1. Equal forces in balance
Chapter 10 - Forces
Question – What causes an object to start moving, stop moving, or change direction
Answer - Forces
Force – a push or a pull
30 lbs lbs
Equal forces in balance
Balanced Forces

2. Equilibrium – when all forces acting on an object are in balance
If all forces are in balance there is no acceleration. (The object is not moving or is at constant velocity
100 lbs
Ground
100 lbs

3. If the forces are not in balance there will be an acceleration
Balanced Forces
The force exerted by the motor in a forward direction is equal the force of friction in the opposite direction.
Frictional force – a slowing force caused by objects or substances rubbing against each other.
If the forces are not in balance there will be an acceleration
50 mph
From wind resistance, gears, etc..

4. NEWTON’S FIRST LAW OF MOTION
An object at rest will stay at rest and an object that is moving will stay in motion unless acted on by an unbalanced force
An object will tend to do what it is already doing.
Inertia – the tendency of an object to resist changes in motion

5. NEWTON’S SECOND LAW OF MOTION
Fnet = m(a) a = Fnet/m
m/sec2
(meter) kg
Newton
Newton – the force required to accelerate a 1 kg mass 1 m/sec2
Acceleration and force are directly proportional . If we double the force, we double the acceleration.
Acceleration and mass are inversely proportional. If we double the mass, the acceleration is cut in half.

6. 3 types of friction Sliding (when objects slide over each other)
Rolling friction (less than sliding)
Fluid – when objects move through a fluid such as water or air

7. Gravity – attraction between all objects that have mass
The greater the mass the greater the gravitational attraction
Free fall – acceleration due to gravity is m/sec2
Air resistance – fluid friction of air
The higher the velocity, the greater the air resistance.
Terminal velocity – when air resistance equals gravitational force

8. Skydiver terminal velocity
120 mph if body is horizontal
Over 200 mph if body is vertical

9. Projectile Motion 9.8 m/sec2 9.8 m/sec2or
9.8 m/sec2
They will strike the ground at the same time. Gravity accelerates them both at the same rate

10. Mass – measure of the amount of matter an object contains
Weight – measure of gravitational pull measured in lbs on a spring scale
Is dependent on location
Mass – measure of the amount of matter an object contains
Measured on a balance scale (with counter weights)
Independent of location

11. NEWTON’S THIRD LAW OF MOTION
If one object exerts a force on another object, then the second object exerts an equal force (but in the opposite direction) on the first object.
These forces can be thought of as action and reaction forces.

12. Examples:
Rocket
Action force – rocket pushing out gases
Reaction force – gases pushing back against rocket
Gun
Action force – powder and gun push against bullet
Reaction force – bullet pushes back against powder and gun
Canoe
Action force – paddle pushes against water
Reaction force – water pushes back against paddle and canoe

13. Momentum Momentum = mass (velocity)
The greater the momentum, the harder an object is to stop. Momentum can be increased by increasing velocity or mass.
Which football player will be hardest to stop?
Light and slow
Heavy and slow
Light and fast
Heavy and fast

14. Law of conservation of momentum – the total momentum of any group of objects remains the same unless outside forces act on the objects. (friction is a force). This is used to analyze collisions.
Examples:
Before After
1 kg 0 m/sec 2 kg m/sec
1 kg 5m/sec 2 kg 0 m/sec
Momentum = 1 kg (5m/sec)
Momentum = 5 kg(m) /sec
Momentum = 2 kg (2.5 m/sec)
Momentum = 5 kg(m) /sec

15. In opposite directions
Before Shot
bullet and gun
momentum = 0
After Shot
Gun Bullet
In opposite directions
Momentum = 1 kg (10 m/sec)
Momentum = 10 kg(m)/sec
Momentum = .01 kg (1000 m/sec)
Momentum = 10 kg(m)/sec

16. Collision of bullet and clay
Bullet Clay
M = .01 kg (1000 m/sec) M = .99 kg (0m/sec)
M = 10 kg(m)/sec M = 0
After Collision Bullet in Clay
Momentum = (.01 kg kg) (10 m/sec)
Momentum = 10 kg(m)/sec

17. Satellites What keeps a satellite in orbit?
Imagine you are standing on the top of a mountain and can throw a rock as fast as you want. Can you put the rock in orbit by throwing the rock straight out? How? Draw a diagram and explain the forces involved.
If the velocity is perfect the drop of the rock due to gravity matches the curvature of the earth.