An Introduction to Forces
Definition: A force is a push or a
pull in a particular direction.
How does it balance?
Textbook p. 53
 You probably would be uneasy
standing under Balanced Rock
near Buhl, Idaho. Yet this unusual
rock stays in place year after
year. The rock has forces acting
on it. So, why doesn’t it fall over?
 Answer: The forces acting on the
rock combine, and the rock does
not move.
Now, let’s read page 54.
Remember… A force is a push or
pull in a particular direction.
Forces Packet p. 1
EXAMPLE of a PUSH:
When you ride a bike, your foot
pushes against the pedal. The
push makes the wheels of the
bike move.
EXAMPLE of a PULL:
When you drop something, it is
pulled to the ground by gravity.
Remember
…
Textbook p. 54, Review from Ch. 2- Lesson 1
There are contact forces.
 Example: Air Resistance, pushing a shopping cart,
pulling on a rope in Tug’o’War
There are non-contact forces.
 Example: Gravity, magnetism, static electricity
The Big Ideas…
Forces Packet p. 1
 Forces can make objects:
start moving.
move faster.
move slower.
stop moving.
change direction.
change shape.
REMEMBER…
Forces Packet p. 1
 Velocity is the speed and direction of a
moving object.
 Acceleration is the measure of the change
in velocity during a period of time.
 Since forces cause a change in the speed or
direction of an object, we can say that
forces cause changes in velocity. SO…
FORCES CAUSE ACCELERATION!
Force Facts…
Forces Packet p. 1
Forces are measured in Newtons
(N).
Forces come in pairs.
Forces act in a particular direction.
Forces usually cannot be seen, but
their effects can be seen.
 FYI- A Newton is equal to the amount of
force required to accelerate a mass of one
kilogram at a rate of one meter per
For the next few slides:
Label each picture as a push or
pull. Then describe whether the
force is causing a change in speed
or direction or both.
Forces Packet p. 2
Lift Off!
 Push
 Change in speed
Parachute
 Pull (gravity)
 Change in speed
and direction
Rolling a Rock Uphill
Push
 Change in speed
Teeter Totter
 Push AND Pull
 Change in speed
and direction
Hitting a Golf Ball
 Push
 Change in speed
Combining Forces
Textbook p. 55-56, Forces Packet p. 3
 Net Force: The combination of all
the forces acting on a object.
 Forces may work together.
This means the forces act in the same direction.
 Forces may be opposite forces.
This means the forces act in opposite directions.
Balanced and Unbalanced Forces
Textbook p. 56, Forces Packet p. 3
Balanced Forces:
Forces that combine
and form a net
force of zero.
 Balanced forces do not
cause a change in motion.
 Unbalanced Forces:
Forces acting on an
object that combine
and form a net
force that is not
zero.
 Unbalanced forces cause a
change in motion.
Balanced Forces
Unbalanced Forces
Video Clip- Balanced and Unbalanced
Forces
2:53
Circle the best answer.
1. The forces above are showing PUSHING/PULLING forces.
2. The forces above are WORKING TOGETHER/OPPOSITE forces.
3. The forces are EQUAL/NOT EQUAL.
4. The forces DO/DO NOT balance each other.
5. The resultant force is 1000N TO THE LEFT, 1000 N TO THE RIGHT, or
ZERO.
6. There IS/IS NO motion.
Forces Packet p. 4
Answers:
1. The forces above are showing PUSHING/PULLING forces.
2. The forces above are WORKING TOGETHER/OPPOSITE forces.
3. The forces are EQUAL/NOT EQUAL.
4. The forces DO/DO NOT balance each other.
5. The resultant force is 1000N TO THE LEFT, 1000 N TO THE RIGHT, or
ZERO.
6. There IS/IS NO motion.
Forces Packet p. 4
Circle the best answer.
7. The forces shown above are PUSHING/PULLING forces.
8. The forces shown above are WORKING TOGETHER/OPPOSITE
forces.
9. The forces are EQUAL/NOT EQUAL.
10. The forces DO/DO NOT balance each other.
11. The stronger force is pulling to the RIGHT/LEFT.
12. The weaker force is pulling to the RIGHT/LEFT.
13. The motion is to the RIGHT/LEFT.
Forces Packet p. 5
Answers:
7. The forces shown above are PUSHING/PULLING forces.
8. The forces shown above are WORKING TOGETHER/OPPOSITE
forces.
9. The forces are EQUAL/ NOT EQUAL.
10. The forces DO/ DO NOT balance each other.
11. The stronger force is pulling to the LEFT/RIGHT.
12. The weaker force is pulling to the LEFT/RIGHT.
13. The motion is to the LEFT/RIGHT.
Forces Packet p. 5
Newton’s 1st Law
(also known as The Law of Inertia)
Textbook p. 57
 A moving object moves in a straight line with constant
speed unless a force acts on it.
 The tendency of an object at rest to remain at rest and
an object in motion to remain in motion unless acted
upon by an unbalanced force.
 Objects do not change their motion unless a force
acts on them
Newton’s First Law of Motion
Textbook p. 57
Objects at Rest
 Objects at rest tend to
stay at rest unless acted
upon by a force. [push or
pull]
 Newton described this
tendency as inertia.
 Inertia can be described
as the tendency of an
object to keep doing
whatever’s it’s doing.
What about objects that are already
in motion?
 Newton stated that
objects in motion tend to
stay in motion until acted
upon by a force (or it hits
an outside force.)
The truck is in motion. What is the force that
causes it to stop?
The push of the stopped car.
The car is at rest. What is the force that
causes it to move?
The push of the truck.
Mass & Inertia
 Mass is the amount of
matter in an object.
 The more MASS an object
has, the more INERTIA
the object has.
 Bigger objects are harder
to start & stop
Which vehicle has more
inertia?
Video Clip: The Science of FootballNewton’s First Law of Motion
3:51
LINK