I A EX S3: Shown below are two force diagrams for a moving cart upon which a force (from a fan unit) is acting. What will the motion of the cart be like in each case? (Increasing speed, decreasing speed, or constant speed.) Explain how you know in each case. FM-25 Unit FM Force applied to Force applied to cart by fan unit cart by fan unit S4: Push a cart so that it moves along the track, and then give it a quick firm tap with your hand to change its direction and send it back toward its starting point. If you did this in Unit EM, you would have seen from the speed-time graph that during your tap the speed of the cart decreased quickly until it stopped for just a moment, and then its speed increased quickly as it started to move back in the opposite direction. Using the ideas you have developed in the first few activities of this unit, you should now be able to explain this quick sequence of events in terms of the force the hand applied to the cart and the effect it had on the cart's motion. Explanation: Why did the cart reverse direction when it was hit by the hand?B U A 5 Summarizing Questions S1: If an object is already moving, what effect on its speed does a single force applied in the direction opposite to its motion have? If such a force continues to act, what else may happen? S2: At the beginning of this activity, you gave a moving cart a very gentle backward tap that was not strong enough to either stop it or reverse its direction. Use the pictures below to draw three force diagrams (including both force and speed arrows where appropriate) for the cart; one for a moment before the tap (for which the speed arrow is already included), one at the mid-point during the short time that you were tapping it, and a third for a moment just after your tap. Briefly explain any force and speed arrows you add. Before the tap During the tap After the tap S3: Shown below are two force diagrams for a moving cart upon which a force (from a fan unit) is acting. What will the motion of the cart be like in each case? (Increasing speed, decreasing speed, or constant speed.) Explain how you know in each case. FM-25BI UA 5 Explanation: Why did the cart reverse direction when it was hit by the hand? Describe the situation using a diagram: (Complete the force diagrams below that represent moments in time while the hand is contact with the cart; the first while it is decreasing in speed, the second at the moment it is stopped, and the third while it is increasing in speed in the opposite direction. On each diagram, draw a motion half-arrow as appropriate and a labeled force arrow representing the interaction with the hand.) Decreasing speed Stopped momentarily Increasing speed Write the narrative: (You will need to break your narrative into two parts. First, explain why the cart slows down and stops. Then explain why it starts moving again and speeds up. Remember to use ideas about forces, not energy.) FM-26 H Activity 3: Pushes and Slowing Down