Unit 3. Forces Part 2

Transcription

1 Unit 3 Forces Part 2 1

2 Vocabulary: Force Acceleration Mass Net Force Balanced Forces Unbalanced Forces Friction Air resistance Gravity Weight Inertia Action Force Reaction Force Concepts: How does a force affect an object s motion? How does force affect acceleration? How does mass affect acceleration? How do you find the net force acting on an object? How does friction affect an object s motion? How does air resistance affect an object s motion? How does gravity affect an object s motion? How does gravity affect an object s mass? How does gravity affect an object s weight? How does mass affect an object s inertia? How do action and reaction forces compare? 2

3 Anticipation Guide: How Seat Belts Work Directions # 1: 1. Before you read the article, read each statement below. 2. If you agree with the statement, place a check mark in the Agree column. 3. If you disagree with the statement, place a check mark in the Disagree column. a. This section will not be graded for correct answers. b. The purpose of this section is to activate what you already know about how seat belts work. Agree Disagree Statement on Seat Belts 1. The property of matter which seatbelts are based on is inertia. 2. Inertia is every object s freedom to move. 3. Air resistance and friction are forces which act with inertia. 4. Without a seat belt, a rider s inertia is independent of the car s. 5. A person can be killed by flying head first through a windshield because the head is a large, sturdy part of the body. 6. It is important to use the lap belt and shoulder belt to spread out the force exerted on your body. 7. Wearing a seat belt makes the rider s inertia the same as the car s. 8. Crumple zones are designed to make the crash look worse. 9. A crumple zone can only do its job if a rider is wearing a seat belt. 10. Wearing a seat belt prevents riders from flying out of a windshield because they will stop when the car stops. 4. Read the article How Seat Belts Work.. 3

4 How Seatbelts Work by Tom Harris According to a research report from the National Highway Traffic and Safety Administration, seatbelts save approximately 13,000 lives in the United States each year. Furthermore, the NHTSA estimates that 7,000 U.S. car accident fatalities would have been avoided if the victims had been wearing belts. According to the NHTSA, seatbelts reduce the risk of death for a front seat car occupant by about 50 percent. When you think about it, this is absolutely amazing: How can a piece of fabric end up being the difference between life and death? What does it actually do? In this article, we'll examine the technology of seatbelts to see why they are one of the most important technologies in any car. Crashing Concepts The basic idea of a seatbelt is very simple: It keeps you from flying through the windshield or hurdling toward the dashboard when your car comes to an abrupt stop. But why would this happen in the first place? In short, because of inertia. Inertia is an object's tendency to keep moving until something else works against it. To put it another way, inertia is every object's resistance to changing its speed and direction of travel. Things naturally want to keep doing what they are doing, whether it be moving or staying at rest. If a car is speeding along at 50 miles per hour, inertia wants to keep it going 50 mph in one direction. Air resistance and friction with the road are constantly slowing it down, but the engine's power compensates for this energy loss. Anything that is in the car, including the driver and passengers, has its own inertia, which is separate from the car's inertia. The car accelerates riders to its speed. Imagine that you're coasting at a steady 50 miles per hour. Your speed and the car's speed are pretty much equal, so you feel like you and the car are moving as a single unit. But if the car were to crash into a telephone pole, it would be obvious that your inertia and the car's were absolutely independent. The force of the pole would bring the car to an abrupt stop, but your speed would remain the same. Without a seatbelt, your inertia is independent of the car s, and you would either slam into the steering wheel at 50 miles per hour or go flying through the windshield at 50 miles per hour. 4

5 It is a given that no matter what happens in a crash, something would have to exert force on you to slow you down. But depending on where and how the force is applied, you might be killed instantly or you might walk away from the damage unscathed. If you hit the windshield with your head, the stopping power is concentrated on one of the most vulnerable parts of your body. It also stops you very quickly, since the glass is a hard surface. This can easily kill or severely injure a person. A seatbelt is made of softer materials that give and stretch. It also applies the stopping force to more durable parts of the body over a longer period of time. In the next section, we'll see how this reduces the chances of major injury. Taking a Hit In the last section, we saw that any time a car comes to a sudden stop, a passenger comes to a sudden stop as well. A seatbelt's job is to spread the stopping force across sturdier parts of your body in order to minimize damage. A typical seatbelt consists of a lap belt, which rests over your pelvis, and a shoulder belt, which extends across your chest. The two belt sections are tightly secured to the frame of the car in order to hold passengers in their seats. This ties passengers to the car, essentially making their inertia the same as the cars inertia. When the belt is worn correctly, it will apply most of the stopping force to the rib cage and the pelvis, which are relatively sturdy parts of the body. Since the belts extend across a wide section of your body, the force isn't concentrated in a small area, so it can't do as much damage. Additionally, the seatbelt webbing is made of more flexible material than the dashboard or windshield. It stretches a little bit, which means the stop isn't quite so abrupt. Safe seatbelts will only let you shift forward slightly. A car's crumple zones do the real work of softening the blow. Crumple zones are areas in the front and rear of a car that collapse relatively easily. Instead of the entire car coming to an abrupt stop when it hits an obstacle, it absorbs some of the impact force by flattening, like an empty soda can. The car's cabin is much sturdier, so it does not crumple around the passengers. It continues moving briefly, crushing the front of the car against the obstacle. Of course, crumple zones will only protect you if you move with the cab of the car -- that is, if you are secured to the seat by your seatbelt. 5

6 Directions # 2: 1. Re-read the statements above. 2. If the information in the article supports your choice, place a check mark in the Supported column. (The statement # s are the same in both charts.) a. Write the paragraph number of where you found the support. b. Quote the article for evidence of support. 3. If the information in the article does not support your choice, place a check mark in the Not Supported column. a. Write the paragraph number of where you found the correct information. b. Quote the article for evidence. Supported Not Supported Parapraph # Quote From Text 6

7 Information: Newton s 1 st Law Directions: Read pg in your text book. Use the space below to write down important ideas about inertia and Newton s 1 st Law of Motion. We will compile notes on the next page together. 7

8 Inertia and Mass 1. What is inertia? 2. What does inertia have to do with mass? 3. As the amount of increases, the inertia of the object. Newton s 1 st Law of Motion 4. What is Newton s 1 st Law of motion? 5. What does that mean in plain english? 6. Give four examples of Newton s 1 st Law from real life. 8

9 First Law Problems 1. How is it possible that an object is not moving when there could be multiple forces acting on it all at once? 2. What is INERTIA? 3. What type of things have the most inertia? 4. The law of inertia states that no force is required to maintain motion. Why then do you have to keep pedaling a bicycle to maintain motion? 5. Many automobile passengers have suffered neck injuries when struck by cars from behind. How does Newton s first law of inertia apply here? 6. How do headrests help guard against this type of injury? 7. Compare and contrast the following pairs of words a. balanced force & net force b. force & inertia c. force & net force 9

10 8. Which term below best describes the forces on an object with a net force of zero? a. inertia b. balanced forces c. acceleration d. unbalanced forces 9. What is the tendency for an object to resist any change in its motion? a. net force b. balanced force c. acceleration d. inertia 10. Explain why a passenger who is not wearing a safety belt will likely hit the windshield in a head-on collision. Teacher s Initials 10

11 Inquiry Lab: Newton s Second Law State the Problem How does mass affect an object s tendency to accelerate, when the same force is applied? Pre-lab: In this experiment 1. What was your independent variable? 2. What was your dependent variable? 3. Name at least two constants in this experiment. 4. Why will we repeat this experiment and find the average distance traveled? Write a Hypothesis Design an Experiment Materials Tennis ball Ring Stand Tennis ball with eyescrew Meter stick Styrofoam ball Electronic Balance 75 cm string Procedure 1. Measure the mass of the tennis ball and Styrofoam ball. Record this data in the data table. 2. Tie one end of the string to the ring stand and the other end to the eyescrew on the tennis ball. 3. Set the ring stand on a table. Adjust the height of the ball so that it hangs barely above the surface of the table. 4. Place the Styrofoam ball at the edge of the table. Adjust the position of the ring stand so that when the tennis ball is swung, it will contact the Styrofoam ball and knocks it off the table. 5. Hold the tennis ball so that it is perpendicular to the table and release it so that it hits the Styrofoam ball. 6. Mark on the floor where the ball lands. 7. Measure the distance the table to the place where the ball landed. 11

12 8. Repeat 4 more times and find the average distance the Styrofoam ball traveled. 9. Repeat steps 1-8, but use a tennis ball in place of the Styrofoam ball. Run the Experiment & Collect Data Mass of tennis ball and eyescrew: Table 1: Distance Traveled by Styrofoam Ball Mass of Styrofoam Ball (g) Distance (m) Trial 1 Distance (m) Trial 2 Distance (m) Trial 3 Distance (m) Trial 4 Distance (m) Trial 5 Average Distance (m) Table 2: Distance Traveled by Tennis Ball Mass of Tennis Ball (g) Distance (m) Trial 1 Distance (m) Trial 2 Distance (m) Trial 3 Distance (m) Trial 4 Distance (m) Trial 5 Average Distance (m) Analyze the Data 1. Was your hypothesis correct? Explain using data. 2. Were there any errors? Explain. 12

13 3. How does increasing the mass affect the object s motion when the force is the same? 4. Explain the difference in the distance traveled by the two balls (which went further). 5. How does this relate to Newton s Second Law? 6. Explain the relationship between mass and acceleration if the force is held constant. 13

14 Information: Mass, Force, and Acceleration Newton s 2nd Law Force, mass and acceleration have a close relationship. This relationship is described in Newton s second law of motion. The law states that the force acting on an object is equal to the object s mass multiplied by the object s acceleration. An object will accelerate (move) in the direction of the net force. For instance, the net force on the object below is acting to the right, so the object will move to the right! 60 N 43 N Movement (net force = 17 N) The force on an object can be calculated using the mass of the object and the acceleration of the object. THINK: If you have two soccer balls of the same mass and you toss one underhand and you throw the other as hard as you can, which one will accelerate faster? o The greater the acceleration of the object, the greater the force acting on the object must be. THINK: If you have two objects of different mass, and you apply the same force to them both which one will accelerate faster, the heavy one or the lighter one? o The greater the mass of the object, the greater the force acting on the object must be. Critical Thinking Questions: 1. What is the relationship between force and acceleration? 2. As force increases, the acceleration. 3. How does the acceleration of an object affect the distance traveled? 14

15 4. What is the relationship between force and mass? 5. As mass increases, the force must. 6. What two variables does force depend on? 7. Write an equation that you could use to calculate the force on an object. Teacher s Initials 15

16 2 nd Law Problems Directions: Answer the following problems about Newton s second law. 1. What is the mass of a wagon accelerating at 4 m/s 2 if the force required to pull it is 120 Newtons? Givens Solving For Equation Substitution Answer with Units 2. An Olympic swimmer pushes through the water with a force of 35 Newtons. What is her acceleration if she has a mass of 70 kg? Givens Solving For Equation Substitution Answer with Units 3. A rock is dropped off the edge of a cliff so that it accelerates down at 9.8 m/s 2. What is the force acting on the rock if its mass is 5 kg? Givens Solving For Equation Substitution Answer with Units 4. A net force of 10 N is applied to an object having a mass of 2 kg. What is the acceleration of the object? Givens Solving For Equation Substitution Answer with Units 16

17 5. A net force of 20 N is applied to an object having a mass of 2 kg. What is the acceleration of the object? Givens Solving For Equation Substitution Answer with Units 6. In questions 4 and 5 above, the mass of the object stayed the same. The net force applied to the object doubled. As the net force doubled, the acceleration. This is because net force is to acceleration according to Newton s Second Law. 7. A net force of 20 N is applied to an object having a mass of 4 kg. What is the acceleration of the object? Givens Solving For Equation Substitution Answer with Units 8. In questions 5 and 7 above, the force applied to the object was help constant. The mass of the object was doubled. As the mass doubled, the acceleration. This is because mass is to acceleration according to Newton s Second Law. 9. What acceleration will result when a 12 N net force is applied to a 3 kg object? Givens Solving For Equation Substitution Answer with Units 17

18 Information: Newton s 3 rd Law Action & Reaction Directions: 1. Read pages Use the space below to write down important ideas about Newton s 3 rd Law. Including but not limited to: a. Action force b. Reaction force c. Newton s 3 rd law d. How does mass affect the reaction to the forces? e. How do action reaction forces compare to each other? 3. Complete the Sequential Alphabet Roundtable on the next page. 18

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20 Sequential Alphabet Roundtable Newton s 3 rd Law Directions: Read the section in your text about Newton s 3re Law. Fill in each square with a main idea, phrase, or word that SPECIFICALLY has to do with the 3 rd Law. The main idea, phrase, or word MUST START WITH THE LETTER IN THE BOX. A B C D E F G H I J K L M N O P Q R S T U V W X Y Z 20

21 3 rd Law Problems 1. What is Newton s 3 rd Law of Motion? 2. Draw a picture of an action/reaction pair of forces. 3. Why don t action and reaction forces cancel? 4. Explain whether there can be any forces acting on a car moving in a straight line with a constant speed. 5. Explain why when you pull a door open, you don t move if the force on the door and you are the same? 6. I push on a wall with a force of 20 N. What are the magnitude and direction of the reaction forces? Draw the reaction force and explain in words. 20 N 7. A mosquito flies along and runs into the windshield of a rapidly moving vehicle. Which force is stronger: the force of the windshield hitting the mosquito or the force of the mosquito hitting the windshield? Why? 21

22 8. Complete the follow action/reaction pairs by drawing in the forces: a. A tennis ball pushes a racket c. A pool stick pushes the cue ball b. A soccer player strikes a ball d. A weightlifter pushes up on a dumbbell 9. A gun recoils when it is fired. The recoil is the result of action-reaction force pairs. As the gases from the gunpowder explosion expand, the gun pushes the bullet forwards and the bullet pushes the gun backwards. The acceleration of the recoiling gun is... (circle one): a. greater than the acceleration of the bullet. b. smaller than the acceleration of the bullet. c. the same size as the acceleration of the bullet. 22

23 10. In the top picture (below), a physics student is pulling on a rope which is attached to a wall. In the bottom picture, the physics student is pulling upon a rope which is attached to an elephant. In each case, the force scale reads 500 Newtons. The physics student is pulling (circle one): a. with more force when the rope is attached to the wall. b. with more force when the rope is attached to the elephant. c. the same force in each case 11. Identify the at least six pairs of action/reaction forces in the picture below: a. b. c. d. e. f. 12. An elephant and a mouse would both have a weight of 0 N in gravitational-free space. However, they would still retain their amount of matter or. If they were moving toward you with the same speed, would they bump into you with the same affect? Why? 23

24 Pop Rocket Demo 1. What happens when baking soda and vinegar are mixed together? 2. How do you know that? 3. What happened to the film canister? Explain why this happened. 4. Can you think of another event where pressure causes a similar "explosion?" 24