1. Motion & forces

2. Motion Motion comes from matter and energy interacting
A force is 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 speed or 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
Distance is 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 mass and 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
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?
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?
Friction is 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 velocity is 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)