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8th Grade
Forces and Motion
Classwork-Homework Slides www.njctl.org
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Classwork #1: Motion and Speed
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1 Define motion.
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2 When you look at the ground you seem to be at
- rest. Why is this? Why does someone in space
see you moving in a circle?
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3 Define speed.
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4 What is the formula for measuring the speed of an
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5 Lance Armstrong can ride his bicycle at speeds of 15 m/s. If he is traveling at this speed, how long will it take him to travel 81.9 meters? Show your work.
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6 John is growing another crop of corn this year. He plows a row of his field in 90 seconds. If each row is 384 meters long, how fast does John drive his tractor? Show your work.
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7 Julia is learning how to ride her tricycle. It takes her 10 seconds to go from one end of the driveway to the other. If she travels at 3 m/s, how far does she travel? Show your work.
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Homework #1: Motion and Speed
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8 How can you tell that an object is in motion?
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9 What is a frame of reference?
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10 How long does it take a snail to travel a distance
- f 3 ft at 0.1 ft/sec? Show your work.
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11 How far does a cat run if it travels at a speed of 3 m/s for 25 seconds? Show your work.
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12 A racecar driver frequently drives her racecar at 67 m/s for as long as she can. If she drives her car at that speed for 22 seconds, how far will she travel? Show your work.
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13 Everyone in Paperville needs their newspaper before 6 a.m. on Monday. If it takes the delivery boy 3 hours to deliver the papers at 12 m/s, what is the distance of the paper route in Paperville? Show your work. (HINT: convert 3 hours to seconds before using the speed equation)
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Classwork #2: Speed and Velocity
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The 100m dash is a sprint race in track and field competitions where athletes are timed on how long it takes them to sprint a distance of 100m. It has been an Olympic sport since 1896. The table below shows some of the record setting times achieved by athletes running the 100m dash. Use the data table to answer the questions.
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Time (s) Name Country Date 9.572 Usain Bolt Jamaica August 16, 2009 9.683 Usain Bolt Jamaica August 16, 2008 9.72 Usain Bolt Jamaica May 31, 2008 9.74 Asafa Powell Jamaica September 9, 2007 9.762 Asafa Powell Jamaica August 18, 2006 9.763 Asafa Powell Jamaica June 11, 2006 9.766 Justin Gatlin USA May 12, 2006 9.768 Asafa Powell Jamaica June 14, 2005 9.78 Tim Montgomery USA September 14, 2002 9.79 Maurice Green USA June 6, 1999 9.835 Donovan Bailey Canada July 27, 1996 9.85 Leroy Burrell USA July 6, 1994 9.86 Carl Lewis USA August 25, 1991 9.90 Leroy Burrell USA June 14, 1991 9.92 Carl Lewis USA September 24, 1988
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14 According to the table, who holds the current world record for finishing the 100 m sprint the fastest?
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15 By how much time did Tim Montgomery beat the previous world record holder?
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16 The first recorded world record is 10.8 seconds set by Luther Cary (USA 1891). How much time has been taken off the world record since it was first set in 1891?
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17 At what average speed did Asafa Powell run the 100 m in 2007? Show your work.
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18 Usain Bolt broke the world record on May 31, 2008. He then broke his own world record about two months later. What was his average speed on each day? Show your work for both days.
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19 Can you use the table to calculate instantaneous speeds? How about velocities? Explain why or why not.
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20 How much faster (in terms of average speed) did Leroy Burrell run on June 14, 1991 to break the record set by Carl Lewis on September 24, 1988? Show your work.
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21 If an athlete were to run the 100 m dash at a speed
- f 10.5 m/s, would they beat Usain Bolt's current
world record? Justify your answer.
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22 What was Justin Gatlin's average speed on the day that he broke the world record? Show your work.
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23 Calculate the speed achieved by Maurice Green in
- 1999. At this average speed, how long would it
take Maurice to run 315 meters? Show your work.
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Homework #2: Speed and Velocity
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24 What is the difference between average speed and instantaneous speed?
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25 Can average speed ever be equal to instantaneous speed? Explain.
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26 What is the average speed of a dog that runs 100 m in 30 s? Show your work.
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27 Describe two examples of average speed and two examples of instantaneous speed.
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28 The Admiral train is traveling east at 100 mph and the Alamo train is traveling west at 100 mph. Do they have the same speed? Do they have the same velocity? Explain why or why not.
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29 Train A leaves New York for California at the same time Train B leaves California for New York. They are 2900 miles apart. They both travel at an average speed of 120 mph. A What is the velocity of train A? B What is the velocity of train B?
C How long will it take before they meet? Show your work.
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Homework #3: Graphing Motion
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30 It is Field Day at your school and everyone is ready for the potato sack race. Each grade has selected one person to enter the race. There are nine checkpoints along the racecourse. The table below shows how long it took each student to get to each checkpoint. Graph the motion of the three contestants using a separate color/line pattern for each contestant. Be sure to plot position on the y- axis and time on the x-axis and label each axis.
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Name 10m 20m 30m 40m 50m 60m 70m 80m 90m Susie (6th) 4s 9s 11s 15s 21s 23s 28s 32s 37s Joe (7
th)
5s 15s 19s 24s 29s 33s 36s 41s 46s Thomas (8th) 6s 8s 11s 15s 20s 24s 28s 31s 35s
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31 Who won the race? Explain how you can tell from the graph.
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32 Who had the fastest speed from 0- 30m? Justify your answer with speed calculations for each student.
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33 Who had the fastest speed from 0-60m? Justify your answer with speed calculation for each student.
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Classwork #3: Acceleration
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34 Define acceleration.
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35 Write the formula for measuring the acceleration
- f an object. Be sure to explain what each part of
the equation is.
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36 What is the SI unit of measurement for acceleration?
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37 Explain 3 different ways that an object can accelerate.
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38 What is the acceleration of a car that starts at rest and increases its speed to 10 m/s in 2 seconds? Show your work.
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39 What is the acceleration of an object that starts at 10 m/s and slows to 2 m/s in 4 seconds? Show your work.
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40 Describe the speed and velocity of an object whose motion is shown in the speed vs time graph below:
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41 Describe the speed and velocity of an object whose motion is shown in the position vs time graph below. What happens to the distance covered during each 5 second time interval?
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Homework #4: Acceleration
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42 A student runs at constant speed on a circular
- racetrack. Does the student accelerate? Justify
your answer.
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43 A car increases its speed. Is this acceleration positive or negative? Explain.
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44 What is the acceleration of a rocket that is traveling at 20 m/s and increases its velocity to 40 m/s in 4 seconds? Show your work.
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45 What is the acceleration of a dragster that deploys a parachute when travelling at 33 m/s and slows to rest in 11 seconds? Show your work.
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46 Describe the speed of an object whose motion is shown in the speed vs time graph below:
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47 Describe the speed of an object whose motion is shown in the position vs time graph below. What happens to the distance covered during each 5 second time interval?
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Classwork #4: Forces and Friction
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48 Define a force.
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49 What do forces do to objects? Describe an example.
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50 What is the “BIG IDEA” about the relationship between forces and acceleration?
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51 What is the unit for measuring force in the SI system of measurement?
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52 Determine the net force in the diagram seen
- below. Show how you came up with your answer.
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53 Define friction and explain its effect on motion.
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54 What is static friction?
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Homework #5: Forces and Friction
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55 A force is a push or a pull acting on an object. How do forces affect motion when the forces are a) balanced? b) unbalanced?
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56 What happens to the motion of any object that feels an unbalanced force?
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57 What types of friction occur when you ride a bike through a puddle? Justify your answer.
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Classwork #5: Gravity and Equilibrium
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58 In your own words, define the law of gravitation.
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59 What is the formula for measuring the weight of any object? Identify each part of the formula.
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60 Explain why the weight of a barbell is smaller on the moon than on Earth.
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61 What happens to the force of gravity as any 2
- bjects move farther apart?
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62 When is an object said to be in free fall? Give an example.
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63 Define equilibrium and relate it to forces .
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64 Calculate the net force acting on the block in the diagram seen below. Show your work. What is the direction of the net force?
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Homework #6: Gravity and Equilibrium
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65 As we move away from the center of the Earth, what happens to the force of gravity?
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66 Explain why Jupiter’s gravitational constant is stronger than Mercury’s gravitational constant.
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67 What is the weight of a 99 kg man on Earth? Show your work.
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68 What happens to the motion of any object that feels an unbalanced force?
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69 If an object is in equilibrium, then the forces must be ___________________?
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70 Calculate the net force acting on the block seen in the diagram below. Show your work.
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71 Is the object in #70 in equilibrium? Explain.
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Classwork #6: Newton's First Law
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72 Who were the early scientists that first studied/explained motion?
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73 What were some of the advances that Sir Isaac Newton made in the area of physics?
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74 Define Newton’s 1st law of motion and give an everyday example of Newton's 1st law of motion at work.
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75 What is the relationship between an object’s mass and the amount of inertia it exhibits? Give an example to answer this question.
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Homework #7: Newton's First Law
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76 Define inertia.
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77 Which object has more inertia? A running boy or an automobile at rest? Why?
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78 In your own words, explain an example of Newton’s 1st law of motion for moving and non- moving objects.
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Classwork #7: Newton's Second Law
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79 How does applying a force to an object affect its motion?
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80 What is the acceleration of a 100 kg canoe that feels a force of 1000N? Show your work.
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81 Suppose the canoe in problem #80 is loaded to have a total mass of 500kg, but the force stays the same at 1000N. Will the canoe slow down or speed up? Why? Justify your answer by calculating the new acceleration. Show your work.
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82 A tennis ball with a mass of 0.314 kg, is accelerated at a rate of 164 m/s2 when hit by a professional tennis player. What force does the player’s tennis racket exert on the ball? Show your work.
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83 You push a friend sitting on a swing. She has a mass of 50 kg and accelerates at a rate of 4 m/s 2. Find the force you exerted. Show your work.
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84 You want to push another friend to accelerate them at the same rate of 4 m/s2. This friend has a mass of 70 kg. Will you push as hard as you did in #83? Why or why not? Justify your answer by calculating the new force. Show your work.
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Homework #8: Newton's Second Law
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85 You pull on a box with 50 N of force. Your friend pulls on an identical box with 100 N of force. Compare the motion of both boxes. Explain your answer.
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86 A person uses 50 N of force to push a cart. If the mass of the cart increases, but the person uses the same amount of force, what is the effect on the resulting acceleration?
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87 What is the acceleration of a 1000 kg car that feels a force of 5000 N? Show your work.
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88 What force causes a 1 m/s 2 acceleration of a 5 kg bowling ball? Show your work.
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89 What is the mass of an object that accelerates at a rate of 3 m/s2 when acted upon by a 30 N force? Show your work.
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90 A worker drops his hammer off the roof of a
- house. The hammer has a mass of 9 kg, and
gravity accelerates it at the usual 9.8 m/s 2. What is the gravitational force acting on the hammer? What is the weight of the hammer? Show your work.
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Homework #9: Newton's Third Law
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91 Explain how a rocket works using Newton's third law of motion.
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92 Compare and contrast momentum and inertia.
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93 Compare the momentum of a fly and a train travelling at the same velocity.
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94 How does sitting in your chair relate to Newton's 3rd law of motion?
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95 A dog digs a hole in the ground by pushing dirt back with his paws. Describe an action reaction pair in this scenario.
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96 What is the momentum of a 13 kg shot put travelling at 3 m/s? Show your work.
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97 What is the momentum of the same shot-put travelling at 1.5 m/s? Show your work.
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98 If the velocity of an object is cut in half, what happens to the momentum of the object? How about if it is doubled?
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99 Define “conservation of momentum” and give an example of this principle in the real world.
