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Formative assessment questions for teaching physics

A2L Item 017

  • Description: Identify the corrrect acceleration graph for a rolling ball kicked back up an incline
  • Goal: Relating physical understanding of an object's behavior to a graphical representation of acceleration.
  • Source: UMPERG
  • Keywords: Mechanics, Graphing, Acceleration

The question for students:

A soccer ball rolls across the road and down a hill as shown below. At the bottom of the hill the ball is given a quick kick so that the ball goes back up the hill and across the road. The initial and final speed of the ball is the same.

Item017_fig1.gif

Which of the following sketches of ax vs. t is a reasonable representation of the horizontal acceleration of the ball as a function of time for period of time shown?

Item017_fig2.gif

Commentary for teachers:

Answer

(2) The acceleration of the ball while on the slope is the same whether it is going down or going up. Also, taking the positive direction to the right, the kick would appear as a negative spike in the acceleration.

Background

This item is related to item 1. See comments there. Students need to note that the plots are for the xcomponent of the acceleration.

Questions to Reveal Student Reasoning

How is the acceleration related to the velocity? Suppose the hill were more inclined. What feature of the acceleration vs. time graph would change? What is the direction of the velocity just before the kick? just after?

Suggestions

Have students make a graph of velocity vs. time for each of the given plots of acceleration vs. time. Have students generate a plot of the acceleration and velocity in the y direction.