Chapter 5

1. Chapter 5 Matter in Motion

2. VOCABULARY • Motion Force • Newton Friction • Net force Gravity • Weight Acceleration • Mass Velocity • Speed

3. Observing Motion You might think that the motion of an object is easy to detect—you just observe the object. But you actually must observe the object in relation to another object that appears to stay in place. The object that appears to stay in place is a reference point. When an object changes positions over time when compared with a reference point, the object is inmotion. When an object is in motion, you can describe the direction of its motion with a reference direction, such as north, south, east, west, or up and down.

4. Common Reference Points The Earth’s surface is a common reference point for determining position and motion. Non-moving objects on Earth’s surface, such as buildings, trees, and mountains, are also useful reference points.

5. Speed Depends on Distance and Time The rate at which an object moves is its speed. Speed depends on the distance traveled and the time taken to travel that distance. The SI unit for speed is meters per second (m/s). Kilometers per hour, feet per second, and miles per hour are other units commonly used to express speed.

6. Determining Average Speed • Most of the time, objects do not travel at a constant speed. For example, you probably do not walk at a constant speed from one class to the next. Therefore, it is very useful to calculate average speed using the following equations: • Average speed = total distance total time

7. Recognizing Speed on a Graph Suppose a person drives from one city to another. The blue line in the graph shows the distance traveled every hour. Notice that the distance traveled every hour is different. This is because the speed (distance/time) is not constant—the driver changes speed often because of weather, traffic, or varying speed limits.

9. Recognizing Speed on a Graph (cont) The average speed can be calculated by adding up the total distance and dividing it by the total time: Average speed = 360 km = 90 km/h 4h The red line shows the average distance traveled each hour. The slope of this line is the average speed.

10. Velocity: Direction Matters The speed of an object in a particular direction is the object’s velocity. Be careful not to confuse the terms speed and velocity: they do not mean the same thing! Because velocity must include direction, it would not be correct to say that an airplane’s velocity is 600 km/h. However you could say the plane’s velocity is 600 km/h south. Velocity always includes a reference direction!

11. Velocity (cont) You can think of velocity as the rate of change of an objects position. An objects’ velocity is constant only if its speed and direction don’t change. Therefore, constant velocity is always along a straight line. An objects’ velocity will change if either its speed or direction changes.

12. Velocity (cont) For example, if a bus traveling at 15m/s south speeds up to 20m/s, a change in velocity has occurred. But a change in velocity also occurs if the bus continues to travel at the same speed but changes direction of travel to the East.

13. Combining Velocities If you’re riding in a bus traveling East at 15 m/s, you and all the passengers are also traveling at a velocity of 15 m/s east. But suppose you stand up and walk down the bus’s aisle while it is moving. Are you still moving at the same velocity as the bus? No! The next slide shows how you can combine velocities to determine the resultant velocity.

15. Acceleration: The Rate at Which Velocity Changes Imagine that you are in-line skating and you see a large rock in your path. You slow down and swerve to avoid the rock. A neighbor sees you and exclaims, “That was great acceleration! I’m, amazed that you could slow down and turn so quickly!”. You’re puzzled. Doesn’t acceleration mean to speed up? But you didn’t speed up—you slowed down and turned. So how could you have accelerated?

16. Define Acceleration Although the word acceleration is commonly used to mean “speed up”. There’s more to its meaning scientifically. Acceleration (ak Sel uhr AY shuhn) is the rate at which velocity changes. To accelerate means to change velocity. You just learned that velocity changes if speed changes, direction changes, or both.

17. Define Acceleration(cont) • So your neighbor was right! Your speed and direction changed, so you accelerated. • Keep in mind that acceleration is not just how much velocity changes. It is also how fast velocity changes. The faster velocity changes, the greater the acceleration is.

18. Calculating Acceleration You can calculate acceleration by using the following equation: Acceleration= final velocity – starting velocity time it takes to change velocity

19. Calculating Acceleration(cont) • Velocity is expressed in meters per second (m/s), and time is expressed in seconds (s). Therefore acceleration is expressed in meters per second per second (m/s/s). • Suppose you get on your bicycle and accelerate southward at a rate of 1 m/s/s. (like velocity, acceleration has speed and direction.) This means that every second, your southward velocity increases by 1 m/s as shown on the next slide.

20. Calculating Acceleration(cont) After 1 second, you have a velocity of 1 m/s south, as shown in Figure 4, on page 113. After 2 seconds, you have a velocity of 2 m/s south. After 3 seconds, you have a velocity of 3 m/s south, and so on.

22. Calculating Acceleration(cont) If your final velocity after 5 seconds is 5 m/s south, your acceleration can be calculated as follows: Acceleration = 5 m/s – 0 m/s = 1 m/s/s south 5s You can practice calculating acceleration by doing the Math Break (next slide).

23. Math Break Use the previous equation to do the following problems: • A plane passes over Point A with a velocity of 8,000 m/s north. Forty seconds later it passes over Point B at a velocity of 10,000 m/s north. What is the plane’s acceleration from A to B? Answer: 50 m/s/s north

24. Math Break (cont) Use the previous equation to do the following problems: 2. A coconut falls from the top of a tree and reaches a velocity of 19.6 m/s when it hits the ground. It takes 2 seconds to reach the ground. What is the coconut’s acceleration? Answer: 9.8 m/s/s downward

25. Examples of Accelerations • Accelerations also occurs when velocity decreases. In the skating example, you accelerated because you slowed down. Acceleration in which velocity decreases is sometimes called negative acceleration or deceleration. • Remember that velocity has direction, so velocity will change if your direction changes. Therefore, a change in direction is acceleration, even if there is no change in speed.

26. More Examples

27. QUIZ • What distinguishes the measurement of speed from that of velocity and acceleration? 2. What is centripetal acceleration? • Speed does not involve direction, as both velocity and acceleration do. • 2. Acceleration that occurs in circular motion.

28. QUIZ (cont) • How do you calculate speed? Velocity? Acceleration? 3. Divide the distance traveled by the time; divide the distance and direction traveled by the time; subtract the starting velocity from the final velocity, and divide by the time it takes to change velocity.

29. Circular Motion:Continuous Acceleration Does it surprise you to find out that standing at Earth’s equator is an example of acceleration? After all, you’re not changing speed, and you’re not changing direction…or are you? In fact, you are traveling in a circle as the Earth rotates. An object traveling in a circular motion is always changing its direction. Therefore, its velocity is always changing, so acceleration is occurring. The acceleration that occurs in circular motion is known as centripetal (sen TRIP uhtuhl) acceleration.

30. Did Acceleration Occur?Why or Why Not? • You are riding your bike at 10 km/h. Ten minutes later, your speed is 6 km/h. • You ride your bike around the block at a constant speed of 11 km/h. • You ride your bike in a straight line at a constant speed of 10 km/h. • Acceleration occurred because speed decreased. • Acceleration occurred because direction changed. • No acceleration occurred because neither speed nor direction changed.

31. Recognizing Accelerationon a Graph Suppose that you have just gotten on a roller coaster. The roller coaster moves slowly up the first hill until it stops at the top. Then you’re off, racing down the hill! The following slide shows your acceleration for the 10 seconds coming down the hill. You can tell from the graph that your acceleration is positive because your velocity increases as time passes. Because the graph is not a straight line, you can also tell that your acceleration is not constant for each second.

33. What Is A Force? • You often hear the word force in everyday conversation: “That storm has a lot of force!” “Our basketball team is a force to be reckoned with” “A flat tire forced me to stop riding my bicycle” “The inning ended with a force-out at second base”

34. What Is A Force? • A force is simply a push or a pull. All forces have both size and direction. • Forces are everywhere. In fact, any time you see something moving, you can be sure that its motion was created by a force. Scientists express force using a unit called the newton (N). The more newtons, the greater the force.

35. Forces in Combination Often more than one force is exerted on an object at the same time. The net force is the force that results from combining all the forces exerted on an object. So how do you determine the net force? The examples on the next couple of slides can help answer this question.

36. Forces in the Same Direction Suppose you and a friend are asked to move a piano for the music teacher. To do this, you pull on one end of the piano, and your friend pushes on the other end. Together, your forces add up to enough force to move the piano. This is because your forces are in the same direction. Because the forces are in the same direction, they can be added together to determine the net force. In this case, the net force is 45 N, which is plenty to move a piano—if it is on wheels, that is!

38. Forces in the Different Directions Consider two dogs playing tug of war with a short piece of rope. Each is exerting a force, but in the opposite direction. Notice that the dog on the left (next slide) is pulling with a force of 10 N and the dog on the right is pulling with a force of 12 N. Which dog do you think will win the tug of war?

40. Forces in the Different Directions Because the forces are in opposite directions, the net force is determined by subtracting the smaller force from the larger one. In this case, the net force is 2 N in the direction of the dog on the right. Give the dog a biscuit. When the forces are in different directions, you subtract the smaller force from the larger force to determine the net force.

41. Unbalanced and Balanced Forces If you know the net force of an object, you can determine the effect the force will have on the object’s motion. Why? The net force tells you whether the forces on the object are balanced or unbalanced.

42. Unbalanced Forces Produce a Change in Motion When the net force on an object is not zero, the forces on the object are unbalanced. Unbalanced forces produce a change in motion (acceleration). In the two previous examples, the receivers of the force—the piano and the rope—move. Unbalanced forces are necessary to cause a non-moving object to start moving.

43. Unbalanced Forces Produce a Change in Motion (cont) Unbalanced forces are also necessary to change the motion of moving objects. For example, consider a soccer game. The soccer ball is already moving when it is passed from one player to another. When the ball reaches the second player, the player exerts an unbalanced force—a kick—on the ball. After the kick, the ball moves in a new direction and with a new speed.

44. Unbalanced Forces Produce a Change in Motion (cont) Keep in mind that an object can continue to move even when the unbalanced forces are removed. A soccer ball, for example, receives an unbalanced force when it is kicked. However, the ball continues to roll along the ground long after the force of the kick has ended.

45. Balanced Forces Produce No Change in Motion When the force is applied to an object produce a net force of zero, the forces are balanced. Balanced forces do not cause a nonmoving object to start moving. Furthermore, balanced forces will not cause a change in motion of a moving object.

46. Balanced Forces Produce No Change in Motion (cont) Many objects around you have only balanced forces acting on them. For example, a light hanging from the ceiling does not move because the force of gravity pulling down on the light is balanced by an elastic force due to tension that pulls the light up.

47. Friction: A Force That Opposes Motion Frictionis a force that opposes motion between two surfaces that are touching.

48. Rougher Surfaces Create More Friction Rougher surfaces have more microscopic hills and valleys. Thus, the rougher the surface, the greater the friction. Pavement is much rougher than grass. Therefore, more friction is produced when you slide on pavement. This increased friction is more effective at stopping your sliding, but it is also more painful! On the other hand, if the surfaces are smooth, there is less friction.

49. Greater Force Creates More Friction The amount of friction also depends on the force pushing the surfaces together. If this force is increased, the hills and valleys of the surfaces can come into closer contact. This causes the friction between the surfaces to increase. Less massive objects exert less force on surfaces than more massive objects do.

50. SCIENCE HUMOR • Tom: This match won’t light. • Jerry: What’s the matter with it? • Tom: I don’t know; it worked a minute ago.