light as to why i need to do this.

    Then again I maybe trying to explain it incorrectly by using momentum.

    Ex. Why is it hard for a person to change directions the faster he runs. For instance lets say a running back is carring the ball. at slower speeds he can change his direction of travel a greater angle than if he was at full velocity.

    Any takers?

  4. Nov 21, 2003 #3
    Part of the reason is that the distance between a swift ballcarrier and tacklers closes faster, so there is less time in which to make a change. Also, changing direction at a larger angle must also be harder to do (more exertion). But a lighter quarterback can do it easier. Obviously, training and good condition are necessary to make it work.

    Momentum might be exhibited better by considering a bigger guy and a smaller guy colliding on the field, or maybe a faster guy and a slower guy?
  5. Nov 21, 2003 #4


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    Welcome to the Forums, marine! Allways an honor to have someone from the armed forces stop in.

    "Momentum" is indeed the correct term. Or at least, a correct term. "Inertia" might be a better way to say it, though.

    A short, easy-to-remember definition would be "resistance to acceleration". The other determining factor, besides the speed at which you want a thing to accelerate, is the mass of the thing. This is just to say that the running back could change directions more easily if he's going slower and maintians a constant mass, but also he could change directions more easily if he keeps the speed and looses some mass.

    Since your example delt directly with how speed increases momentum, let's focus on that. Now I'll warn you, I'm a big freak for relativity, so I'm going to explain this in relativistic terms, I hope your friend finds this sort of explanation helpfull.

    From a relativistic perspective, it is useful to view a situation from different points of view, or "reference frames". When we say that the running back is "moving fast", we are using the ground underneath him as our reference frame. It is holding still, and he is moving. But for a moment, let's switch reference frames. If we follow the running back with a camera that hangs from a cable above the field and travels along with him, matching his speed precisely and looking straight down at him, then we would say that he is holding still, and the field under his feet is "scrolling by".

    From this new viewpoint, if the ball carrier makes a 90o turn to the right (but the camera keeps going straight), we will see that he, who was holding stationery, has now accelerated toward the bottom of our screen, and slightly to the right. In fact, once he has completed the turn, his progress toward the bottom of the screen is at exactly the same speed as the field underneath him. So, if the field underneath him was scrolling by slowly, and he accelerates to match speed with it, he does not need to accelerate very much. However, if it is scrolling by very quickly, he must accelerate more to match up with it. This means that, assuming he uses the same amount of muscle and achieves the same rate of acceleration, it will take him a longer amount of time to get up to the speed at which the field is moving by.

    Keep in mind that the time it takes him to match speed with the field (as its scrolls by toward the bottom of our screen) is the amount of time it takes him to complete his 90o turn.

    Does that help any?
  6. Nov 22, 2003 #5

    Doc Al

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    Staff: Mentor

    Any change in momentum or velocity requires a force. The quicker you want to change it, the greater the force you need. (If you don't care how slowly you change velocity, any force will do!)

    To change direction, you apply that force sideways. The faster you are going, the more force you need if you want to make a sharp turn. Think of a car going around a curve. At 20 mph, no problem; At 100 mph, lots of luck! Same with the running back. To turn sharply, he's really got to dig into the ground.

    It's the same with slowing down. At 20 mph, no problem; at 100 mph you really have to jam on the brakes! That running back needs to change speed and direction fast; not easy.
  7. Nov 22, 2003 #6


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    In fact, the simplest way to think of force itself is to define force as "rate of change of momentum".

    Momentum is a vector quantity so changing direction is a change in momentum. Greater weight (i.e. greater mass) means greater momentum so requires greater force to change.
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