Here is a quick experiment… first, find a wall. Then hit it with your bare fist. (Take it easy, just hit it with enough force that you feel the impact.) Now put a pillow in front of the wall and hit it with about the same force as you hit it before. With the pillow in front of the wall, you can hit it a little harder if you like but again, don’t go nuts!

What did the pillow do? It slowed the time of impact. Remember our formula Ft = mv. When the momentum of your moving fist struck the wall directly, the momentum was cut to zero instantly and so you felt enough force to hurt a bit. When the pillow was in the way it took longer for your momentum to come to zero. So you could hit the pillow fairly hard without feeling much force. Basically a bike helmet is like a pillow for you head. It slows the time of impact, so when you fall off your bike, there is much less force on your head. Just be glad your mom doesn’t make you wear a pillow on your head!

So let’s go back to momentum for a minute. Momentum is inertia in motion. It is how much force it takes to get something to slow down or change direction. One more concept I’d like to give you this month, is conservation of momentum. This is basically momentum equals momentum or mathematically mv = mv. (Momentum is mass times velocity.) When objects collide, the momentum that both objects have after the collision, is equal to the amount of momentum the objects had before the collision. Let’s take a look at this with this experiment.

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The video above shows you another quick experiment you can do with momentum: Put one penny on the table. Put another penny on the table about 6 inches away from the first one. Now, slide one penny fairly fast towards the first penny. What you want to have happen, is that the moving penny strikes (or gives impulse to) the stationary penny head on. The moving penny should stop and the stationary penny will move. Now, try that with other coins. Make big ones hit small ones and vice versa. It’s also fun to put a line of 5 coins all touching one another. Then strike the end of the line with a moving penny.

This is conservation of momentum. If you were able to strike the penny head on, you should have seen that the penny that was moving, stopped, and the penny that was stationary moved with about the same speed of the original moving penny. Conservation of momentum is mv = mv. Once the moving penny struck the other, all the moving penny’s momentum transferred to the second penny. Since the pennies weighed the same, the v (velocity) of the first penny is transferred to the second penny and the second penny moves with the same velocity as the first penny. What happens if you use a quarter and a penny? Make the quarter strike the penny. That penny should really zip! Again mv = mv. The mass of the quarter is much greater then the mass of the penny. So for momentum to be conserved, after impact, the penny had to have a much greater velocity to compensate for its lower mass.

Mathematically it would look like this… after collision:
(Mass of Quarter) x (Velocity of Quarter) = (Mass of Penny) x (Velocity of Penny)

5g x 10m/s = 1g x v
50 = 1 x v
50/1 = v
50m/s = v
or 5g x 10m/s = 1g x 50 m/s
50 momentum = 50 momentum

After the collision, the penny is moving at 50 m/s, 5 times faster then the quarter was moving because the penny is 5 times lighter then the quarter.

Click here to go to next lesson on Wagons.

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