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83 84 Newton’s Three Laws Last year we learned about Isaac Newton and all the different things he invented and discovered. This year we will be learning about his three laws of motion. We learned about the first one last year, but we didn’t name it. His first law of motion is called the Law of Inertia. It states that objects in motion tend to stay in motion unless acted on by an unbalanced force. It also states that an object at rest will stay at rest unless act on by an unbalanced force. When we played with the toy car and penny we observed the penny continue moving when the car stopped. Let’s do the same activity but change some variables. Materials: Toy car, penny, 3 books same height, small book, ramp, meter stick or ruler What To Do: 1. Set up your ramp to look like the picture except use only one book for the height of the ramp. 2. Place the small book at the end of the ramp to stop the car. 3. Place the penny on top of the car and let it roll down the ramp. 4. Use the ruler or meter stick to measure how far the penny goes from the end of the ramp. 5. Add another book and repeat. 6. Add a third book and repeat. 7. Record your measurements in the data table. Number of Books Distance Penny Traveled in cm 1 2 3 Questions: 1. In this activity we changed the number of books that were under the ramp. How did this change the motion of the car? ________________________ 2. What happened to the penny? _____________ Watch the animation called Car and the Wall from the website http://www.physicsclassroom.com/mmedia/newtla ws/cci.cfm Then watch the Inertia video from www.missdoctorbailer.com. On the lines below write 3 sentences telling why Newton’s Law of Inertia means we need to wear seatbelts in a car. ______________________________________ ______________________________________ ______________________________________ ______________________________________ ______________________________________ 85 Newton’s second law is commonly known as F=ma where F is the force in Newtons, “m” is the mass in kilograms and “a” is acceleration. So, the Force need to move an object can be calculated by multiplying the Mass times the Acceleration you want the object to have. An easy way to think about this is if you wanted to kick a goal with a soccer ball or with a bowling ball. To make the bowling ball move fast you would have to kick it much harder than the soccer ball. Let’s see what happens when we add mass to one of our cars. Materials: two like cars, 50 g mass, masking tape, ruler, ramp, three books. What To Do: 1. Set up your ramp and 3 books as in the previous activity. Do not put a small book at the bottom of the ramp. 2. Measure 30 cm from the bottom of the ramp and place a piece of masking tape on the table like a finish line on a racetrack. 3. Notice that one car has the 50 g mass taped to it. 4. Place both cars at the top of the ramp with the ruler acting as a starting gate. 5. Lift the ruler so that the cars start down the ramp together. 6. Observe which car crosses the finish line first. Do this two more times for a total of three times. 7. Place a check mark in the data table below for the car the crosses the finish line first. Car With mass Without mass Trial 1 Watch the video F=ma from www.missdoctorbailer.com Trial 2 Trial 3 We can use the formula F= ma to calculate the forces on an object. Remember the unit for force is the Newton. 86 1. A train’s mass is 50kg and its acceleration is 5 m/s. What is the net force on the train? __________ 2. A car’s mass is 500 kg and its acceleration is 20 m/s. What is the net force on the object? ______________ What if we wanted to know the acceleration? We can rearrange the formula like you do in math. F=ma can change to F = a m 3. A bowling ball has a mass of 40kg and is kicked with a force of 10 N. What is its acceleration? ______ 4. A soccer ball has a mass of 10 kg and is kicked with a force of 10 N. What is its acceleration? __________ What if we wanted to know the mass? We can rearrange the formula like you do in math. F= ma can change to F = m a 5. A soccer ball is kicked with a force of 20N and accelerates at 2m/s2. What is its mass? 6. A bowling ball is kicked with a force of 50N and accelerates at 1 m/s2. What is its mass? __________ 87 The third law of motion states that for every action there is an equal and opposite reaction. It is called the Law of Action-Reaction. When you sit in a chair you push down on the chair and the chair pushes back on you. This is action-reaction. If you were at the Galleria, with your friends at the ice skating rink and you pushed your friend’s shoulder, your friend’s shoulder will push back on you. Labe l each action and reaction in the picture above. Let’s try it with 2 skateboards. Materials: 2 skateboard, 2 student volunteers What To Do: 1. Two student volunteers will each stand on a skateboard both facing the same direction. 2. The student in back with GENTLY push the other student. 3. Observe what happens to both students. 4. Draw it below. Label the action and the reaction. 88 Rockets are able to blast off because of Newton’s third law. The rocket carries “propellants” which are basically things that explode when you mix them together and ignite them. The gasses from the controlled explosions are forced out of the rear of the rocket. The rocket pushes the gases out at a really high speed. The gases push back and the rocket accelerates. Because rockets push on these gases instead of pushing on air they don’t need air molecules and can cruise along just fine in outer space. Materials: vinegar, baking soda, water bottle, beaker with measurements, medicine cup with measurements, tissue, spoon, rocket pattern, index card, cork, paper clip, tape What To Do: 1. Use the medicine cup to measure 2 ounces of baking soda. 2. Place it in the tissue and roll it up to a size that will fit into the water bottle opening. But don’t put it in the bottle yet. 3. Place 100 mL of vinegar in the water bottle. 4. Use the rocket pattern to trace a rocket on the index card. 5. Unbend the paper clip one time and push it into the cork. Tape the rocket pattern to the paper clip. )See picture on next page.) 90 89 6. Go outside with your teacher. 7. One group at a time will launch their rocket by putting the baking soda packet into the bottle and putting the cork firmly into the bottle. 8. For safety everyone will stand back from the launch pad and watch. 9. After each group has launched their rocket gather up all materials and return to the classroom. Summing Up 1. What is Newton’s 1st Law called? ______________ ____________________________________________ 2. What does it state? _________________________ ____________________________________________ 3. What is an example of it in our lives? ___________ ____________________________________________ Questions: 1. What happened inside the water bottle? __________ ____________________________________________ 2. What did the mixing of baking soda and vinegar produce? ____________________________________ 3. How did this help the rocket launch? ___________ ___________________________________________ 4. The gas from the baking soda and vinegar pushed on the rocket and the rocket pushed back. Which had the stronger 4. What is Newton’s 2nd Law called? ______________ ____________________________________________ 5. What does it state? _________________________ ____________________________________________ 6. What is an example of it in our lives? ___________ ____________________________________________ force? ______________________________________ 5. What was the action? __________________________ 7. What is Newton’s 3rd Law called? ______________ 6. What was the reaction? ________________________ ____________________________________________ 7. What kind of force caused the rocket to launch? 8. What does it state? _________________________ ____________________________________________ ____________________________________________ 9. What is an example of it in our lives? ___________ Watch the Action-Reaction video from www.missdoctorbailer.com. ____________________________________________ 91 Name __________________ period ____ 91 Name __________________ period ____ Exit Ticket Exit Ticket Newton’s Three 1. In a series of controlled experiments, a student measures the force acting on a car and the car’s acceleration. The student’s data are shown below. Newton’s Three 1. A student argued that she would not slide backward on here roller skates when she pushed her little sister on her roller skates. Which of Newton’s laws is she defying?__________________________________ 2. In a series of controlled experiments, a student measures the force acting on a car and the car’s acceleration. The student’s data are shown below. Based on the recorded observations and Newton’s 2nd law (F=ma) what is the mass of the cart in kilograms? ________________________ 2. A student argued that he did not have to wear a seatbelt in the car because he was strong enough to stop himself from flying out of the car if it stopped suddenly. Which of Newton’s laws is he defying? Based on the recorded observations and Newton’s 2nd law (F=ma) what is the mass of the cart in kilograms? ________________________________________ 3. A student argued that he did not have to wear a seatbelt in the car because he was strong enough to stop himself from flying out of the car if it stopped suddenly. Which of Newton’s laws is he defying? 3. A student argued that she would not slide backward on here roller skates when she pushed her little sister on her roller skates. Which of Newton’s laws is she defying? ________________________ ________________________________________ _________________________________________