Conceptual Questions

  1. You find yourself in the middle of a frozen lake with a surface so slippery (μs=μk=0) you cannot walk. However, you happen to have several rocks in your pocket. The ice is extremely hard. It cannot be chipped, and the rocks slip on it just as much as your feet do. Can you think of a way to get to shore? Use pictures, forces, and Newton’s laws to explain your reasoning.

  2. How does a sprinter sprint? What is the forward force on a sprinter as she accelerates? Where does that force come from? Your explanation should include an interaction diagram and a free-body diagram.

  3. How does a rocket take off? What is the upward force on it? Your explanation should include an interaction diagram and free-body diagrams of the rocket and of the parcel of gas being exhausted.

  4. How do basketball players jump straight up into the air? Your explanation should include an interaction diagram and a free-body diagram.

  5. A mosquito collides head-on with a car traveling 60 mph. Is the force of the mosquito on the car larger than, smaller than, or equal to the force of the car on the mosquito? Explain.

  6. A mosquito collides head-on with a car traveling 60 mph. Is the magnitude of the mosquito’s acceleration larger than, smaller than, or equal to the magnitude of the car’s acceleration? Explain.

  7. A small car is pushing a large truck. They are speeding up. Is the force of the truck on the car larger than, smaller than, or equal to the force of the car on the truck?

  8. A very smart 3-year-old child is given a wagon for her birthday. She refuses to use it. “After all,” she says, “Newton’s third law says that no matter how hard I pull, the wagon will exert an equal but opposite force on me. So I will never be able to get it to move forward.” What would you say to her in reply?

  9. Teams red and blue are having a tug-of-war. According to Newton’s third law, the force with which the red team pulls on the blue team exactly equals the force with which the blue team pulls on the red team. How can one team ever win? Explain.

  10. Will hanging a magnet in front of the iron cart in FIGURE Q7.10 make it go? Explain.

    FIGURE Q7.10

    The figure illustrates an iron cart carrying a horseshoe magnet in the front. The north and south poles face the cart.

  11. FIGURE Q7.11 shows two masses at rest. The string is massless and the pulley is frictionless. The spring scale reads in kg. What is the reading of the scale?

    FIGURE Q7.11

    The figure illustrates two 5 kilograms masses hanging by the ends of a rope. The rope moves over a pulley. A spring balance is attached above the 5 kilograms mass on the right.

  12. FIGURE Q7.12 shows two masses at rest. The string is massless and the pulleys are frictionless. The spring scale reads in kg. What is the reading of the scale?

    FIGURE Q7.12

    The figure illustrates two 5 kilograms masses hanging by the ends of a rope. The rope moves over two pulleys attached to the corners of a horizontal surface. A spring balance is attached to the rope between the pulleys.

  13. The hand in FIGURE Q7.13 is pushing on the back of block A. Blocks A and B, with mB>mA, are connected by a massless string and slide on a frictionless surface. Is the force of the string on B larger than, smaller than, or equal to the force of the hand on A? Explain.

    FIGURE Q7.13

    The figure illustrates two blocks on a horizontal surface connected to each other by a string.

  14. Blocks A and B in FIGURE Q7.14 are connected by a massless string over a massless, frictionless pulley. The blocks have just been released from rest. Will the pulley rotate clockwise, counterclockwise, or not at all? Explain.

    FIGURE Q7.14

    The figure illustrates two 1 kilogram masses hanging by the ends of a rope. The rope moves over a pulley. The mass on the left is labeled "A" and the mass on the right is labeled B. B hangs lower than “A”.

  15. In case a in FIGURE Q7.15, block A is accelerated across a frictionless table by a hanging 10 N weight (1.02 kg). In case b, block A is accelerated across a frictionless table by a steady 10 N tension in the string. The string is massless, and the pulley is massless and frictionless. Is A’s acceleration in case b greater than, less than, or equal to its acceleration in case a? Explain.

    FIGURE Q7.15

    The figure illustrates two cases labeled case "a" and case b. In both the cases a mass labeled "A" is connected to a rope that moves over a pulley.