1. Why does a child in a wagon seem to fall backward when you give the wagon a sharp pull?
  1. If the acceleration of a body is zero, are no force acting on it?
  1. Why do you push harder on the pedals of a bicycle when first starting out than when moving with a constant speed?
  1. Only one force acts on an object. Can the object have zero acceleration? Can it have zero velocity?
  1. When a golf ball is dropped to the pavement, it bounces back up. (a) Is a force needed to make it bounce back up? (b) If so, What exerts the force?
  1. Why might your foot hurt if you kick a heavy desk or a wall?
  1. When you are running and want to stop quickly, you must decelerate quickly. (a) What is the origin of the force that causes you to stop? (b) Estimate (using your own experience) the maximum rate of deceleration of a person running at top speed to come to rest.
  1. The force of gravity on a 2 kg rock is twice as great as that on a 1 kg rock. Why then doesn’t the heavier rock fall faster?
  1. Compare the effort (or force) needed to lift a 10 kg object when you are on the moon as compared to lifting it on the Earth. Compare the force needed to throw a 2 kg object horizontally with a given speed when on the moon as compared to on the Earth.
  1. Whiplash sometimes results from an automobile accident when the victim’s car is struck violently from the rear. Explain why the head of the victim seems to be thrown backward in this situation. Is it really?
  1. A person exerts an upward force of 40 N to hold a bag of groceries. Describe the “reaction” force (Newton’s third law) by stating (a) its magnitude, (b) its direction, (c) on what body it is exerted, and (d) by what body it is exerted.
  1. hen you stand still on the ground, how large a force does the ground exert on you? Why doesn’t this force make you rise up into the air?
  1. According to Newton’s third law, each team in tug of war pulls with equal force on the other team. What, then, determines which team will win?
  1. Why is the stopping distance of a truck much shorter than for a train going the same speed?
  1. Can a coefficient of friction exceed 1.0?
  1. A block is given a push so that it slides up a ramp. After the block reaches its highest point, it slides back down. Why is the magnitude of its acceleration less on the descent than on the ascent?
  1. A heavy crate rests on the bed of a flatbed truck. When the truck accelerates, the crate remains where it is on the truck, so it too, accelerates. What force causes the crate to accelerate?
  1. You can hold a heavy box against a rough wall and prevent it from slipping down by pressing only horizontally. How can the application of a horizontal force keep an object from moving vertically?

PROBLEMS:

  1. (I) A net force of 255 N accelerates a bike and rider at 2.20 m/s2.  What is the mass of the bike and rider?
  1. (I) How much force is required to accelerate a 9.0-g object at 10,000 “g’s” (say, in centrifuge)?
  1. (I) How much tension must a rope withstand if it is used to accelerate a 1050-kg car horizontally at 1.20 m/s2?  Ignore friction.
  1. (I) What is the weight of a 66-kg astronaut (a) on Earth, (b) on the Moon (g=1.7 m/s2), (c) on Mars (g= 3.7 m/s2), (d) in outer space traveling with constant velocity?
  1. (II) A 20.0-kg box rests on a table. (a) What is the weight of the box and the normal force acting on it? (b) A 10.0-kg box is placed on top of the 20.0-kg box, as shown in Fig. 4-35.  Determine the normal force that the table exerts on the 20.0-kg box and the normal force that the 20.0-kg box exerts on the 10.0-kg box.
  1. (II) What average force is needed to accelerate a 7.00-gram pellet from rest to 175 m/s over a distance of 0.700 m along the barrel of a rifle?
  1. (II) How much tension must a rope withstand if it is used to accelerate a 1200-kg car vertically upward at 0.80 m/s2? Ignore friction.
  1. (II) A 10-kg bucket is lowered by a rope in which there is 63 N of tension.  What is the acceleration of the bucket?  Is it up or down?
  1. (II) An elevator (mass 4850 kg) is to be designed so that the maximum acceleration is 0.0600 g.  What are the maximum and minimum forces the motor should exert on the supporting cable?
  1. (II) A person stands on a bathroom scale in a motionless elevator.  When the elevator begins to move, the scale briefly reads only 0.75 of the person’s regular weight.  Calculate the acceleration of the elevator, and find the direction of acceleration.
  1. (II) (a) What is the acceleration of two falling sky divers (mass 120.0 kg including parachute) when the upward force of air resistance is equal to one forth of their weight? (b) After popping open the parachute, the divers descend leisurely to the ground at constant speed.  What now is the force of air resistance on the skydivers and their parachute?  See Fig. 4-36.
  1. (I) A box weighing 70 N rests on a table.  A rope tied to the box runs vertically upward over a pulley and a weight is hung from the other end (Figure 4-37).  Determine the force that the table exerts on the box if the weight hanging on the other side of the pulley weighs (a) 30 N (b) 60 N, and (c) 90 N.
  1. (I) Draw the free-body diagram for a basketball player (a) just before leaving the ground on a jump, and (b) while in the air. (Fig. 4-38)
  1. (I) Sketch the free-body diagram of a baseball (a) at the moment it is hit by that bat, and again (b) after it has left the bat and is flying toward the outfield.
  1. (II) The two forces F1 and F2 shown in Fig. 4-39a and b (looking down) act on a 27.0-kg on a frictionless tabletop.  If F1 = 10.2 N and F2 16.0 N, find the net force on the object and its acceleration for each situation, (a) and (b).
  1. (II) A person pushes a 14.5-kg lawn mower at constant speed with a force of 88.0 N directed along the handle, which is at an angle of 45.0o to the horizontal (Fig. 4-40).  (a) Draw the free-body diagram showing all forces acting on the mower.  Calculate (b) the horizontal retarding force on the mower, then (c) the normal force exerted vertically upward on the mower by the ground, and (d) the force the person must exert on the lawn mower to accelerate it from rest to 1.5 m/s in 2.5 seconds (assuming the same retarding force).
  1. (II) A 6500-kg helicopter accelerates upward at 0.60 m/s2 while lifting a 1200-kg car. (a) What is the lift force exerted by the air on the rotors? (b) What is the tension in the cable (ignore its mass) that connects the car to the helicopter?
  1. (II) A window washer pulls herself upward using the bucket-pulley apparatus show in Fig. 4-42. (a) How hard must she pull downward to raise herself slowly at constant speed? (b) If she increases this force by 10 percent, what will her acceleration be?  The mass of the person plus the bucket is 65 kg.
  1. (II) A pair of fuzzy dice is hanging by a string from your rearview mirror.  While you are accelerating from a stoplight to 20 m/s (in 5.0 seconds), what angle q does the string make with the vertical?  See Fig. 4-43.
  1. (III) The two masses shown in Fig. 4-45 are each initially 1.80 m above the ground, and massless frictionless pulley is 4.8 m above the ground.  What maximum height does the lighter object reach after the system is released? [Hint: First determine the acceleration of the lighter mass and then its velocity at the moment the heavier on hits the ground.  This is its “launch” speed]
  1. (I) If the coefficient of kinetic friction between a 35-kg crate and the floor is 0.30, what horizontal force is required to move the crate at a steady speed across the floor?  What horizontal force is required if m k is zero?
  1. (I) A force of 40.0 N is required to start a 5.0-kg box moving across a horizontal concrete floor. (a) What is the coefficient of static friction between the box and the floor? (b) If the 40.0-N force continues, the box accelerates at 0.70 m/s2.  What is the coefficient of kinetic friction?
  1. (I) (a) A box sits at rest on a rough 30o inclined plane.  Draw the free-body diagram, showing all the forces acting on the box. (b) How would the diagram change if the box were sliding down the plane? (c) How would it change if the box were sliding up the plane after an initial shove?
  1. (II) A 2.0-kg silverware drawer does not slide readily.  The owner gradually pulls with more and more force.  When the applied force reaches 8.0 N, the drawer suddenly opens, throwing all the utensils to the floor.  Find the coefficient of static friction between the drawer and the cabinet.
  1. (II) A box is given a push so that it slides across the floor.  How far will it go, given that the coefficient of kinetic friction is 0.20 and the push imparts an initial speed of 4.0 m/s?
  1. (II) Two crates, of mass 75 kg and 110 kg, are in contact and at rest on a horizontal surface (Fig. 4-47). A 730-N force is exerted on the 75-kg crate.  If the coefficient of kinetic friction is 0.15, calculate (a) the acceleration on the system, and (b) the force that each crate exerts on the other.
  1. (III) A child slides down a slide with a 28o incline, and at the bottom her speed is precisely half what it would have been if the slide had been frictionless.  Calculate the coefficient of kinetic friction between the slide and the child.
  1. (II) A wet bar of soap (mass = 150 grams) slides without friction down a ramp 2.0m long inclined at 7.3o.  How long does it take to reach the bottom?  Neglect friction.   How would this change if the soap’s mass were 250 grams?
  1. (II) The block shown in Fig. 4-48 lies on a smooth plane tilted at an angle q = 22.0o to the horizontal. (a) Determine the acceleration of the block as it slides down the plane. (b) If the block starts from rest 9.10m up the plane from its base, what will be the block’s speed when it reaches the bottom of the incline?  Ignore friction.
  1. (II) A block is given an initial speed of 3.0 m/s up the 22.0o plane shown in Fig. 4-48. (a) How far up the plane will it go? Ignore friction.  (b) How much time elapses before it returns to its starting point?  Ignore friction. (c) Repeat part (a) but assume a coefficient of friction of 0.08.
  1. (II) An 18.0-kg box is released on a 37.0o incline and accelerates down the incline at 0.270 m/s2.  Find the friction force impeding its motion.  How large is the coefficient of friction?
  1. (II) Figaro the cat (5.0 kg) is hanging on the tablecloth, pulling Cleo’s fishbowl (11 kg) toward that edge of the table (Fig. 4-50).  The coefficient of kinetic friction between the tablecloth (ignore its mass) under the fishbowl and the table is 0.44. (a) What is the acceleration of Figaro and the fishbowl? (b) If the fishbowl is 0.90 m from the edge of the table, how much time does it take for Figaro to pull Cleo off the table?
  1. (III) A small mass m is set on the surface of a sphere, Fig. 4-51.  If the coefficient of static friction is m s = 0.60, at what angle f would the mass start sliding? [Hint: compare to Fig. 4-48; how are q and f related?
  1. Police lieutenants, examining the scene of an accident involving two cars, measure the skid marks of one of the cars, which nearly came to a stop before colliding, to be 80 m long.  The coefficient of kinetic friction between rubber and the pavement is about 0.80.  Estimate the initial speed of that car assuming a level road.
  1. An elevator in a tall building is allowed to reach a maximum speed of 3.5 m/s going down.  What must the tension be in the cable to stop this elevator over a distance of 3.0 m is the elevator has a mass of 1300 kg including occupants?
  1. A motorcyclist is coasting with the engine off at a steady speed of 17 m/s but enters a sandy stretch where the coefficient of friction is 0.80.  Will the cyclist emerge from the sandy stretch without having to start the engine if the lasts for 15 m?  If so, what will be the speed upon emerging?
  1. A city planner is working on the redesign of a hilly portion of a city.  An important consideration is how steep the roads can be so that even low-powered cars can get up the hills without slowing down.  It is given that a particular small car, with a mass of 1100 kg, can accelerate on a level road from rest to 21 m/s (75 km/h) in 14.0 s.  Using this data, calculate the maximum steepness of a hill.
  1. Jean, who likes physics experiments, dangles her watch from a thin piece of string while the jetliner she is in takes off from the Dulles Airport (Fig. 4-55).  She notices that the string makes an angle of 25o with respect to the vertical while the aircraft accelerates for takeoff, which takes about 18 seconds.  Estimate the takeoff speed of the aircraft.
  1. A 28.0-kg block is connected to an empty 1.00-kg bucket by a cord running over a frictionless pulley (Fig. 4-57).  The coefficient of static friction between the table and the block is 0.450 and the coefficient of kinetic friction between the table and the block is 0.320.  Sand is gradually added to the bucket until the system just begins to move. (a) Calculate the mass of the sand added to the bucket. (b) Calculate the acceleration of the system.

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