Newton's First and Second Laws

Transcription

1 Name Class _ Date Newton's First and Second Laws KEY IDEAS As you read this section, keep these questions What makes an object's motion change? What is inertia? What affects how much an object speeds up or slows down? in mind: What Are Newton's. Laws of Motion? Imagine rolling a basketball across a smooth wooden floor. How far will the ball roll before it stops? Now imagine rolling the ball across a floor covered with carpet. The ball will not roll as far on the carpet as it did on the smooth floor. 'What causes this? Hundreds of years ago, an English scientist named Sir Isaac Newton noticed similar effects. He also wondered what caused them. He discovered the relationship between motion and forces. He described the relationships in three laws. Today, we call these laws Newton's laws of motion. What Is Newton's First Law of Motion? A ball that stops after rolling along the floor is an example of Newton's first law of motion. This law states that an object's motion will not change until an unbalanced force acts on the object. The law also states that an object will not start to move until an unbalanced force acts on it. A rolling ball stops moving because the force of friction acts on it and slows it down. If there were no friction, the ball would keep rolling. ~ Ii~MJJ;GtlO:d.ll Find Examples As you read,. make a table listing examples of Newton's first and second laws. ~ 1. Describe State Newton's first law of motion in your own words. This bowling ball slows down as it rolls because of the force of friction. If no forces acted on the ball, it would not stop rolling. Copyright ~ by Holt, Rinehart and Winston. All rights reserved. Interactive Reader 249

2 Name _ Class _ Date _ Newton's First and Second Laws continued ~ 2. Explain How is mass related to inertia? What Is Inertia? Another way of stating Newton's first law is that matter resists any change in motion. All objects resist changes in motion. However, not all objects resist changes in motion by the same amount. For example, it does not take very much force to change the motion of a baseball. It takes much more force to change the motion of a bowling ball. The bowling ball resists changes in motion more than the baseball does. The tendency of an object to resist changes in motion is called!!iit!t~.all objects have inertia. The amount of inertia an object has depends on the.object's mass. The greater the mass of an object, the greater its inertia. ~ SEAT BELTS AND CAR SEATS Think about the last time you rode in a car. What happened when the car stopped suddenly? You probably felt like you were being pushed toward the front of the car. Objects in the seats near you may have fallen over. Newton's first law of motion explains why this happens. When a car stops, the inertia of the objects inside the car prevents them from stopping right away. Therefore, they may slide or fall over as the car stops suddenly. Seat belts help to protect people inside a car from being hurt when the car stops. When the car stops, you start to slide toward the front of the car. The seat belt catches you and 'stops you from sliding. Car seats help keep babies from being hurt when a car stops. These seats face toward the back of the car, as you can see in the figure below. This kind of car seat keeps the baby from sliding forward. LOOKlNG CLOSER 3. Describe How does the car seat protect the baby? Car seats like this one help prevent babies from being hurt when a car stops. Interactive Reader 250

3 Name Class _ Date Newton's First and Second Laws continued What Is Newton's Second Law of Motion? Newton's first law describes what happens when no unbalanced forces act on an object: the object's motion does not change. What happens when an unbalanced force does act on an object? Newton's second law states that the larger the unbalanced force on an object, the more the object's motion will change. It also states that the larger the object's mass, the smaller its acceleration will be. Acceleration is the change in motion of an object. Think about a baseball. If you throw the baseball gently, it will not move very fast. If you throw it more forcefully, it will move much more quickly. Now, imagine using the same force to throw a bowling ball. What will happen? The bowling ball will move much more slowly than the baseball. We can write Newton's second law of motion as a mathematical equation:. net force = mass X acceleration or F=ma You can use this equation to calculate how much force is needed to make an object accelerate. You can also use it to calculate how much an object will accelerate when a force acts on it. Before you can use the equation, though, you need to know how scientists measure force. What Are the Units of Force? The 81 unit scientists use to measure force is the newton en). One newton is the amount of force needed to make a one-kilogram mass accelerate at one meter per second squared: 1 N = 1 kg x 1 m1s 2 Although scientists use newtons to measure force, sometimes people use pounds instead. One pound (lb) is equal to 4.45 N. One newton is equal to lb. Critical Thinking 4. Apply Concepts In many cases, large trucks move more slowly up hills than small cars. What do you think is the reason for this? LOOKING CLOSER 5. Calculate How many newtons are equal to 10 Ib? Units of force newtons (N) 1 N = 1 kg x m/s 2 1 N = O.2251b pounds (Ib) 1 Ib = 4.45 N 1 Ib = 4.45 kg x m/s 2 Interactive Reader 251

4 Name _ Class _ Date _ ~ Newton's First and Second Laws continued How Does Mass Affect Acceleration? Look at the two pictures below. The football players are pushing with the same amount of force in both pictures. However, the mass of the sled is smaller in the picture on the left than the one in the picture on the right. Therefore, the sled on the left accelerates more than the one on the right. LOOKING CLOSER 6. Infer What could the football players pushing the sled do to make it move more quickly? The sled on the left accelerates more because its mass is smaller. Talk About It Give Examples In a small group, talk about times you have seen examples of Newton's second law of motion. How Does Force Affect Acceleration? Look at the other two pictures below. In the picture on the left, one person is pushing the car. The car has a large mass, and it does not accelerate very much. In the picture on the right, several people are pushing the same car. The mass of the car is the salne as in the left-hand picture. However, the force is greater. Therefore, the car on the right accelerates more. LOOKING CLOSER 7. Infer What would happen if the mass of the car on the right was greater? The car in the right-hand picture accelerates more because the force acting on it is greater. Interactive Reader 252

5 Name _ Class _ Date _ Newton's First and Second Laws continued CALCULATING FORCE FROM ACCELERATION Zoo keepers lift a lion on a stretcher. The total mass of the lion and the stretcher is 175 kg. The acceleration of the lion and the stretcher is m1s 2 upward. What force do the zoo keepers use to produce this acceleration? Step 1: List the given and Civen: Unknown: unknown values. mass, force, F m = 175 kg Step 2: Write the equation. acceleration, a = m/s 2 F=ma Step 3: Insert the known values and F = (175 kg) x (0.657 mjs2) solve for the unknown value. F= 115 kg x mjs2 = 115 N [JViath.~ 8. Calculate An object with a mass of 10.0 kg accelerates upward at 5.0 m/s 2 What force acts on the object? Show your work. So, the zoo keepers apply 115N of force to lift the lion and the stretcher. CALCULATING ACCELERATION FROM FORCE A sailboat has a mass of 655 kg. A force of 895 N pushes the sailboat forward. What is the sailboat's acceleration? Step 1: List the given and Given: Unknown: unknown values. mass, acceleration, a m = 655 kg Step 2: Write the equations. force, F= 895 N F=mo o=~ [MatH~Em 9. Manipulate Equations Show how to rearrange the equation for Newton's second law of motion in order to solve for mass. Step 3: Insert the known values and 895 kg x m/s2 a = 895 N = solve for the unknown value. 655 kg 655 kg a = 1.37 m/s 2 So, the sailboat accelerates at 1.37m1s 2 Interactive Reader 253

6 Name Section 1 Review _ Class _ Date _ SECTION VOCABULARY inertia the tendency of ~mobf~ct'to resist a.. ':-. change in motion unless an 'outside 'force' acts.. "on the obje~.,,;,'. :... \:;.(l;:}jir.!';..'./f,.~~.:i.g.::..:r, " Apply Concepts Which has more inertia, a car with a mass of 900 kg or a car with a mass of 1,500 kg? Explain your answer. 2. Infer Newton's first law of motion is sometimes called the "law of inertia." Why? 3. List Give two examples of Newton's second law of motion. 4. Calculate A baseball accelerates downward at 9.8 m/s 2 The force pulling the ball downward is 1.4 N. What is the mass of the baseball? Show your work. 5. Calculate A model airplane has a mass of 3.2 kg. Its propeller pulls it forward with a force of 7.0 N. What is the airplane's acceleration? Show your work. 6. Make Predictions Two cars are driving at the same speed. The cars have the san1e mass. One car is driving on an icy road. The other is driving on a dry road. Which car will be able to stop more quickly? Explain your answer. Interactive Reader 254

7 Name Class Date Newton's Third Law As you read this section keep these questions in' mind: What happens when one object exerts a force on another object? How can you calculate the momentum of an object? How does momentum change after a collision? What Is Newton's Third Law of Motion? Imagine kicking a soccer ball. The ball would move in a different direction. From Newton's first law, you know that the ball's motion could not have changed unless a force acted on it. Therefore, there must be a force acting on the ball. This force came from your foot. However, if you kicked a soccer ball, you would probably also feel a force on your foot. Where did this force come from? When you kick a soccer ball, your foot exerts a force on the ball. This force is called an action jorce. At the same time, the ball exerts a force on your foot. That force is called a reaction jorce. Sir Isaac Newton described the relationship between action forces and reaction forces in his third law of motion. Newton's third law of motion states that action forces ahyays produce reaction forces. It also states that action forces and reaction forces are always equal in size, but act in opposite directions. The figure below shows the sizes and directions of action and reaction forces when a person kicks a soccer ball. ') READINGTOOLBOX( - Organize After you read this section, create a Concept Map for momentum. Include the words momentum, mass, velocity, and direction in your map. According to Newton's third law, the foot and the soccer ball exert equal and opposite forces on each other. LOOKING CLOSER 1. Identify On the figure, label the action force and the reaction force. Interactive Reader 263

8 Name Newton's Third Law continued _ Class _ Date _ Critical Thinking- 2. Apply Concepts A student has a weight of 534 N. The student sits in a chair. When the student sits in a chair, how much force does the chair apply to the student? FORCE PAIRS An action force and the reaction force that results are called aforce pair. Newton's third law states that the forces in a force pair are equal in size, but opposite in direction. Youmay wonder why these forces do not cancel each other out, since they happen at the same time. The answer is that the forces act on different objects. For example, in the figure below, the action force acts on the water. The reaction force acts on the swimmer. The action force is the swimmer pushing the water backward. LOOK1NG CLOSER 3. Explain Why don't the action and reaction forces in the figure cancel each other out? The reaction force is the water ~. pushing the swimmer forward. ~ 4. Explain Why don't we notice Earth moving upward when an object falls toward the ground? EQUAL FORCES, UNEQUAL EFFECTS Imagine dropping a soccer ball. Earth's gravitational force pulls the soccer ball toward the ground. This is the action force. At the same time, the soccer ball exerts an equal gravitational force on Earth. This is the reaction force. The action force and the reaction force are the same size, but opposite in direction. It is easy to see the effect of the action force-the ball falls to the ground. Why don't you notice the effect of the reaction force-earth being pulled upward? The answer is that these two equal forces act on objects with very different masses. Recall Newton's second law of motion: A large mass will accelerate less than a small mass when you apply the sanle anlount of force. For example, the sanle amount of force acts on the soccer ball and Earth, but Earth's mass is much greater than the soccer ball's. Therefore, Earth's acceleration is much smaller than that of the soccer ball. Earth does move upward, but this acceleration is so small it is almost impossible to measure. ~ Interactive Reader 264

9 Name Newton's Third Law continued Class _ Date SUMMARY OF NEWTON'S THIRD LAW OF MOTION To summarize, there are four main ideas in Newton's third law of motion: ~ occur in pairs made up of an action force and a reaction force. Action and reaction forces are equal in size, but opposite in direction.. Action and reaction forces act on different objects. Equal forces acting on different objects may have different effects. DETERMINING THE EFFECTS OF FORCES You can use Newton's third law of motion to determine how an object will move. For example, think again about the falling soccer ball. You can measure its weight, which is equal to the gravitational force on it-the action force. Once you know the weight of the soccer ball, you know the size of the reaction force acting on Earth. If you know Earth's mass, you can calculate Earth's acceleration using Newton's second law of motion. What if you do not know the size of the force acting on an object? You may still be able to predict the object's motion using a quantity called momentum. Talk About It Brainstorm Think of 10 examples of force pairs. Describe each action force with a partner. Ask your partner to try to think of the reaction force. Then, have your partner share his or her list of action forces with you. Try to think of the reaction force for each of your partner's action forces. 2. Identify \Vhat is Newton's third law of motion? 3. Apply Concepts A skier pushes.her ski poles against the ground. She begins to move across the snow. Earth does not seem to move. Identify the action and reaction forces in this example, and e1l..'plainwhy the skier moves but Earth does not seem to.

10