1. Forces

2. What causes things to move?
Forces!
What is a force?
A push or a pull that one body exerts on another.

3. Newtons SI unit of force is the Newton (N).
Defined as the amount of force that when acting on a 1 kg object produces an acceleration of 1m/s2.
Therefore, 1N = 1 kg • m s2

4. Balanced

6. Unbalanced Forces
If the forces are "unbalanced" this means that there is a "net" or "resultant" force.
The body will do one of the following:
Speed Up
Slow Down
Change the Direction of its Motion
In other words the body will accelerate

8. Newton’s First Law What is a “Net Force”?
An object at rest stays at rest, and an object in motion stays in motion unless acted upon by a net external force.
What is a “Net Force”?
Net means: The sum of all the forces acting on the object.

9. Also called the Law of Inertia.
Newton’s First Law
Also called the Law of Inertia.
Inertia is the tendency of an object to resist any change in its motion.

10. Why does the ladder continue to move?
Inertia!

11. Why does the person continue to move?
Inertia!

12. Force = mass x acceleration
Newton's 2nd Law of Motion states: the net force acting on an object causes the object to accelerate in the direction of the net force
Force = mass x acceleration
F = (ma)
Mass is in kg
Acceleration is in m/s2
Force is in kg x m/ s2 (or N)

14. Force Problems:
What force is needed to give an object with a mass of 25.0 kg an acceleration of 15.0 m/s2 ?
Formula:
F = ma
Given:
Unknown:
m = 25.0 kg
a = 15.0 m/sec²
F = ?
F = (25 kg)(15 m/sec²)
F = 375 kg•m/sec² = 375 N

15. -Air resistance is the force air exerts on a moving object.
-Friction is a force that opposes motion between two surfaces that are touching each other.

16. Friction What Causes Friction?
Microscopic, electrostatic interactions between contacting surfaces

17. Types of friction:
1. Static – force required to overcome inertia of a stationary object
-requires the most force to overcome

18. Static Friction (FS)

19. Types of Friction:
2. Sliding – force required to keep an object sliding at a constant speed
3. Rolling – force required to keep an object rolling at a constant speed
-To decrease friction, surfaces can be lubricated with a liquid such as oil.

20. GRAVITY!! Force of gravity=an attractive force between objects.
The amount of GRAVITATIONAL FORCE between objects depends upon their:
1- Masses (greater mass means more gravity)
2- Distance apart (greater distance means less gravity)
*All objects fall toward the earth at the same rate of acceleration, regardless of their masses!*

21. Free fall and Terminal Velocity
The opposing force of air resistance increases with speed.
Objects accelerate towards the Earth until the force of gravity is cancelled or balanced by this air resistance.
This is the highest speed an object can reach.
Different objects have different terminal velocities.

22. Weight is the force of gravity between the earth and a body on its surface.
Weight ≠ mass!
Weight is measured in Newtons (N) because it is a force!

23. g=acceleration due to gravity
Weight
W = (g)(m)
g=acceleration due to gravity
M=mass
W=weight
Ex. Mass = 100kg Acc.= 9.81 m/s2
Weight = 981 N

24. Example Problem What is your weight if your mass 4.52 kilograms?
Given: Unknown:
m = 4.52 kg W = ?
g = a = 9.8 m/sec²
Formula: W = mg
W = (4.52kg)(9.8 m/sec²)
W = kg•m/sec² = N

25. Newton’s Third Law of Motion
…to every action force there is an equal and opposite reaction force
Action-Reaction forces act on different objects

26. Projectile Motion
Projectiles follow a curved path because of the Earth’s gravitational pull.
They have 2 types of motion that are independent of each other:
1. Horizontal motion
2. Vertical motion

27. Projectile Motion Which will hit the ground first?
A ball that is dropped straight down or a ball that is “shot” out horizontally?
* An object launched horizontally will land on the ground at the same time as an object simply dropped from the same height!!!!

28. Another Example of Projectile Motion

29. Circular Motion
An object moving in a circular motion at the same speed is accelerating toward the center because its direction is constantly changing.
Centripetal acceleration is acceleration toward the center of a curved path.

30. Centripetal force is the force that causes a moving object to move in a curved path
Without a centripetal force, an object in motion continues along a straight-line path.
With a centripetal force, an object in motion will be accelerated and change its direction.