This week we covered  Conservation of Momentum.  I started by asking the students what momentum means…. does an object at rest have momentum?  No,  it has motion. If its going faster, it has more momentum, so momentum depends on speed and direction is important so its a vector.  I then asked which has more momentum, a semitruck or a smart car traveling at the same velocities – they correctly answered the semitruck. So momentum depends on mass, the greater the mass, the greater the momentum.  That’s pretty much it.  Momentum, p = mass x velocity.

p = mv

The nice thing about momentum, is that, like energy and mass, it is conserved in a closed system.  If you have a cannon sitting on the grass with a shell inside, its total momentum is zero since both the cannon and the shell are at rest. Once you fire the cannon the shell will be moving at a large velocity away from the cannon and will therefore have momentum.  Since the total momentum of the system (the cannon and shell) must still be zero, the cannon must have momentum equal and opposite to the shell’s momentum.  Obviously the cannon has a much greater mass than the shell so it will only need a small recoil velocity to get a considerable momentum.

mc  vc  = ms vs

large # x small # = small # x large #

For the lab, we played with the airtrack and gliders. If initially one glider is at rest and hit by a second glider of the same mass, the momentum is transferred to the glider that was initially at rest. If the gliders start with equal and opposite velocities and have an inelastic collision (meaning they stick together – with the help of velcro), then the total momentum before the collision was zero and has to remain zero, so the two gliders come to rest. This experiment is shown in the video below.

Students used the Video Physics app to record the motion of the gliders during a collision. They could choose any type of collision, inelastic, elastic, one cart initially at rest, or both moving, or they could change the mass of one of the carts to see what happens. The goal was to show conservation of momentum in each case, which they did. The video physics app makes it pretty simple to get the velocities of the carts.

Here are some interesting videos I found on youtube with Newton’s Cradle, the first one showing some cool tricks and the second one explaining the physics behind the toy.

Advertisements