At the bottom of the swing, the child will be moving exactly horizontally. Since

Question

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 upward between A and B. So that is the direction of the acceleration, and also of the net f orce. To get a net f orce 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 acts toward the center of the circle. Since both the tension and the centripetal force point upward, and the weight points downward, to get zero net force the 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. A ball with a weight of 2 N is attached to the end of a cord of length 2 m. The ball is whirled in a vertical circle counterclockwise. The tension in the cord at the top of the circle is 7 N and at the bottom it is 15 N. [Do not assume that the speed of the ball is the same at these points.) Three students discuss the net forces on the ball at the top: Deante: The net force on the ball at the top position is 7 N since the net force is the same as the tension. Edgar: The net force on the ball at the top position is 9 N. Both the tension and the weight are acting downward so you have to add them. Fiona: No, you are both wrong. You need to figure out the centripetal force (mv2/r) and include it in the net force. Which, if any, of these students do you agree with? Deante: Edgar: Fiona: None of them: Explain your reasoning. Now the students discuss the net force at the bottom. Deante: The net force on the ball at the bottom position is 15 N since the net force is the same as the tension. Edgar: The net force on the ball at the bottom position is 17 N, since you need to add the weight of 2 N to the tension of 15 N. Fiona: The net force on the ball at the bottom position is 13 N. I agree that you need to take into account both the weight and the tension but they are in different directions so they will subtract. Which, if any, of these students do you agree with? Deante: Edgar: Fiona: None of them: Explain your reasoning.

Explanation / Answer

3)
ayliah = wrong T = m*v*v/r + mg

blaise = correct

conrad = wrong

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