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Circular Motion
Contributed by: Council Chief State School Officers (CCSSO)

ME126 Form

Item Description:

This exercise demonstrates three ideas about circular motion.

  1. Forces are needed to cause circular motion.
  2. When a force causing an object to move with circular motion stops, the object will move with straight line motion tangent to the circle.
  3. Objects moving straight will continue to move straight until an outside force causes them to turn.

The first part of the assessment is designed to teach the students some of the concepts involved. This section calls for students to make observations about the activities and while insight can be gained about their performance, this section will not affect the students' score.

The second part contains three questions which will be used to score this event. The first question is about an activity where students swing a ball in a plastic bag over their head and try to release the bag so that it flies to their partner. For a complete answer, the student should show the correct location where the ball is released and that the ball travels in a straight line. Either one of these is usually enough to get the student one of the points. As always though, the student should make the ideas clear.

For the second question, students should show a marble moving around the edge of a 3/4 pie plate. When the ball reaches the edge it should roll off tangent to the plate. This is the direction of its final motion on the plate. This question must be totally correct to pass criteria 1 due to its simplicity.

In question 3, students explain why we feel like we are being pushed against the car door when a car turns. When the car moves into a turn, our bodies naturally tend to continue in straight line motion. We move straight until some force acts to change our direction. When the car turns right, the left side of the car pushes us toward the new direction. Students must express the idea that the car pushes us and that we tend to go straight to get full credit.


Criteria 1:
Question 2 must demonstrate that the ball curves around the inside of the plate and then moves straight out toward the top of the page.


  2. The ball rolls around then comes straight out toward the top.

    Insufficient or unacceptable

  3. They didn't explain what happens next.


  4. Ball is moving in wrong direction.

  5. The ball comes straight out: not enough without showing us which way out.


Criteria 2:

Student clearly shows straight motion in two of the three questions.

For question 1 they must state that the ball flies straight after being released. Any mention of the ball flying circularly after release is not acceptable.

For question 2, the criteria are the same as for criteria 1.

For question 3, the student mention that the bodies tendency to go straight or forward. This may be attributed to inertia or momentum as long as the person's straight motion is mentioned.

Examples:  (Q1)

  1. You have to let go at the right point so it will fly straight to your partner.



  3. Note: Release is not correct but the ball is shown traveling straight.


Examples: (Q3)


B. Your body wants to go in a straight line.

C. The car turns but you go straight. Usually this will be satisfied by having Q2 and one of the others.


The student has the release point correct for question 1. A correct release point is anywhere in the following range.

Students may describe this as on the side or to their right (or left) as long as the ball is not behind them at all. Students may mention curved motion after release.


  1. I let the ball go at my side so it would curve around to my partner: not correct, but sufficient.


Criteria 3: Idea of straight motion is communicated in question 1 or 3 and is not contraindicated in any of the three questions. If the students mention straight motion in Q1 or Q3 and don't describe motion at all in the other, that is acceptable. However, any mention of the ball curving after being released (Q1) or the marble curbing after moving off of the plate (Q2) prevents getting this point.


Criteria 4: Student explains that the body in the car moves in a straight line forward until the car acts on it. (Q3) Eventually the person will move in the same direction as the car. Simply saying that the person moves with the car is not sufficient.


  1. "When the car turns you want to go straight, but the car forces you to turn with it."
  2. "Your body goes straight until it hits the side. Then the car pushes it in the new direction."
Both ideas are required

The student identifies a correct point and that the ball flies straight (Q1). The method for assessing these separately is given in the description for criteria 2.

  C1   Q2 correct
C2 Straight motion is correct in 2 of 3 OR
Correct point of release in Q1.
C3 Straight line in Q1 or Q3 and no contradiction of straight motion in any answer. And Q2 correct.
C4 Straight and forced to turn (Q3) OR
Straight and correct release point (Q1).


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