Vocabulary. Force. Inertia. Newton. Net Force. Newton s 1 st Law. Newton s 2 nd Law. Newton s 3 rd Law. 1 Page

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1 Vocabulary Term Definition Force Inertia Newton Net Force Newton s 1 st Law Newton s 2 nd Law Newton s 3 rd Law 1 Page

2 Newton s 1 st Law of Motion Every body persists in its state of rest or of uniform motion in a straight line unless it is compelled to change that state by unbalanced forces impressed on it. ~ISAAC NEWTON The principle we describe as Newton s first law was actually first discovered by Galileo. (Many of Newton s discoveries originated with people other than Sir Isaac.) Before Galileo, the explanation for natural motion was the one put forth by Aristotle. Aristotle theorized that things move to find their natural place in the universe and come to rest when they get there. Galileo, on the other hand, proposed that objects maintain their state of motion: If they are at rest, they remain at rest unless acted on by an external force. Easy enough. But Galileo went on to say that objects moving in a straight line with constant speed (constant velocity, in other words) remain moving in a straight line with constant speed, unless acted upon by an external force. That s so dumb, charged Galileo s many antagonists (Galileo had a knack for making enemies), you re saying that if I set a brick into motion on a tabletop, it will maintain constant velocity? Get outta town! Of course not, propeller-heads! Galileo explained. In the case of the brick, a frictional force acts on the brick to bring it to rest. But consider an experiment I ve done: Of course, the ball does not continue forever. Air resistance and rolling friction forces slow the ball to a stop. But if a ball were given a push in deep space, it would travel an exceedingly long distance before being acted on by external forces. To understand a concept like this we need to undress nature. We must look past superficial complexity to see the simple characteristics of nature. Once the fundamental properties are understood, we can take into account other, more complex factors. The simple characteristic of nature that Newton points out is that objects are lazy. If they are at rest, they have no desire to get up and move. If they are to move, something else will have to kick them in the rear (providing the unbalanced external force). If they are moving at a constant velocity, they will continue at constant velocity. If a force pushes them along, they will go faster. If a force pushes against them, they will slow down. The laziness of matter is called inertia. Formally, inertia is defined as the resistance a body offers to changes in its state of motion and applies to moving and non-moving objects. 2 Page

3 Newton s 1 st Law of Motion Reading Questions 1. Who actually discovered Newton s 1 st law of motion? 2. What did Aristotle theorize about the natural motion of objects? 3. Did Galileo agree or disagree with Aristotle s theory? Explain. 4. One of Galileo s conclusions was that moving bodies would continue to move as long as nothing interfered with them. This conclusion became s first law of motion. It states that: 5. What did Newton call the laziness of matter? 6. What did Newton mean when he referred to objects as lazy? 7. Many automobile passengers have suffered neck injuries when struck by cars from behind. How does Newton's law of inertia apply here? How do headrests help to guard against this type of injury? 8. Explain and sketch a picture of one demonstration conducted in class. 3 Page

4 Newton s 1 st Law of Motion Newton s 1 st Law of motion Main Ideas is also know as the law of Definition of inertia: Unless acted upon by an Applying Newton s 1 st Law of motion Explain according to Newton s 1 st law of motion what would happen to the person in the car below if they crashed into the wall using the terms inertia, state of motion, and unbalanced force. NEWTON S FIRST LAW OF MOTION (aka: ) States: An Object is the property of an object that resists changes in motion. Class Work 4 Page

5 Newton s 1 st Law of Motion Check Questions 1. Which has more inertia a bowling ball or a golf ball? 2. Objects with more mass have more and are more to changes in their motion. 3. Mac and Tosh are arguing in the cafeteria. Mac says that if he flings the Jell-O with a greater speed it will have a greater inertia. Tosh argues that inertia does not depend upon speed, but rather upon mass. Who do you agree with? Explain why. 4. Fred spends most Sunday afternoons at rest on the sofa, watching pro football games and consuming large quantities of food. What effect (if any) does this practice have upon his inertia? Explain. 5 Page

6 Notes Newton s Second Law Explain the following observations: d. When a grocery cart is pushed by a person, a(n) acts on the cart and the cart. e. The acceleration of an object is directly proportional to the net force acting on it. This means that, as the net force acting on the object increases, the acceleration of the object. 6 Page

7 Newton s 2 nd Law of Motion states: The acceleration of an object is directly proportional to the net force acting on the object and inversely proportional to the object s mass. 2 nd law abbreviated using symbols: F = ma F = sum of the forces m = mass a = acceleration Newton s 2 nd law relates the net force on an object, the mass of the object, and acceleration. It states that the stronger the net force on an object, If twice the net force is applied, the acceleration will be as great. The acceleration of an object is proportional to the net applied force and proportional to the mass. a F m NEWTON S 2 ND LAW Formula Want to find What you know acceleration (a) force (F) and mass (m) force (F) mass (m) acceleration (a) and mass (m) acceleration (a) and force (F) One of Galileo s surprising findings was that heavy bodies and light bodies fall with the same acceleration. How does Newton s second law explain that observation? 7 Page

8 Conceptual Problems For questions 1-6, consider the cart on a track below. A force is applied acting to the right. Assume that friction is negligible. For each question, one or more features of the system have been changed. You are to indicate what effect the change will have on the acceleration. Use the following answer key. a. The acceleration will be greater. b. The acceleration will be less. c. The acceleration remains the same. d. It's not possible to tell. 1. The mass of the cart is increased to 2 kg. If I double the mass the acceleration will... 2 kg 2. The mass of the cart is increased to 3 kg. If I triple the mass the acceleration will... 3 kg 3. The mass of the cart is decreased to ½ kg. If I reduce the mass by ½ the acceleration will... ½ kg 4. The towing force is increased to 1.0 N. If I double the force the acceleration will N 5. The towing force is increased to 1.5 N. If I triple the force the acceleration will N 6. Both the mass of the cart and the towing force are double. If I double the mass and double the force the acceleration will... 8 Page

9 Newton s 2 nd Law Formula Net Force No Acceleration Net force is. Objects are at or moving with constant. Same Direction Opposite Direction 9 Page

10 Newton s 3 rd Law states that Forces are equal in Forces are opposite in 10 Page

11 Conceptual Problems Create your own example. Draw and write your own here. Draw the picture, the arrows for both action and reaction and the descriptions for the action and reaction forces. 11 Page

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