Forces and Newton's first law

1. Today’s Objectives
-Learn what a force is
-Learn what balanced and unbalanced forces are
-Learn Newton’s first law of motion

2. What is a Force?
A force is a push or pull on an object
A force has a size AND a direction
Is a force a scalar or vector quantity?
Examples of forces:
-Pushing your book across a table
-Pushing a box
A force must have a
direction, so it is a vector quantity

3. The Units of Force
Force is measured in Newtons
Newton is abbreviated “N”
Newton (N) = kg x m/s2
m/s2 is acceleration
So, one Newton is the amount of force that will accelerate a 1 kg
object at 1m/s2

Force is mass x acceleration f ma

4. Vector Notation
Is force a scalar or vector?
Vector.
We label vector quantities by putting an arrow over the letter.
For example, force = mass x acceleration…….

f ma
Arrow indicates that it is a vector quantity

5. Forces are shown as arrows, or vectors
The longer the arrow (vector), the greater the force
The direction of the arrow shows you the direction of the force
10 N east
N
5 N east
Vectors

6. Adding Forces
More than one force on an object will be added.
So, if two people are pushing something east, one pushing with
5N, the other with 10N, what will be the net force on the box?
10 N east
+
5 N east
15 N east = net force

7. Balanced and Unbalances Forces
Balanced forces: when 2 forces cancel each other (no movement)
 10 N force east 10 N force west
f 10 N 10 N 0N
There is no movement in balanced forces
Example of a balanced force??

8. Balanced and Unbalances Forces
Unbalanced forces: forces that do not cancel (create movement)
10 N force east 15 N force west
5 N west

f 15 N 10 N 5 Nwest
Object would move to the west in this example
Example of an unbalanced force??

9. Newton’s First Law of Motion
Newton’s first law – an object in motion will stay in motion; an
object at rest will stay at rest
Newton’s first law explains why the motion of objects change.
Inertia is an object’s resistance to stop moving or start moving
Examples of inertia:
-A person flies forward when a car stops
-A person flying over bike handlebars
Inertia in action…..

10. Complete Questions in Notebook
1. What is a force?
2. Is force a scalar or vector quantity?
3. Are you exerting a force on the ground right now?
4. What are the units of force? 
5. What does the arrow indicate in the following equation? f ma
6. How much acceleration will there be when a 1 Newton force pushes a 1
kilogram object?
7. Due to the earth’s size, objects accelerate toward the earth at a constant
speed of 9.81 m/s2 . If you drop a basketball from an airplane, how fast will
it be going after 5 seconds??
8. Will a balanced force accelerate an object?
9. If an 18-wheel truck weighing 9000kg and accelerating at 2m/s2 east hits a
pickup truck weighing 900 kg and accelerating at 21m/s2 west, which way
will the 2 trucks travel after the collision, east or west?
10. In a car, what is used to stop a person’s inertia during an accident?
Write your answers and the questions in your notebook