Question

Activity 4: Explaining Phenomena Using Force Ideas You are the expert witness You will need Low-friction cart and track Small block (to represent a case) A passenger is suing a bus company for injuries she claims were sustained when the bus had to brake sharply to avoid hitting an obstruction in the road. The passenger was seated in the row of seats behind the luggage rack. She claims that when the driver braked sharply, this caused a case to fly backwards off a luggage rack in front of her and hit her head. As an expert in Newton's Laws, you have been called to testify whether this story is credible or not. As part of your testimony, you will do a live demonstration in the courtroom, but you need to practice it and explain the results before you 'take the stand' Use the small block to represent the case and the top of the cart to represent the luggage rack in the bus. Devise a way to demonstrate what happens to the case when it is in the middle of the luggage rack at the moment the bus stops suddenly Describe how you will conduct your courtroom demonstration. When you perform your demonstration, what happens to the block DURING the very short period of time that the cart is coming to a stop? Does this support or refute the passenger's claim? It is possible that someone in the court will ask you some questions about your demonstration similar to those below. Use ideas that connect force and motion in your responses. ric When an object is already moving, what is needed to make it slow quickly and stop? When you have two objects moving together (like the cart and the block), what would happen if a strong force acted on one of them in the opposite direction to its motion, but no such force acted on the other? Did a strong force act on the cart to slow and stop it quickly? If so, hovw was that force exerted on the cart? During the very short period of time while the cart was stopping suddenly, did a strong force also act on the block to slow and stop it? If so, what object exerted that force on the block? If no such force acted on the block, what would happen to its speed? Activity 4: Explaining Phenomena Using Force Ideas Lawyers sometimes try to clarify/summarize a witness's statements for the benefit of the jury. Suppose two lawyers said the following to you in court. Lawyer 1: "So what you are saying is that a backward' force acts on the cart so it stops moving, but as it does so a forward' force also acts on the block, so it is pushed to the front of the cart." Lawyer 2: "I disagree with my colleague. I think that what you are saying is that a 'backward' force acts on the cart, but no force acts on the block, so it just keeps moving foruard at the same speed while the cart stops." Which lawyer do you agree with, or do you agree with neither of them? Finally, the block does stop when it reaches the front lip of the already stopped cart. Why is it that the block now stops, rather than stopping at the same time as the cart? Finally, you should write a scientific explanation for why the case would slide forward, not backward, when the bus stopped suddenly. Copies of this explanation will be given to the jury who must decide the case. First think about the case and the bus separately, drawing separate force diagrams for each. Then combine your ideas in the written narrative to answer the question Explanation #4: Why would the case slide toward the front of the bus when the bus is stopped suddenly? Represent the situation using a diagram: (On the next page, draw two separate force diagrams, one for the case and one for the bus, for the same moment in time while the bus is in the process of stopping but before the sase hit anything in fro nt of it) CF-43 Unit CF Case: Bus: Write the narrative: (Use general ideas about the connection between the forces acting on an object and its motion to explain why the bus and the case behave as they do.) Suggest some circumstance under which the passenger's claim might be credible. Briefly explain why Participate in a class discussion about all these explanations CF-44

Answer 1

Case Study -

Take a cart and block. Place block on the cart. Start moving cart with your hand and suddenly stops the cart after moving some distance. You will observe that block will continue moving forward for some distance and will stop.

Compare it with Bus luggage system. This clearly demonstrate that women's claim was false.

Let's explore what's happening -

When the bus suddenly stops, the lower portion of our luggage comes to rest with the bus whereas upper part continues to be in motion due to inertia of motion. So it will move forward eventually. But as it start motion, kinetic force of friction start acting on it in backward direction until it stops due to relative motion between bus and luggage surface. Hence it will stop after moving some distance forward.

As for force on bus or cart , By Newton's third law; Forces between two bodies acts in pair( equal and opposite indirection). So there is force in forward direction on bus or cart due to friction. But this force is significantly low to cause any noticeable effect on motion of bus or cart.