Laymens definition of momentum

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?

 

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 90^o 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 90^o turn.

Does that help any?

 

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.