1. Forces & motion
2012

2. force Define: Push or a pull
Describe: a force is described by its direction in which it acts and its strength
Strength of force is measured in the Newton (N)
Direction and strength can be represented by an arrow
Force is in direction of the arrow
If you push a door you exert a force in a different direction than if you pull the door

3. Combining Forces
- The Nature of Force
net force = The combination of all forces acting on an object
If you exert a force of 120 N on a desk and your friend exerts a force of 150 N in the same direction. What is the net force on the desk?

4. Unbalanced Forces
- The Nature of Force
Unbalanced forces = result in a net force and cause motion.

5. Balanced Forces Balanced forces = do not cause motion. Net force = 0
- The Nature of Force
Balanced forces = do not cause motion.
Net force = 0
Balanced forces in opposite directions?
How will that affect motion?

6. Questions A force is a push or pull. What is a force?
- The Nature of Force
Questions
Question
Answer
A force is a push or pull.
What is a force?
What happens when forces combine?
Forces combine to produce a net force.

7. **Two main forces – friction and gravity**

8. Friction
A force that two surfaces exert on each other when they rub together
Acts in the opposite direction of the object’s motion
Depends on 2 factors:
1. how hard the surfaces push together
2. Types of surfaces involved

9. friction Measured in Newtons
Friction will eventually cause everything to stop
Rough surfaces = more friction

10. Important uses Walking Drive a car Slowing down – brakes Warm up your
hands
Erase a mistake
You could not walk without the friction between your shoes and the ground. As you try to step forward, you push your foot backward. Friction holds your shoe to the ground, allowing you to walk. Consider how difficult it is to walk on slippery ice, where there is little friction.

11. Problems with Friction
Make movement difficult - Go to push a box across the floor or drag something
Fall off your bike and stop yourself by sliding on your knees
Wastes energy: use extra fuel
Creates heat (conservation of energy energy changes form – heat)
Wears things out (moving parts, tires, shoes, use oil to reduce)

13. gravity Force that pulls object toward each other 9.8 m/s2
You are attracted to all the objects around you and all the objects around you are attracted to you.

14. Law of Gravitational Attraction
Any two objects are attracted to each other
Two factors that affect the gravitational attraction between objects:
1. mass
2. distance

15. Continued
We don’t notice our gravitational attraction between other objects because these forces are small compared to the Earth’s attraction.

16. Gravity: mass & distance

17. Mass
Amount of matter in an object
Units for mass: kilograms

18. weight
- Friction and Gravity
The force of gravity on a person or object at the surface of a planet is known as weight.
I have a revolutionary weight loss plan.
A trip to the moon, where you would weigh 1/6 of what you do on Earth
Why wouldn’t this weight-loss plan have the effect most people want (the amount of matter or mass of a person’s body would not change)

19. 1. What is the scientist’s mass?
2. What is exerting the greater gravitational force – Earth or the moon?
3. Which is a more accurate measurement – mass or weight?
What is the scientist’s mass?
What is exerting the greater gravitation force

20. Free fall Only force acting on an object is gravity
In free fall gravity is an unbalanced force  causes the object to accelerate

21. Free fall Acceleration:
change in direction
All objects regardless of their mass accelerate at the same rate.

22. Free Falling & velocity
For every second a falling object’s velocity increases by 9.8 m/s
Velocity: speed with a direction (down, SE, etc.)
Start at 0 sec = 0 m/s
1 sec = 9.8 m/s
2 sec = 19.6 m/s
3 sec = ?
29.4 m/s

23. Air Resistance
- Friction and Gravity
Falling objects with a greater surface area experience more air resistance.

24. Terminal velocity
Greatest velocity that can be attained by an object in free fall
Air resistance and velocity = balanced
Humans = ~ 124 miles per hour

25. Comparing and Contrasting
Gravity
Friction
Pulls objects toward one another
Effect on motion
Opposes motion
Types of surfaces involved, how hard the surfaces push together
Depends on
Mass and distance
Measured in
Newtons
Newtons