1. Forces
Chapter 4

2. Motion Acceleration describes a change in velocity of an object
Velocity describes a change in the object’s position
What causes acceleration in the first place? Why do things move? What is the cause of motion?

3. Sir Isaac Newton 300 years ago he gave us the answer
Explained the way in which forces- pushes and pulls- influence motion
Summed up his explanations in three laws

4. Forces A force is a push or a pull on an object
The object is the system
The world around the object is the environment

5. Types of Forces Contact force acts on an object only by touching it
Long-range force is exerted without contact
Examples: magnets, gravity
Draw force diagrams

6. Newton’s Second Law of Motion
F = ma (a= Fnet / m)
Forces are vectors, so the total force on an object is the vector sum of all forces exerted on the object
The vector sum of 2 or more forces is called the net force

7. Measuring Forces
F = ma
Where mass is measured in kg and acceleration is measured in m/s2
So Force is measured as (kg x m)/ s2
The SI unit for Force is the newton, (N)

8. Newton’s First Law of Motion
An object that is at rest will remain at rest or an object that is moving will continue to move in a straight line with constant speed, if and only if the net force acting on that object is zero (unless acted upon by an outside Force)
Also called the law of inertia

9. Inertia The tendency of an object to resist change
If an object is at rest, it tends to remain at rest
If it is moving at a constant velocity, it tends to continue moving at that velocity
If the net force on an object is 0, it is in equilibrium
Either at rest or moving at constant velocity

10. Causes of motion
A net force is something that disturbs a state of equilibrium
A change in velocity is caused by an unbalanced force
Changes in velocity or acceleration are the result of a net force acting on an object

11. Types of Forces
Friction
Normal
Spring
Tension
Thrust
Weight

12. Mass and Weight Mass is how much stuff is in an object
Weight is how the acceleration due to the Earth’s gravity pulls on the mass of an object
So weight is a force
F=ma so the force of gravity is mg
1 lb = N & 1 N = lb

13. Causes of friction
When two surfaces touch, the high points on each are in contact and temporarily bond
When you try to move one of objects, you must break the bonds
This is the cause of static friction

14. As the surfaces move past each other, the electrostatic forces, that caused the bonds, continue to create an attraction between the high points on the moving surfaces and this results in the weaker kinetic friction

15. Drag and resistance
When an object moves through the air or any other fluid, the fluid exerts a friction like force on the moving object.
This force depends upon the speed of the motion, size and shape of the object, and the density and kind of fluid

16. As velocity increases, so does the drag force
No net force means no acceleration, so velocity becomes constant
The constant velocity that is reached when the drag force equals the force of gravity is terminal velocity

17. Video- crushing barrel

18. Periodic Motion
There is one position in which the net force is zero- this is in equilibrium
When pulled away from equilibrium position, the net force on the system becomes nonzero, and pulls back toward the equilibrium point.

19. The force that restores the object is directly proportional to the displacement of the object, the motion is simple harmonic motion
The time needed to repeat one complete cycle of motion is the period (T)
Maximum distance the object moves from equilibrium is amplitude

20. Drawings
The upward force of the spring is directly proportional to the amount stretched- Hooke’s Law

21. Pendulum P.379 (1-3) A pendulum is also simple harmonic motion
Bob is suspended
Drawing
T= 2¶ √(l/g)
Mass does not matter
Often used to measure gravity of Earth

22. Interaction Forces system 1 system 2 system External Forces
interactions
system 1
system 2

23. Newton’s Third Law
For every action force there is an equal and opposite reaction force.
Interaction pair
FA on B = -FB on A
Page 124 (1-2) practice A
P. 132 (1-5) & P. 379 (1-3)