In Experiment II, you explored the speed needed to keep an object moving in a ci
ID: 1426318 • Letter: I
Question
In Experiment II, you explored the speed needed to keep an object moving in a circle. Based on Newton’s II law for circular motion, would this speed need to increase or decrease if the hanging mass M were increased? Unit 7 Post-lab- 2 2. Suppose one of your partners missed this week’s lab. Explain what you found in Experiment III, and how this related to Newton’s Laws, with enough detail that your partner could understand what he missed.
A child is swinging back and forth on a tire swing that is attached to a tree branch by a single rope.The child swings from right to left and has a speed of 2 m/s at a position just to the right of thelowest position, and the same speed of 2 m/s at a position just to the right of the lowest position.Three students are discussing the tension in the rope at the bottom of the swing (the lowestposition).
Ayliah: At the bottom of the swing, the child will be moving exactly horizontally.Since she is not moving vertically at that instant, the vertical forces cancel.The tension in the rope at that instead equals the weight.
Blaise: Just looking at the velocity vectors, the change in velocity points upwardbetween A and B. So that is the direction of the acceleration, and also ofthe net force. To get a net force pointing upward, the tension would have to
be a greater than the weight.
Conrad: But there aren’t just two forces acting on her at the bottom of the swing.Since she’s moving in a circle, there’s also the centripetal force, which actstoward the center of the circle. Since both the tension and the centripetalforce point upward, and the weight points downward, to get zero net forcethe tension actually has to be less than the weight. The tension plus the
centripetal force equals the weight.
Which, if any, of these students do you agree with?Ayliah: _____ Blaise: _____ Conrad: _____ None of them: _____Explain your reasoning.
Explanation / Answer
In Experiment II, you explored the speed needed to keep an object moving in a circle. Based on Newton’s II law for circular motion, would this speed need to increase or decrease if the hanging mass M were increased?
speed needed to keep an object moving in a circle v = r2/g
This speed is independent of mass, so speed does not need to be changed if hanging mass M were increased.
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A child is swinging back and forth on a tire swing that is attached to a tree branch by a single rope.
I agree with Conrad. Reason being had the swing been in a stable (rest) position, the weight will equal to the tension in the rope. But since swing is moving to and fro in a circular motion, the centripetal force also balances the weight of the child. Hence
Forces acting upwards = Forces acting downwards
Centripetal force + Tension in string = Weight of child
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