1. Chapter 8 Lesson 3
Forces

2. What is Force? A force is a push or pull on an object
An example would be pushing the classroom door open
The unit for force is the newton (N)

3. Contact Forces A contact force is a push or pull
one object applies to another
object that is touching it
This contact force can be
small such as a finger pushing
the button on an elevator or as
big as a wrecking ball crashing in
to a building

4. Noncontact Force A noncontact force is a force
that one object applies to
another object without touching it
An example would be the attraction
between metal and a magnet
and gravity

5. Gravity
Gravity is an attractive force that exists between all objects that have mass
Mass is the amount of matter in an object
The Law of Gravity was developed by Sir Isaac Newton
This law states that all objects are attracted to each other by a gravitational force.
The strength of the force depends on the mass of each object and the distance between them

6. Gravitational Force Depends on Mass
If the mass of an object increases, so does its gravitational force between it and another object
The gravitational force between you and Earth is large because Earth’s mass is large. That is what holds you to Earth.
The GF between you and your pencil is small because your mass and the pencil’s mass is small compared to the Earth’s mass. You do not feel the attraction even though it is present

7. Gravitational Force Depends on
Distance
As 2 objects move apart, the GF between them decreases

8. Mass and Weight are DIFFERENT
Weight is a measure of GF acting on an object’s mass
Weight depends on the mass of the 2 objects and the distance
between them
When the weight of 2 objects on Earth are compared,
the one with the greater mass will weigh more
Those same two objects will have a different weight on
the moon but their masses will be the same as they are
on Earth but the weight will be different because the
mass of the moon is less than the mass of the Earth.

9. Friction Friction is a contact force that resists the sliding motion
of 2 surfaces that are touching
Rough surfaces or materials tend to produce more friction

10. Effects of Friction Friction causes things to slow down
The heavier the object, the more friction it experiences
Friction acts in the opposite direction of the object’s motion

11. Air Resistance
Air resistance is the frictional force between air and object’s
moving through it
The more surface area an object has, the more air resistance it
experiences
For example, a flat piece of paper compared to a
crumbled ball piece of paper

12. Combining Forces When more than one force acts on an object,
the forces combine and act as one force
The sum of all the forces acting on the object
is called the net force

13. Combining Forces (cont)
When 2 forces are acting on an object but
in opposite directions, the direction of the force
must be included when you add them
The positive direction is usually to the right
The negative direction is usually to the left
If the force to the right is the same as the force to the left, the object does not move

14. Balanced Forces
If the net force on an object is 0N, the forces acting on the object are called balanced forces
There is no movement of the object

15. Unbalanced Forces
When the net forces acting on an object are not 0, the forces acting on the object are called
unbalanced forces
The object will move in the direction in which the force is greater.
Unbalanced forces
cause a change in
the object’s velocity
It will change the
objects speed or
direction

16. Newton’s 1st Law of Motion
If the net force acting on an object is zero, the motion of the object does not change
This is also known as the law of inertia
The person in the car flies out because the car and the person are traveling at 70mph and then the car comes to a sudden stop because it hits the wall but the person in the car is still traveling at the 70mph

17. Newton’s 2nd Law of Motion
States that acceleration of an object is equal to the net force exerted on the object divided by the object’s mass
This means that the greater the mass, the greater the force needed to accelerate the object at the same rate
Acceleration is the same direction as net force
A=f/m

18. Newton’s 3rd Law of Motion
States that for every action there is an equal and opposite reaction
When one object exerts a force on a second object, the second object exerts a force of the same size but in the opposite direction
These equal and opposite forces are called force pairs

19. Which one of Newton’s laws is also known as the law of inertia?
Newton’s 2nd law
Newton’s 1st law
Newton’s 3rd law
Newton’s 4th law

20. Your mass will change if you go to the moon
True or false:
Your mass will change if you go to the moon
False, your weight will change but your mass will stay the same

21. A person pushes a cart forward with a force of 20N
A person pushes a cart forward with a force of 20N. A friction force of 5N pushes the cart backwards. What is the net force on the cart?
15N backward
15N forward
25N backward
25N forward