31. An airplane making a routine trip is flying northward 40. The cart in Figure I has a mass of 0.4 kg and is with a horizontal force from the engines of 42 000 N. attached, by a string and a pulley, to a weight that has A wind is blowing to the south and pushes the plane a mass of 0.20 kg. (3.3) my with a horizontal force of 1200 N. What is the net 0.4 kg horizontal force on the plane? (3.1) mu 32. A book sits on a desk. Use Newton's first law to O- explain why the normal force must be equal to the force of gravity in order for the book to remain at rest when no other forces are acting on it. (3.2) mule 0.20 kg 33. A moving ballistics cart fires a ball straight up. Use Newton's first law and the concept of inertia to Figure 1 explain why the ball follows an arched path instead of going straight up and down even though the force (a) Calculate the acceleration of the cart, assuming acting on the ball was vertical. (3.2) mule there is no friction. 34. Students are trying to see if they can pull apart a rope (b) Calculate the acceleration of the cart if there is a they made out of a bunch of vines. The two students frictional force of 0.10 N acting on the cart. on the right are able to pull with forces of 95 N and (c) Calculate the magnitude of the tension in the 87 N, and one of the students on the left is able to pull string. with a force of 104 N. The rope does not break and the students remain stationary. What is the force that 41. Use Newton's third law to explain each situation. (3.4) the fourth student is pulling with? (3.3) Du (a) A student jumps off a skateboard, causing it to 35. Use Newton's second law to explain each situation. roll in the opposite direction. (3.3) (b) A person is leaning against a wall and does not (a) Two cars are pushed by an equivalent net force, move. but one car has more mass. Which car will accelerate faster? (c) A ball rolls forward, hits a group of balls, and then rolls backwards. (b) A person pulls a cart with a constant force while his friends keep loading more boxes onto it. What 42. A hockey player pushes off the boards with a force of will happen to the acceleration of the cart? 68 N. The combined mass of the hockey player and her skates is 80 kg. (3.4) ma 36. Calculate the net force in each situation. (3.3) am (a) Calculate the acceleration of the player as she (a) A sprinter with a mass of 68 kg accelerates at a pushes off if there is a 30 N frictional force from rate of 2.4 m/s. the ice. (b) A 425 g baseball falls after being hit. (b) Explain why the boards do not move. 37. For each situation, calculate the acceleration. (3.3) Du 43. A 0.50 kg weight is hanging from a string attached to (a) A 2200 kg car, starting from rest, accelerates the ceiling of a helicopter. Calculate the tension in the northward while experiencing a net force of 4500 N. string for the following situations: (3.5) (b) A skydiver with a mass of 71.2 kg experiences a (a) The helicopter is stationary- net force of 245 N upward when she opens her (b) The helicopter is accelerating straight up at parachute. 0.80 m/s. 38. After a hockey player takes a shot, the puck slows (c) The helicopter is accelerating straight down at down at a rate of 1.3 m/s'. The mass of the puck is 0.92 m/s. 175 g. What are the magnitude and direction of the frictional force acting on the puck? (3.3) a 44. A small brick is dropped out of a hot air balloon. (4.1) ME 39. A little-league player hits a baseball from a tee. The (a) What forces are acting on the brick as it falls? baseball experiences a net force of 8.0 x 10' N [S]. (b) Which of the forces you listed for (a) is acting The mass of the ball is 145 g. What is the acceleration of the ball? (3.3) mu more strongly in the instant the brick is dropped? (c) Once the brick reaches its terminal speed, what is the relationship between the two forces? (d) The brick has a mass of 220 g. What is the force of air resistance acting on the brick when it reaches terminal speed? 210 Unit 2 . Forces45. (a) Describe two different methods that are used to 51. An ice-fishing hut weighing 52 N sits on a dry, frozen measure the strength of gravitational fields. lake. (4.2) (b) The force of gravity acting on a 2.50 kg object is (a) What is the magnitude of the initial horizontal 24.475 N. What is the gravitational field strength force required to make the ice-fishing hut start acting on the object? sliding across the surface of the frozen pond? (c) For your answer to (b), would this value of Earth's (b) Once the ice-fishing hut has begun sliding, what gravitational field strength be likely for an altitude force is required to maintain the hut sliding at a above sea level or below sea level? Explain. constant speed? (4.1) me 52. A 12 kg box at rest on the floor requires a horizontal 16. (a) In your own words, describe the difference force of 47 N to start moving it. Once it starts between mass and weight. moving, the same horizontal force makes the box (b) Describe how to change the mass of an object. accelerate at 1.1 m/s'. (4.2) m (c) Describe how to change the weight of an object (a) Find the coefficient of static friction. without changing its mass. (b) Find the coefficient of kinetic friction. (d) Is it possible for mass and weight to be the same? 53. A runner with a mass of 72 kg is running sprints Explain. (4.1) Bum to train for a track meet. The coefficient of friction 47. Two cylinders with different masses are dropped from between the runner's shoes and the track is 0.79. If the top of a building. (4.1) me the runner starts from rest, what is the maximum rate (a) Which one has a larger cross-sectional area at which he can accelerate? (4.3) m if both of them hit the ground at the same 54. A man is pulling a block with a mass of 6.2 kg across time-the lighter one or the heavier one? a horizontal surface and accelerates the block at a (b) Which one will hit the ground first if they both rate of 0.50 m/s'. The coefficient of kinetic friction have the same cross-sectional area? between the block and the surface is 0.24. What is (c) Describe what would happen if the objects were the magnitude of the force with which the man pulls? dropped in a vacuum (assume there is no air (4.3)m resistance)? 55. A person pushes a box across a horizontal floor with 18. An electrician holds a 3.2 kg chandelier up against a an initial velocity of 5.2 m/s. The box has a mass of ceiling with a force of 53 N. What is the normal force 22 kg, and the coefficient of kinetic friction between acting on the chandelier from the ceiling? (4.1) m the box and the floor is 0.44. How far does the box 49. A man tries to push a large rock across a flat horizontal slide before coming to rest? (4.3) m surface but is unable to move the rock. (4.2) m 56. Examine Figure 2 showing a crash test. (a) List the forces acting on the rock. (a) Describe at least two safety features visible in this (b) Draw a free-body diagram of the rock. photograph. (c) If the rock has a mass of 210 kg, what is the (b) Describe the type of information that an engineer normal force acting on the rock? would gather during a crash test. 50. An object at rest on a flat horizontal surface (c) What evidence is there in the photograph that the experiences a maximum static frictional force of crumple zone was very effective in protecting the 29.89 N. The object has a mass of 5.0 kg. (4.2) mumm people in the car? (4.4) (a) What is the coefficient of static friction? (b) From the materials in Table 1, which two could be involved in this interaction? Table 1 Material steel on steel (dry) 0.78 0.42 leather on oak 0.61 0.52 steel on ice 0.1 0.01 Teflon on Teflon 0.04 Figure 2 HEL Unit 2 Review 211