Forces and Motion

1. Forces and Motion Unit 19

2. What words can you think of to describe the motion of an object?

3. VELOCITY The word scientists use to describe how fast an object is moving in a certain direction.

4. Velocity is measured as: a change in distance per change in time change in distance change in time What are some things you can think of that are measured like this?

5. What are some examples of objects that move at high velocities? 220 mph 1600 mph

6. Extreme Velocities 17,000 mph 40,000 mph

7. What are some examples of lowvelocities? 50 yds/hr (.03 mph) 1.2 mph

8. What units would you use to measure the velocity of an object moving in this classroom? meters per second m s Meters per second tells us how many meters something can move in one second.

9. Converting Velocities 1 highway, 0 city 1 hwy, 0 city m/s into km/hr m s “What kind of mileage does this get?” 1000 m/km 3600 seconds/hr Just take the m/s multiply by 1000 and divide by 3600

10. NSF North Mississippi GK-8

12. Scientists have a special way to measure a change in velocity: Acceleration Acceleration is a change in velocity.

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. If you’re speeding up, you have positive acceleration. If you’re slowing down, you have negative acceleration.

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. Answers Negative Acceleration High Rate

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. Or you are a drag racer

19. Answer High positive acceleration because you’re speeding up quickly.

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. Answer Low negative acceleration because you’re slowing down slowly.

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. Answer Zero, because you are at constant velocity That velocity can be fast, slow, or not moving at all.

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 acceleration Hint: There are multiple steps

25. Answer First, high positive acceleration because it speeds up quickly as it leaves your hand.

26. Answer Then, as it rises, low negative acceleration, because it begins to slow down slowly because of gravity.

27. Answer At the top of the arch, it has zero acceleration, because it briefly stops moving upward and has constant velocity.

28. Answer As it begins to fall again, low positive acceleration, because it speeds up slowly.

29. Answer Finally, when you catch it, high negative acceleration because it slows down very quickly in your hand.

30. How Do I Graph the Motion of a Particle?

31. Car 1 is moving with a velocity of 1 m / sec There is zero acceleration

32. Car 2 has constant velocity throughout at 1 m /sec Its acceleration remains zero

33. Car 3 has a changing velocity throughout, finishing at 7 m / sec Acceleration is 1 m / sec2

34. Quick Review Velocity: speed of motion in a direction Acceleration: change in velocity What causes an object’s velocity to change?

37. Only a force can change the velocity of an object. That force can act to accelerate it NSF North Mississippi GK-8

38. Only a force can change the velocity of an object. That force can act to decelerate it

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. 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.