1. Forces
SPS8. Students will determine relationships among force, mass, and motion.
a. Calculate velocity and acceleration.
b. Apply Newton’s three laws to everyday situations by explaining the following:
Inertia
Relationship between force, mass and acceleration
Equal and opposite forces
c. Relate falling objects to gravitational force
d. Explain the difference in mass and weight.

2. Forces
S8P3. Students will investigate relationship between force, mass, and the motion of objects.
b. Demonstrate the effect of balanced and unbalanced forces on an object in terms of gravity, inertia, and friction.
S8P5. Students will recognize characteristics of gravity and that it is a major kind of force acting in nature.
a. Recognize that every object exerts gravitational force on every other object and that the force exerted depends on how much mass the objects have and how far apart they are.

3. Forces Forces Newton’s 1st Law of Motion and Inertia
Balanced and Unbalanced Forces
Newton’s 2nd Law of Motion
Gravity and Weight
Centripetal force and acceleration
Newton’s 3rd Law of Motion
Friction

4. Force Fkick Fgravity Force
A push or a pull exerted on an object in order to change the motion of the object
Forces do NOT cause motion; they can cause accelerations (changes in motion).
To describe a force, you need to know two things:
The size of the force.
The direction of the force.
Force is measured in Newtons (N).
Fkick
Fgravity
What forces are being exerted on the football?

5. Newton’s First Law Newton’s First Law of Motion
An object at rest will remain at rest and an object in motion will remain in motion at a constant velocity (speed and direction) unless acted upon by an unbalanced force.
This law is sometimes called the Law of Inertia.

6. Newton’s First Law Newton’s First Law of Motion “Law of Inertia”
tendency of an object to resist any change in its motion
increases as mass increases
mass = amount of matter or stuff in an object
Mass is a measure of the amount of inertia.
Inertia is NOT a force.

7. Inertia
An object that has more mass is harder to move and harder to stop than an object that has less mass. The reason is that the object with the larger mass has greater inertia.
The larger the mass, the greater the inertia.

8. Inertia Why doesn’t an object stay moving forever?*
Inertia
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Why doesn’t an object stay moving forever?
If the Law of Inertia is true, then any moving object should move forever!
In outer space this is true because there is no friction.
Therefore, friction stops objects from moving forever.
Forces are NOT needed to keep an object in motion! *

9. Balanced Forces
Forces acting on an object that are equal in size and opposite in direction
Do not change an object’s velocity (no acceleration)
Do not cause an object to have a change in motion
Examples:
The forces acting on the box are balanced because they are equal in size and opposite in direction. Therefore, the box does not move.
A car moving in a straight line (no direction change) that moves at a constant speed has balanced forces acting on it.
A falling object that reaches terminal velocity (Fgrav = Fair) has balanced forces acting on it.

10. Unbalanced Forces = Unbalanced Forces
-Forces acting on an object that are not equal in size and/or not opposite in direction
-These forces cause an object to have a change in motion (speed and/or direction = acceleration).
An unbalanced force is a net force.
Examples:
Sliding a box across the floor
A car accelerating away from a stop sign
Someone kicking a soccer ball + = +

11. Net Force Fnet Ffriction Fpull Fnormal Fgravity Net Force
The sum of all forces acting on an object (balanced and unbalanced)
Balanced forces have a net force of 0 Newtons = No motion/No change in motion
Example: A tug-of-war match with both teams pulling with equal and opposite forces = No motion/Change in motion
Unbalanced forces have a net force that is greater than 0 Newtons = Change in motion (acceleration)
Example: A tug-of-war match with unequal teams with the stronger team pulling the other team in its direction
Fnet
Fgravity
Ffriction
Fpull
Fnormal

12. F = ma Newton’s Second Law Newton’s Second Law of Motion
The acceleration of an object depends on the mass of the object and the size and direction of the net force acting on it.
It is often expressed as:
Force = Mass x Acceleration
F = ma

13. Newton’s Second Law
The greater the force applied to an object, the more the object will accelerate.
It takes more force to accelerate an object with a lot of mass than to accelerate something with very little mass.
When mass increases and the amount of force is the same, acceleration decreases.
The player in black had more acceleration, thus he hit the opponent with a greater amount of force. (F=ma)

14. Newton’s Second Law
The greater the mass, the greater the force needed for the same acceleration: F = ma
The object will accelerate in the direction of the net force.

15. F = ma F m a F m Newton’s Second Law F: force (N) m: mass (kg)
a: accel (m/s2)
1 N = 1 kg ·m/s2

16. Gravity
Gravity: the force of attraction in the universe between all objects due to their masses
Gravity increases as:
mass increases
distance decreases between the centers of two objects
Gravity is also known as gravitational force.

17. Gravity The measure of the force of gravity on an object is weight.
Gravitational force (weight) increases as mass increases.
Weight is different from mass!!!
Mass is not a force. It is a quantity. It is always the same (constant) regardless of the location.
Weight depends on gravity. It changes based upon an object’s location.
A mass of 1 kg weighs 9.8 N.

18. W = mg Weight Weight the force of gravity on an object MASS WEIGHT
W: weight (N)
m: mass (kg)
g: acceleration due to gravity (m/s2)
MASS
always the same
(kg)
WEIGHT
depends on gravity
(N)

19. Gravity Acceleration Due to Gravity (g):
Acceleration due to gravity is the same for all objects regardless of mass.
On Earth, gravity causes all objects to accelerate toward its center at 9.8 m/s2 or 10 m/s2 if no other force (air resistance) is acting on them.
Ex. Does a feather fall as fast as an elephant?
No. This is because of air resistance.
However, in the absence of air, all objects (regardless of mass) fall at the same rate.

20. Free-Fall
Free-Fall
when an object is influenced only by the force of gravity
Weightlessness
the “sensation” produced when an object and its surroundings are in free-fall
the object is not weightless!
surroundings are falling at the same rate so they do not exert a force on the object

21. Uniform Circular Motion
Centripetal Acceleration
acceleration toward the center of a circular path
caused by centripetal force
Uniform Circular Motion

22. Newton’s Third Law Newton’s Third Law of Motion
For every action force, there is an equal and opposite reaction force.
When one object exerts a force on a second object, the second object exerts an equal but opposite force on the first.

23. Friction Friction Force that opposes the motion between 2 surfaces
Depends on the:
Types of surfaces
The amount of force
between the surfaces

24. Friction
When two surfaces are in contact, the microscopic hills and valleys of one surface stick to the tiny hills and valleys of the other surface. This contact causes friction.

25. Friction Friction is greater: Between rough surfaces
When there is a greater force between the surfaces (e.g. more weight)
What are some Pros and Cons of Friction?
Pros
Provides safety when walking
Traction for tires
Ability to stop a car
Cons
Injuries
Engine parts to wear out
Holes in clothing

26. Friction Ways to Reduce Friction:
Lubricants: substances that are applied to surfaces to reduce the friction between the surfaces.
Ex: motor oil, wax, grease
Ball bearings: can be placed between wheels and axels to make it easier for wheels to turn by reducing friction.
Make the objects smoother

27. Friction
Though friction occurs in different forms, all forms of friction have one thing in common: they will cause the motion of an object to slow down.
Types of friction
Static friction: a filing cabinet sitting on the floor, book on a table
Sliding friction: ice skating, sliding a desk
Rolling friction: soccer ball rolling on field, cars, bicycles
Air resistance – feather and leaf falling

28. *
Friction
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Static Friction: Frictional force that prevents two surfaces from moving past each other
Static = “not moving” *

29. *
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Friction
Sliding Friction: Frictional force that opposes the motion of two surfaces sliding past each other *

30. *
Friction
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Rolling Friction: Frictional force between a rolling object and the surface it rolls on *

31. *
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Friction
Air Resistance: Frictional force that opposes the motion of objects that move through the air *

32. Air Resistance Two objects will only fall at the
same rate if no other force is
present.
Example: Outer space
On Earth, we have air resistance: the force that air exerts on
a moving object
This force acts in the opposite direction of the object’s motion.

33. Air Resistance
The amount of air resistance on an object depends on the speed, size, and shape of the object.
Galileo Demonstration

34. Air Resistance  no acceleration  constant velocity  balanced forces
Terminal Velocity
maximum velocity reached by a falling object
increasing speed  increasing air resistance until…
Fair = Fgrav
 no acceleration
 constant velocity
 balanced forces