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. Balanced

5. The Body will Accelerate
Unbalanced
If the forces acting on a body 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

7. 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

8. Newton’s First Law

10. Newton’s First Law What do you mean by “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 do you mean by “Net Force”?
Net means: Final Sum, so net external force is the sum of all the forces acting on the object.

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

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

13. Why does the person continue to move?
Inertia!
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14. Newton’s Second Law F=ma
The acceleration of an object is directly proportional to the net external force acting on the object and inversely proportional to the object’s mass.
F=ma
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16. Force = mass x acceleration
Newton's 2nd Law of Motion:
Force = mass x acceleration
F = (ma)
Mass is in kg
Acceleration is in m/s2
Force is in kg x m/ s2 (or N)

17. 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

18. Forces that oppose motion

19. -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.
-Air resistance is the force air exerts on a moving object.

20. Friction What Causes Friction?
Microscopic, electrostatic interactions between contacting surfaces
Three types of friction:
Static Friction
Sliding Friction
Rolling Friction

21. Types of Friction Types of friction:
1. Static – force required to overcome inertia of a stationary object

22. Static Friction (FS)

23. Types of Friction (cont’.)
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.

24. CLIP
The amount of force required to overcome static friction is always greater than the force required to overcome sliding or rolling friction.

25. GRAVITY!! 1- Masses (greater mass means more gravity)
The force of gravity is 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!
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26. Calculating the Gravitational Force

28. Weightlessness
Gravity in
Space

29. 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.

30. Weight is measured in Newtons (N).
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’S A FORCE!

31. Could your weight change by simply going to the top of a mountain?

32. Weight W = (g)(m) Ex. Mass = 100kg Acc.= 9.81 m/s2 Weight = 981 N
When the mass of an object and the acceleration due to gravity are known, the weight of an object can be calculated.
W = (g)(m)
Ex. Mass = 100kg Acc.= 9.81 m/s2
Weight = 981 N

33. 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

34. Newton’s Third Law of Motion
…to every action force there is an equal and opposite reaction force

35. Action-Reaction forces act on different objects

37. 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

38. 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!!!!

39. Another Example of Projectile Motion

40. 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.

41. Circular Motion cont.’
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.

43. Amusement Park Forces