Motion & forces

1. Motion & forces

2. Motion • Motion comes from matterand energyinteracting • A forceis a push or a pull • Examples of forces: gravity, magnetism, electricity, friction • Unit of force: Newton (N)

3. Motion • If forces are balanced, an object’s motion will stay the same (constant speed)

4. Motion • When forces are unbalanced, an object’s motion changes (either in speedor direction)

5. Reference • Motion is always judged with respect to a reference point (or frame of reference) • Are you sitting still right now? • No—you are traveling through space at 30 km/sec!

6. practice • If you walk on a moving train, is your speed measured relative to the train or relative to the ground? • Either way!

8. Motion can be described by its position, direction, and speed • Position: its location • Ex: 8181 State Route 294 • Distanceis the separation between two positions • Direction: which way it is going • Ex: Northwest • Speed: how fast it is going (distance divided by time) • Ex. 55 km/hr

9. Velocity: speed in a specific direction • Ex. 55 km/hr Northwest • Speed and velocity are rates. A rate is a change in one quantity compared with a change in another quantity (usually time).

10. practice • If you travel in a circular path at constant speed, is your velocity changing? Why or why not? • Your velocity is changing because your direction is changing

11. Constant velocity is the same speed in the same direction • Acceleration: change in velocity per unit time (distance/time/time) • Speeding up or slowing down • Final velocity – initial velocity time • Average speed: total distance traveled divided by total time

13. practice • The cheetah sped up from 5m/s to 7 m/s in 2 seconds. What was its acceleration? • (7-5)/2 = 2/2 = 1m/s2

14. Position vs time graphs • What does the flat line represent? • Standing still • What does a diagonal represent? • Moving forwards or backwards • Describe what is happening in this graph. • Move forward for 10 s, then stand still for 5 s, then move backwards for 25 s, then move forward for 15 s

15. Velocity vs time graphs • Constant speed: flat line • Both acceleration and deceleration look like a diagonal on a velocity/time graph

17. gravity • Which falls faster, a feather or a bowling ball? • Video • In space • Gravity acts on all falling objects the same (10 m/s2). The reason why a feather normally falls slower is because of air resistance (a type of friction). Without air resistance, they fall at the same rate.

18. Gravity • Newton’s Law of universal gravitation: all bodies in the universe are attracted to each other • Gravity is a force of attraction between all objects in the universe • Attractive force between all objects in the universe • Why things are pulled to the center of the Earth • Why does an apple fall towards the earth instead of the earth towards the apple? • The earth has much more mass so stronger gravity pull

19. gravity • Depends on massand distance • Larger masses exert more gravity • Smaller masses still exert gravity (it is just hard to tell) • Force of gravity decreases as distance between objects increases (get farther apart)

20. The force of gravitational attraction between two objects depends on the distance between the objects and their (1) buoyancies (3) masses (2) temperatures (4) shapes

21. A battery-powered cart pulls an 800-gram load across the floor. If the load were reduced to 400 grams, and the force used to move the cart remained the same, the cart would • (1) move slower • (2) move faster • (3) move at the same speed • (4) stop moving

22. gravity • What force keeps the moon hanging around? • GRAVITY • Orbits happen because an object’s forward motion keeps it from falling straight down (balance between forward motion and downward pull gives planets and satellites their circular paths)

24. gravity • Does the moon exert gravity on the earth? • How do we know? • G forces refer to the amount of acceleration experienced (1 G = normal acceleration due to gravity; 9 G is max humans can experience and survive)

25. friction • Why does the book stop? • Frictionis a force that opposes (goes in the opposite direction of) motion • Static friction: friction while not moving • Must be overcome for object to move • Sliding friction: friction while sliding • Makes things in motion slow/stop • Air resistance is friction in the air

26. friction • What tire surface would you rather have?

27. friction • Friction makes motion possible! • List things that would not be possible without friction! • What happens to motion when friction is decreased? • Think slippery roads, sidewalks, or oily paper.. • Motion speeds up or changes direction

28. friction • But too much friction can be a bad thing—why? • What if there is too much friction inside an engine? • Heat, malfunction (things break) • Or too much friction on your feet? • Blisters

29. friction • How do you move things against friction? • You need to apply a force greater than the friction force for something to move

30. friction • If you are wearing socks, are you more likely to slip on a carpeted floor or a polished linoleum floor? • How can you change the amount of friction? • Less friction: • Rolling friction (use rollers or wheels) • Lubricants

31. friction • To have more friction: • Sand • Tread • Non-slip surfaces • Etc.

32. Free fall & terminal velocity • Another word for a thrown object? • Projectile • Near the surface of the earth, acceleration due to gravity is about 10m/s2 • Every second you fall, you speed up by 10 m/s (10 meters per second, per second)

33. Free fall & terminal velocity • Free fall occurs when gravity (weight) is the only force acting on an object • The moon is in free fall • A skydiver is NOT • Terminal velocityis the maximum speed an object reaches while falling • In terminal velocity, the force of gravity equals the air resistance, so there is no acceleration (net force = 0)