1. Forces and Motion A Force is a push or a pull.
A force can change the motion of an object.
Not always

2. Net Force When two or more forces act on an object, they combine.
This is called the net force.
Two forces acting equally in opposite directions are called balanced forces.
No change in motion occurs with a balanced force.

3. Net Force Cont. When forces are not equal . . .
This is an unbalanced force.
Now a change in motion occurs.

4. Inertia The tendency of an object to resist any change in its motion.
This is Newton’s First Law of Motion.
An object in motion remains in motion, an object at rest remains at rest
. . . unless acted upon by an outside unbalanced force.

5. Questions What’s happening in this picture?
How is it related to Inertia?

6. Newton’s 2nd Law
If an object is accelerating, there must be a force acting on it.
F = ma
F is force, measured in kg.m/s2 or Newtons(N).
m = mass, a = acceleration

7. Calculating Force a = 10m/s/s m = 15kg
What is the force required to accelerate this object?
F = 10 x 15
150 N

8. You Try One!!
Turn to page 69 in your book.
Try problem # 2
Now try #1

9. Friction Friction force opposes motion.
Sliding friction – two objects sliding against each other
Rolling friction – one object rolling across the other (wheel)
Fluid friction – resistance in a fluid, like water or air

10. Friction Cont. Sliding  Rolling  Fluid Decreasing magnitude.
You can turn sliding into rolling by adding wheels.
Turn sliding into fluid by adding a lubricant

11. Air Resistance Opposes motion of object through the air.
Terminal Velocity – when the pull of gravity is equaled by air resistance.
Balanced forces = no change in motion. (no acceleration)

12. This skydiver, with little surface area, will soon reach a terminal velocity fast enough to splatter him all over the ground.

13. So the skydiver increases his surface area that will come into contact with the resistant force.
Now he can land safely.
Skydiver

14. Gravity
An attractive pulling force between any two objects
Depends on two factors
Mass of objects
Distance between objects
Galileo and the Tower 
On the Moon

15. Calculating with Gravity
Use the same formula F=ma
The acceleration of a falling object near the earth’s surface is around 9.8m/s/s
a is always 9.8!
We can calculate the Force of gravity on an object by using this acceleration and the mass of the object.

16. Force of Gravity cont.
What will the force be of an elephant falling to the earth with a mass of 1000kg? (before his parachute opens, obviously)
F = 1000 x 9.8
9800 N
Can actually think of it as F=mg
This is the formula for weight.

17. Weight SI units are N Weight and mass are not the same!!
Weight = mass x gravity
Weight is a force! Mass is not!

18. Weight and Mass

19. Obey the Law

20. Newton’s 3rd Law
To every action force, there is an equal and opposite reaction force.
You push on the wall, the wall pushes back.
Forces act in pairs.

21. 3rd Law Cont. Think about rain. Gravity pulls on the drops.
Fluid friction – air resistance – pushes back.

22. Your feet push on the floor . . .
The floor pushes back.
Think of an example on your own.
Friction?
Brain POP

23. Momentum Shows the relationship between mass and velocity.
p = momentum
Why can’t we use m?
m = mass, v = velocity

24. A Conundrum
A train moving at .2km/hr can have as much momentum as a bullet moving at 2000km/hr.
How can this be?
These two object cannot be much more different!

25. The train has a lot of mass, but little velocity.

26. The bullet has a lot of velocity but very little mass.

27. Try one! Labeling momentum. . . p = mass(kg) x velocity(m/s)
So the units used are kg*m/s
Kilogram meters per second
What is p for a car with a mass of 1300kg moving at 28m/s?
36,400 kg*m/s

28. Momentum Cont. Momentum can be transferred.
This is the Law of Conservation of Momentum.
When two objects collide, their total momentum stays the same.