1. Motion, Forces and Energy
Chapter 2 – Forces
Section 1 – The Nature of Force

2. Vocabulary
Force
Newton
Net force
Unbalanced forces
Balanced forces

3. What is a force?
Common word, so need to be careful about a definition.
A force is any push or a pull.
If I push a chair, I say I exert a force on the chair.
Forces are always done by one object, and exerted on another.
Like velocity and acceleration, force has a direction.
Direction of the force is in the direction of the push or the pull.

4. Unit of force in U.S. is the pound.
The strength of the force is measured in the SI unit of the Newton (N).
Unit of force in U.S. is the pound.
Forces can be represented by arrows.
The direction of the arrow corresponds to the direction of the force.
Length of the arrow indicates the strength of the force.
Big arrow for big force
Small arrow for small force

5. Combining Forces
It is extremely rare for only one force to act on an object.
The combination of all of the forces on an object is called the net force.
The net force determines how an object moves as well as in which direction it moves.
More on this in section 3

6. Adding Forces
When two (or more) forces are in the same direction, we add them up to find the net force.
5N N = N
5N N = N

7. Subtracting Forces
When forces act in opposite directions, they also produce a net force.
Subtract the forces to find their net force.
5N N = 0
The net force will always be in the same direction as the greater force.
5N N = N

8. Unbalanced Forces
Whenever there is a non-zero net force acting on an object, the forces are considered unbalanced.
Unbalanced forces cause an object to accelerate.
Unbalanced forces result in a net force.

9. Balanced Forces
When forces are exerted on an object, but the forces cancel out, we call them balanced forces.
Balanced forces do not change an object’s motion (they don’t cause acceleration).

10. Motion, Forces and Energy
Chapter 2 – Forces
Section 2 – Friction and Gravity

11. Vocabulary Friction Static friction Sliding friction Rolling friction
Fluid friction
Gravity
Mass
Weight
Free fall
Air resistance
Terminal velocity

12. Friction
The force that two surfaces exert on each other when they rub against each other is called friction.
Note that friction does not need to involve two moving objects.
Two non-moving objects, like shoes yesterday deal with friction as well.

13. Causes of Friction
In general, smooth surfaces produce less friction than rough surfaces
The strength of the force of friction depends on two factors
How hard the surfaces push together
The types of surfaces involved
If surfaces pushed together harder, friction increases.
If surfaces are rougher, friction increases.
Friction ALWAYS acts in the direction opposite of the objects motion, or attempted motion.
Without friction, a moving object might never stop.

14. Types of Friction Static Friction
This type of friction acts on objects that are not moving.
Like with your shoes yesterday
Think: Is it harder to push something heavy while its moving or before its moving?
Should be harder before its moving.
Before its moving, you must overcome the static friction before it will move.
Once you do, static friction goes away, but another takes its place…

15. Sliding Friction Rolling Friction Also known as kinetic friction
This friction occurs when two solid surfaces slide over each other
Less friction than static friction.
Rolling Friction
When an object rolls across a surface
Even easier to overcome than sliding friction
Which is why things are on wheels

16. Last one Fluid friction
Occurs when a solid object moves through a fluid
Remember, a fluid is a liquid or gas
Easier to overcome than sliding friction
Air resistance is an example
Tends to increase as the speed of the object passing through it increases
Remember, all of these work OPPOSITE the desired direction of motion!

17. Gravity
Gravity is a force that pulls every object with mass towards every other object with mass
Gravity is universal
Applies everywhere
Responsible for apples falling from trees to the planets going around the Sun.
All objects with mass are affected by the gravity of all other objects with mass.
However, gravity is weak, so we don’t tend to notice small gravitational forces.

18. Factors Affecting Gravity
There are two factors that affect the force of gravity on an object
Mass of the objects
As mass increases, so does force of gravity
Distance between the objects
As distance increases, force of gravity decreases

19. If gravity is a weak force, why does the Earth exert a large gravitational force on us?
Because Earth’s mass is very big
More mass = more gravity
Because the distance between us and the Earth is so small
Less distance = more gravity

20. Weight vs. Mass
We talked about this 1st semester, but time for a re-cover.
Mass is the amount of matter in an object
Weight is the force of gravity acting on an object.
They are NOT the same thing.
Your mass does not change with location, weight does.

21. Gravity and Motion
When the only force acting on an object is gravity, that object is said to be in free fall.
In free fall, would the object experience balanced or unbalanced forces?
So would the object be accelerating or not accelerating?
Near the surface of the Earth, the acceleration of objects due to free fall is 9.8 m/s2.
Any two objects near the surface of the Earth in free fall will have this acceleration.

22. Air Resistance Free-fall only works if we ignore air resistance.
Air resistance is a type of fluid friction an object experiences as it moves through the air.
As an object speeds up, air resistance increases.
As an object falls, it speeds up, so air resistance increases. When the force of gravity = air resistance, no acceleration.
Once the falling object is no longer accelerating, we say it is at terminal velocity, which is the fastest speed a falling object will reach.

24. Motion, Forces and Energy
Chapter 2 – Forces
Section 3 – Newton’s First and Second Laws

25. Vocabulary
Inertia

26. What changes motion?
Since ancient Greece, people have been trying to determine how an object moves
Aristotle and “natural” vs. “violent” motion
Galileo Galilei (one of the world’s first real scientists), an Italian suggested that once an object is in motion, no force is necessary to keep it in motion.
In other words, force is only necessary to CHANGE motion
His ideas paved the way for Newton nearly 100 years later

27. First Law of Motion
Newton’s first law of motion restates (i.e. stole) Galileo’s ideas of motion.
Newton’s first law can be broken down into a few parts
An object at rest will remain at rest and
an object moving at a constant velocity will continue moving at a constant velocity
unless the object is acted upon an unbalanced force

28. In parts
If an object is not moving, it will not move until a force acts on it
Cleaning
If already moving, it will continue to move at a constant velocity until a force acts on it
Tennis ball with racquet
Ball rolling across floor
On Earth, gravity and friction often are the unbalanced forces that will change an object’s motion

29. Inertia
Whether an object is in motion or at rest, it will resist changes to its motion.
This resistance to changes in motion is called inertia.
Inertia is NOT a force
Newton’s first law is sometimes called the Law of Inertia.
Knowing about inertia allows us to explain certain aspects of motion
Stuff flying forward in the car when the brakes are hit

30. Inertia and mass The inertia of an object depends on mass
In fact, mass is also considered to be a measurement of inertia
So, more mass = more inertia
More inertia = more resistance to changes in motion
Think about the effects of pushing a heavy student vs. pushing a light student.

31. The Second Law of Motion
Newton’s second law relates two concepts
Relates how mass (inertia) is related to an object’s acceleration
Relates how acceleration is related to the net force acting on it
It’s an equation. 2 forms of the same equation:

32. To use these equations Mass must be measured in kilograms (kg)
Acceleration must be measured in m/s2
Net force must be measured in newtons (N)

33. Using this equation (math)
A speedboat pulls a 55-kg water-skier. The force causes the skier to accelerate at 2.0 m/s2. Calculate the net force that caused this acceleration.
First we must use the correct equation
Then plug numbers in
Does this mean the boat is pulling him with 110 N of force?

34. Changes in force and mass
If you want to increase your acceleration, two things you can do
If you decrease your mass, you will increase your acceleration
Or increase your force to increase your acceleration

35. Motion, Forces and Energy
Chapter 2 – Forces
Section 4 – Newton’s 3rd Law

36. Vocabulary
Momentum

37. Newton’s 3rd Law of Motion
Probably one of the most misunderstood laws in science
Remember, every force involves TWO objects
In fact, no force exists alone.
Forces come in pairs.
Think of one force as the ACTION force
And one force as the REACTION force
Doesn’t matter which is which

38. When object A exerts the action force on object B… This ALWAYS happens
Object B exerts an equal reaction force on object A
This ALWAYS happens
Examples
I push down on floor, floor pushes up on me (jumping)
Chair pushes up on me, I push down on chair
Earth pulls me down (gravity), I pull Earth up (gravity)
Fun in car rides with siblings
I push floor back, floor pushes me forward (walking)
So, forces ALWAYS exist in an action-reaction pair

39. However, a quandary… Do action-reaction forces cancel?
I push down on an object, object pushes up on me
Two forces, equal strength, in opposite directions
Do they balance each other?
Write down your answer.
No, they do NOT.
Why? Because each force occurs ON A DIFFERENT OBJECT.
You can only have balanced forces if forces for a single object cancel out.

40. Momentum We’re going to focus less on this than the book does
Whenever an object has mass and is in motion it has momentum.
To find momentum, multiply mass by velocity
But again, to have momentum, you must be moving.
Any object with mass and is moving has momentum.