Conceptual Questions

  1. In uniform circular motion, which of the following are constant: speed, velocity, angular velocity, centripetal acceleration, magnitude of the net force?

  2. A car runs out of gas while driving down a hill. It rolls through the valley and starts up the other side. At the very bottom of the valley, which of the free-body diagrams in FIGURE Q8.2 is correct? The car is moving to the right, and drag and rolling friction are negligible.

    FIGURE Q8.2

    The figure illustrates a car moving on a concave path and its five probable net force vectors labeled ("a") to (f) when the car is at the lower most point.

  3. FIGURE Q8.3 is a bird’s-eye view of particles on strings moving in horizontal circles on a tabletop. All are moving at the same speed. Rank in order, from largest to smallest, the tensions TA to TD. Give your answer in the form A>B=C>D and explain your ranking.

    FIGURE Q8.3

    The figure consists of four parts labeled ("A") to (D) that depict four particles in a circular motion.

  4. Tarzan swings through the jungle on a massless vine. At the lowest point of his swing, is the tension in the vine greater than, less than, or equal to the gravitational force on Tarzan? Explain.

  5. FIGURE Q8.5 shows two balls of equal mass moving in vertical circles. Is the tension in string A greater than, less than, or equal to the tension in string B if the balls travel over the top of the circle (a) with equal speed and (b) with equal angular velocity?

    FIGURE Q8.5

    The figure illustrates a particle of mass m moving in two circular paths of different radii made by two strings labeled “A” and B.

  6. Ramon and Sally are observing a toy car speed up as it goes around a circular track. Ramon says, “The car’s speeding up, so there must be a net force parallel to the track.” “I don’t think so,” replies Sally. “It’s moving in a circle, and that requires centripetal acceleration. The net force has to point to the center of the circle.” Do you agree with Ramon, Sally, or neither? Explain.

  7. A jet plane is flying on a level course at constant speed. The engines are at full throttle.

    1. What is the net force on the plane? Explain.

    2. Draw a free-body diagram of the plane as seen from the side with the plane flying to the right. Name (don’t just label) any and all forces shown on your diagram.

    3. Airplanes bank when they turn. Draw a free-body diagram of the plane as seen from behind as it makes a right turn.

    4. Why do planes bank as they turn? Explain.

  8. A small projectile is launched parallel to the ground at height h = 1 m with sufficient speed to orbit a completely smooth, airless planet. A bug rides inside a small hole inside the projectile. Is the bug weightless? Explain.

  9. You can swing a ball on a string in a vertical circle if you swing it fast enough. But if you swing too slowly, the string goes slack as the ball nears the top. Explain why there’s a minimum speed to keep the ball moving in a circle.

  10. A golfer starts with the club over her head and swings it to reach maximum speed as it contacts the ball. Halfway through her swing, when the golf club is parallel to the ground, does the acceleration vector of the club head point (a) straight down, (b) parallel to the ground, approximately toward the golfer’s shoulders, (c) approximately toward the golfer’s feet, or (d) toward a point above the golfer’s head? Explain.