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# Force and motion

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### Force and motion

1. 1. Forces and Motion Unit 19
2. 2. What words can you think of to describe the motion of an object?
3. 3. VELOCITYThe word scientists use to describe how fast an object is moving in a certain direction.
4. 4. Velocity is measured as: a change in distance per change in time change in distance change in timeWhat are some things you can think of that are measured like this?
5. 5. What are some examples of objects that move at high velocities? 220 mph 1600 mph
6. 6. Extreme Velocities17,000 mph 40,000 mph
7. 7. What are some examples of low velocities? 50 yds/hr (.03 mph)1.2 mph
8. 8. What units would you use to measure the velocity of an object moving in this classroom? meters per second m sMeters per second tells us how many meters something can move in one second.
9. 9. 11 hwy, 0 city highway, 0 city Converting Velocities m/s into km/hr“What kind of mileage does this get?” m 1000 m/km s 3600 seconds/hrJust take the m/s multiply by 1000 and divide by 3600
10. 10. NSF North Mississippi GK-8
11. 11. What else can we know about anobject’s motion besides velocity? •What if the object speeds up? • What if the object slows down?What if the object’s velocity changes?
12. 12. Scientists have a special way to measure a change in velocity: Acceleration Acceleration is a change in velocity.
13. 13. Visualizing Acceleration• Here you can see how the ball travels a further and further distance each second.• That is because its velocity is continuing to increase.
14. 14. • If you’re speeding up, you have positive acceleration.• If you’re slowing down, you have negative acceleration.
15. 15. Example:You’re driving along in your car and suddenly have to slam on your brakes for a red light.Negative or positive acceleration?High or low acceleration?
16. 16. Answers• Negative Acceleration• High Rate
17. 17. Example:You’re an astronaut waiting in your shuttle for takeoff. Suddenly the rockets fire and you’re on your way.High or low positive or negative acceleration?
18. 18. Or you are a drag racer
19. 19. AnswerHigh positive acceleration becauseyou’re speeding up quickly.
20. 20. Example:You’re driving down the highway when your car runs out of gas and you slowly roll to a stop on the side of the road.
21. 21. AnswerLow negative acceleration becauseyou’re slowing down slowly.
22. 22. Example:• You are at cruising speed on the highway. You no longer feel any pressure against the seat or are lurching forward.• What is your acceleration?
23. 23. AnswerZero, because you are at constant velocityThat velocity can be fast, slow, or notmoving at all.
24. 24. Challenge Question: You throw a basketball up in the air. It rises to a certain height and then falls back to earth. Describe its accelerationHint: There are multiple steps
25. 25. AnswerFirst, high positive acceleration because it speeds upquickly as it leaves your hand.
26. 26. AnswerThen, as it rises, low negative acceleration, because itbegins to slow down slowly because of gravity.
27. 27. AnswerAt the top of the arch, it has zero acceleration, because itbriefly stops moving upward and has constant velocity.
28. 28. AnswerAs it begins to fall again, low positive acceleration,because it speeds up slowly.
29. 29. AnswerFinally, when you catch it, high negative accelerationbecause it slows down very quickly in your hand.
30. 30. How Do I Graph the Motion of a Particle? 30 Car 1 Car 2 Car 3 25 20 15 10 5 0 0 1 2 3 4 5 6 7
31. 31. 30 Car 1 Car 2 Car 325201510 5 0 0 1 2 3 4 5 6 7 Car 1 is moving with a velocity of 1 m / sec There is zero acceleration
32. 32. 30 Car 1 Car 2 Car 32520151050 0 1 2 3 4 5 6 7 Car 2 has constant velocity throughout at 1 m /sec Its acceleration remains zero
33. 33. 30 Car 1 Car 2 Car 325201510 5 0 0 1 2 3 4 5 6 7 Car 3 has a changing velocity throughout, finishing at 7 m / sec Acceleration is 1 m / sec2
34. 34. Quick Review• Velocity: speed of motion in a direction• Acceleration: change in velocity What causes an object’s velocity to change?
35. 35. • If you’re riding your bike and want to speed up, what do you do?• If you’re driving a car and want to slow down, what do you do? • If you’re playing soccer and want to move the ball, what do you do?
36. 36. Nothing will change speed unless acted upon by a force.If it’s sitting still, it will keepsitting still.If it’s moving fast, it willkeep moving fast.
37. 37. Only a force can change the velocity of an object.• That force can act to accelerate it NSF North Mississippi GK-8
38. 38. Only a force can change the velocity of an object.• That force can act to decelerate it
39. 39. Then why do things slow down?• Flying balls slow down and eventually stop because of the force of gravity.• Cars and bikes eventually slow down and stop because of the force of friction.
40. 40. Then why do things slow down?• In space with no friction and no gravity, objects will keep going at the same speed forever without a force to slow them down or speed them up.