Forest PHYS100 Demos Week4
Balloon Cart:
For this demonstration I will push the cart and the students will observe the system. At this point I will ask students to write down what they think is happening to the balloon and the cart during each step in terms of Newton’s Laws.
Step 1: The cart and the balloon are both stationary.
Step 2: I push the cart and the cart starts to move, but the balloon remains stationary.
Step 3: The cart and the balloon are moving at a constant velocity.
Step 4: The balloon is stopped, but the cart keeps moving.
Step 5: The cart comes to a stop.
The simple answers for most of these are that there is an applied force that makes the cart start and stop.
Toilet Paper Roll:
For this demonstration I will ask the students what they expect will happen when both rolls of toilet paper (full roll and an almost empty roll) will behave if I quickly pull a piece off. The full roll will allow the piece to be torn off with little to no rotation, while the almost empty roll will spin and unravel. The reason for this is that the larger roll has more inertia and is more resistant to a change in motion. The almost empty roll will spin because it has less inertia.
Table Cloth Tug:
Since most people have already seen this demonstration, the result will be well known. I will ask the students why in terms of Newton’s First Law the table cloth is able to be removed with the dining arrangement left unchanged.
Atwood’s Elevator with Scales:
For this demonstration I will write down Newton’s Second Law and ask the students to come up with a reason as to why mathematically it feels as if your weight is increasing as an elevator goes up, and why it feels like your weight decreases as the elevator descends. The reason is that for an elevator going upward, the force must be greater than that of your body weight in order to lift you, so the total force felt by you is F = ma+mg, so the force you feel is greater than your body weight (as it must be) to accelerate upward. Similarly for when the elevator is descending we have F = mg – ma, so the force you feel is less than your weight, mg. I will then do the demonstration to show the students that this is indeed the case.
Two Carts on the Same Spring with Different Masses:
For this demonstration I will ask the students what the behavior of the system will be in terms of the acceleration of each cart. I will let them stress about this one for a bit if no one comes up with an answer. The heavier cart will accelerate more slowly than the lighter cart because we have the same force on each cart. Since Ma = F = ma’, we can conclude that the acceleration must be less for the left hand side of the equation since both ma’ and Ma must equal the same force. The lighter cart will accelerate more quickly.
Carts Pushing Against Each Other:
For this demonstration I will state Newton’s Third Law. Bryce and I will both sit on the carts and ask the students what will happen if I push Bryce away. Then I will show them the motion of the carts. After this I will ask them what would happen if Bryce pushed me away. I will show them the motion of the carts in this scenario. Finally both Bryce and I will push off each other and the distance that we travel will be greater than if only one of us pushed, why is this? The reason is that both Bryce and I are now applying a force, so the opposite force is added for both pushing forces.
Bridge Building:
For this demonstration I will ask the students how I should build a bridge in terms of Newton’s Third Law. I will ask for some volunteers to place the wooden blocks as they see fit. The bridge is stable because we can balance the force of gravity making sure that the vital points of the bridge that have a large acceleration due to gravity are cancelled out somewhere else on the bridge. We can make sure that we cancel these forces so that we have a static situation. It may be a little tough to do this demonstration without the concept of torque, but I can provide a brief free body diagram that shows the forces (torques technically) sum to zero as well as the forces.