Objectives:
– To show coherent kinetic energy get converted to incoherent kinetic energy
– To give intuition for dissipation as a process
Apparatus:
- A PASCO Smart Cart
- PASCO level track with Velcro end stop
- BB/airsoft pellets
- phone camera with slow motion feature
Method:
Put about 20 airsoft/bb pellets in the top “trunk” space of the Smart Cart . Place the cart on the track, with the Velcro end of the Smart Cart pointed towards the Velcro end of the end stop, at some distance. Give the car a push. Observe what happens when the car hits the end stop. The kinetic energy of the car is partially transferred to the kinetic energy of the pellets.
However, the kinetic energy of the pellets is not coherent – they are not all moving at the same speed in the same direction – and is quickly further dissipated via inelastic collisions with one another and with the walls of the Smart Cart trunk. The point to make here is that while it is in some sense “easy” to convert the motion of the car into useful work, doing so with the motion of the airsoft pellets is substantially more difficult
OPTIONAL:
The conservation of momentum and of energy can be quantitatively explored using PASCO Capstone. This is best done by making a single velocity vs. time graph and plotting the velocities of both the red and blue carts on the same graph. Note that the position sensors on the carts are directional, so if you are going to collide magnet-to-magnet or Velcro-to-Velcro, one of the position sensors has to have its sign changed (go to Hardware Setup, pick settings for one of the position sensors, and check “Switch Sign”)
Notes:
– Cover the screw holes in the with a small piece of green tape to avoid pellets getting stuck there during the demo.
– The success of this demonstration depends greatly on the track itself being level, both in the direction of motion and perpendicular to it, so it shall be the responsibility of the technician to ensure a level, stable track.
– The behaviour of the pellets is hard to see live, so it is better to take a slow motion video of the process and make observations of the motion of the movement of the pellets in the video.
– Don’t push the car too hard to start.
– Excessive initial kinetic energy can cause the pellets to spill out upon collision.