1. Laws of Motion
Chapter Two

2. What is force? Force A push or pull on an object.
Can always be applied to another person or object through either touch or no touch
SI Unit for force is newton (N)
Two types of force:
Contact force
Non-contact force

3. What is Force? Contact force Non-contact force
The push or pull of one object by another that is touching it
Can be either weak or strong
Weak Example: pressing keys on a keyboard
Strong Example: Tectonic plates
Force that one object can apply to another object without touching it
Examples:
Magnetic force
Gravity
Static electricity

4. What is Force? Forces have Both
Strength
Direction
Represented by arrows
Length of the arrow shows how much strength is being applied to an object
Different things can happen when applying force from different directions

5. What is gravity?
An attractive force that exists between all objects that have mass
Mass
The amount of matter in an object
Measured in kilograms
So, what is the relationship between the Earth and gravity?
Law of Universal Gravitation
Developed in the late 1600s by Sir Isaac Newton
States that all objects are attracted to each other by a gravitational force
The strength of the force depends on the mass of the object

6. What is Gravity? Gravitational Force and Mass
Gravitational Force and Distance
As the mass of an object increases, so does the gravitational force
The attraction between objects decreases as the distance increases
In other words, the further away objects are from each other, then the lesser the attraction between the two objects

7. Weight-A Gravitational Force
Relationship between Weight and Mass
Definition
An object’s weight is proportional to its mass
Example:
If one object has twice the mass of another object, then it also means that it has twice the weight of the second object
The gravitational force exerted on an object

8. What is Friction Friction Different types of friction:
A force that resists the motion of two surfaces that are touching
In other words, friction is what slows you down
Different types of friction:
Static friction
Sliding friction
Fluid friction

9. What is Friction? Static friction Sliding friction
Prevents surfaces from sliding past each other
The strength of static friction changes to match the applied force
In other words, when the two forces are equal to each other, then no movement will occur
Opposes the motion of surfaces sliding past each other
In other words, when the object is moving because there is a greater applied force, then the sliding friction will not change

10. What is friction? Fluid friction
Is friction between a surface and a fluid such as water or air
Examples:
Air resistance
Friction between a surface and air

11. What is Friction What causes friction? Reducing friction
Caused through two different methods
Microscopic roughness
Microscopic dips and bumps of one object rubbing against the dips and bumps of the other object
Weak electrical charges
When the positive charge of one surface comes in contact with the negative charge of the other surface, an attraction occurs between the two particles
Two ways to reduce friction
Using some sort of lubricant
Examples:
Soap
Oil
Reducing the surface area of an object can help in reducing fluid friction

12. What is Net Force?
The combination of all the forces acting on an object
The way in which these forces are combined depends on the directions of the forces applied to an object

13. What Is Net Force? Combining Forces in the Same Direction
Combining Forces in Opposite Directions
When applied in the same direction, then the net force is the sum of the individual forces
In other words, the direction of the net force is the same as the direction of the individual forces
When calculating in the same direction, then you would add them together
Both will be going in a positive direction
When the forces are being applied in opposite directions, then the net force is still the sum of the forces
However, one force would be in the positive direction, while the other force would be in the negative direction

14. What is Net Force? Balanced Forces Unbalanced Forces
Forces acting on an object that combine and form a net force of zero
Doesn’t change the motion of an object
Forces acting on an object that combine and form a net force that is not zero
Will change the motion of an object

15. Newton’s First Law of Motion
Developed by Isaac Newton
States that if the net force of an object is zero, the motion of the object is zero

16. Newton’s First Law of Motion
Balanced Forces and Motion
Unbalanced Forces and Motion
Cause no change in an object’s velocity
True whenever an object is either at rest or in motion
Newton’s law only applies to this concept
When unbalanced forces act on an object at rest, the object starts moving
When unbalanced forces act on an object in motion, then the object’s speed, direction of motion, or both will change

17. Newton’s First Law of motion
Inertia
Why do objects stop moving?
The tendency of an object to resist change in its motion
In other words, inertia is what helps keep an object in place
Inertia helps the object to stay in place, while friction helps in resisting movement
An object can only move when a greater force is applied to the object in order to overcome friction.

18. Unbalanced Forces Unbalanced Forces and Velocity
Unbalanced Forces on an Object at Rest
Recall: What is velocity?
Only unbalanced forces will change an object’s velocity
Example:
Ball resting in your hand
Is the ball moving?
So therefore, it is what?
However, if a force acts on the ball, what’s going to happen?
When unbalanced forces act on an object, the object begins moving in the direction of the net force.

19. Unbalanced Forces on an Object in Motion
Speeding Up
Slowing Down
If the net force acting on a moving object is in direction that the object is moving, the object will speed up.
In other words, if the object and force are going in the same direction, then object speeds up.
If the net force acting on the object is in the opposite direction of the moving object, then the object slows down.
In other words, if the net force is in the opposite direction, then the object slows down.

20. Unbalanced Forces Changes of Direction of Motion
Unbalanced Forces and Acceleration
Unbalanced forces can change an object’s velocity by also changing its direction.
Example:
Tectonic plates
Change the direction of the Earth’s crust by pushing against each other
Unbalanced forces can make an object change its acceleration also by changing its speed, direction, or both.

21. Newton’s Second Law of Motion
Developed by Isaac Newton
States that the acceleration of an object is equal to the net force acting on the object divided by the object’s mass
SI Units:
Acceleration: m/s²
Mass: kg
Force: N
Helps to describe the relationship between an object’s change in velocity over time, or acceleration, and unbalanced forces acting on an object
Applies the concept to forces acting in a line

22. Circular Motion
Any motion in which an object is moving along a curved path
Newton’s Second Law of Motion also applies to this concept as well

23. Circular Motion The Motion of Satellites and the Planets
Centripetal Force
The Motion of Satellites and the Planets
A force that acts perpendicular to the direction of motion toward the center of the curve
Example:
Ball on a string
Satellites also experience centripetal force
Satellite
An object in space that orbits another object
What keeps a satellite orbiting?
Gravity
Is the centripetal force that helps to keep an object in motion by changing its direction

24. Opposite Forces
When an object applies a force on another object, the second object applies a force of the same strength on the first object, but the force is in the opposite direction.

25. Newton’s Third Law of Motion
Recall: Newton’s first two laws of motion describe the effects of balanced and unbalanced forces on one object.
Looks at the forces between two objects
States that when one object exerts a force on a second object, the second object exerts an equal force in the opposite direction of the first object
Example:
Gymnast pushing against a vault

26. Newton’s Third Law of Motion
Forced Pairs
Action and Reaction
The forces two objects apply to each other
Usually equal in size and opposite in directions
Won’t cancel each other out when calculating net force because it acts on two different objects
In a force pair, there are action and reaction forces.
Action forces
The force that you apply
Reaction forces
The force that pushes back against you
In other words, for every action, there is an equal and opposite reaction.

27. Newton’s Third Law of Motion

28. Momentum
Because action and reaction forces don’t cancel each other, they can change the motion of the objects.
Useful way to describe changes in velocity in action and reaction forces is by looking at the momentum
What is momentum?
A measure of how hard it is to stop a moving object
Newton’s first two laws of motion relate to momentum

29. Momentum Newton’s Second Law of Motion Newton’s First Law of Motion
Recall:
If the net force of an object is zero, its velocity doesn’t change
In relation to momentum, it means that the momentum of the object won’t change either
Recall:
The net force of an object is the product of its mass and its change in velocity
In relation to momentum, momentum is also the product of mass and velocity, and, therefore, the force of the object is equal to the change in momentum.

30. Conservation of Momentum
Example:
A cue ball hitting the other balls on a pool table
When you hit a cue ball with a pool stick, does it have momentum?
If so, how do you know?
When that cue ball hits the other balls, what do you think is going to happen to the cue ball’s velocity and momentum?
Why?

31. Conservation of momentum
Law of Conservation of Momentum
Types of Collisions
In any collision, one object transfers momentum to another object.
However, the overall momentum won’t change.
Why?
Law of Conservation of Momentum
States that the total momentum of a group object stays the same unless outside forces act on the objects
Outside forces include:
Friction
Because it decreases the velocities and causes momentum to be lost
Objects collide with each other in different ways.
Two types of collisions:
Elastic
When the colliding objects bounce off each other
Example:
Two balls hitting each other and bouncing off each other
Inelastic
When the colliding objects stick together
Two football players tackling each other