Forest PHYS100 Demos Week7
Pasco Carts:
For this demonstration we will have a light cart and a heavy cart with springs attached to them. I will begin with a football player analogy. A light football player will have a hard time moving a heavy football. I will ask the students why this is. The light football player has less momentum and therefore cannot transfer enough momentum to move the heavier cart that much. The lighter cart will hit the heavy cart and probably bounce back. On the other hand the more massive cart has more momentum that it can transfer, so the heavy cart will run into the lighter cart and move it more. The heavy cart will probably keep going because it has enough momentum to transfer to the light cart and still keep its velocity.
Astro Blasters:
I will use two equal mass balls (with the ball fixed to the post at the bottom) and ask the students what they expect the motion of the balls to be. In this case the balls will transfer momentum, but since the balls are of equal mass, the momentum transfer will be less than if we have the full set up of astroblaster balls on the peg. Once the balls hit the ground, the momentum of the system switches directions and will launch the uppermost ball into the air. If I do this with the full astroblaster ball set up, what will be the motion then? The light ball on the top will receive momentum from the balls under it as the momentum of the system switches directions and the light ball with fly high into the air.
Tennis Ball Cannon:
I will ask the students what they expect the motion of the system to be. What quantities are conserved? The answer is that the cannon will deliver an impulse to the tennis ball which will accelerate the tennis ball forward and conserve momentum. Similarly the impulse delivered by the cannon onto the ball will also be delivered to the cannon. The cannon will recoil, but it won’t travel as fast as the tennis ball. Why is this? This is because the mass of the ball is less, so to conserve momentum; it must have a higher velocity. The tennis ball cannon is much more massive than the tennis ball, so it will accelerate to a lower velocity to match the momentum of the tennis ball.
Happy/Sad Ball:
I will demonstrate to the students the difference in the elastic properties between the happy ball and the sad ball by dropping them on the table. The happy ball will bounce back up and the sad ball will barely bounce at all. I will ask the students which ball they expect to knock over a piece of wood when launched off a small ramp. The happy ball will knock over the block of wood. To understand this we must look at the impulse delivered to the block of wood by each ball. For the happy ball we can write the impulse as J = mv + m(-v) = 2mv. For the sad ball J = mv – m(0). So the impulse delivered to the block of wood for the happy ball is twice that of the sad ball. Therefore the happy ball will bounce back and knock over the block of wood, while the sad ball will simply hit the block of wood and fall to the ground without knocking over the wood.
Pasco Experiment:
For this experiment we will set up a ramp with a ball on it connected to a PASCO track. The ball will roll down the ramp and hit a cart on the pasco track that will be connected to a spring. I will ask the students to calculate the initial velocity of the cart after the ball hits the cart. To do this we must conserve energy as the ball rolls down the ramp to find the velocity of the ball right before it hits the cart. Then we must conserve momentum to find the initial velocity of the cart. I will only give the students a meter stick and a scale to measure the mass of the ball and the cart. I will also give them the rotational portion of the ball’s energy.