1. Lesson 19 All motion is due to forces acting on objects
often, more than one force
acts on an object at the
same time
When more than one force acts on an object,
scientists can determine the net force, or total combination of these forces

2. Balanced and Unbalanced Forces
A balanced force is one in which
the net forces equal zero
There is no motion

3. Balanced and Unbalanced Forces
During a tug of war, if the
teams are equal in strength
the forces pulling on both
ends of the rope are equal
and there is no movement
The forces are balanced, with
a net force of zero

4. Balanced and Unbalanced Forces
An unbalanced force has a
net force greater than zero
The object acted on shows
movement in one direction
Only an unbalanced force
can change the motion of an object

5. Balanced and Unbalanced Forces
An unbalanced force acting on
a motionless object can cause
the object to move
An unbalanced force that acts
on an object that is already in motion can change the peed or direction of the object

6. Balanced and Unbalanced Forces
Unbalanced forces can act in the same direction
Sometimes unbalanced forces
act in opposite directions

7. Balanced and Unbalanced Forces
Only an unbalanced force can
change the motion of an
object
An unbalanced force acting on
a motionless object can cause
the object to move

8. Balanced and Unbalanced Forces
An unbalanced force that
acts on an object that is
already in motion can
change the speed or
direction of the object
* Unbalanced forces can act in the same direction

9. Balanced and Unbalanced Forces
Sometimes unbalanced
forces act in opposite
directions

10. Mass and Inertia Objects have a tendency to resist
any change in motion
If an object is at rest, the object
will stay at rest until an unbalanced
force causes it to move

11. Mass and Inertia If an object is in motion, the
object will keep moving in
the same direction and at
the same speed until an
unbalanced force acts on it
to change its direction or speed

12. Mass and Inertia Inertia is the tendency of objects
to resist a change in motion
Inertia is why you wear seat
belts

13. Mass and Inertia If a car is going 50 kilometers
per hour and it comes to a
sudden stop, the people
inside continue moving 50
kilometers per hour unless a
force prevents their forward
motion through the windshield

14. Mass and Inertia Mass affects inertia An object with a large mass has
more inertia than an object with
a small mass

15. Mass and Inertia Changing the motion of an
object with a large mass is harder
than changing the motion of an
object with small mass

16. Mass and Inertia Pulling a wagon full of rocks
is harder than pulling an
empty wagon because the
full wagon has more mass

17. Mass and Inertia A train takes longer to accelerate to 60
kilometers per hour
than a car because the
train has more mass

18. Mass and Inertia Stopping a train that is moving at 60 kilometers per
hour also takes longer than
stopping a car moving at the
same speed, this is because the train has much more mass

19. Friction and Gravity Two forces that can always affect
the motion of an object are
Friction
Gravity

20. Mass and Inertia Friction is a force that opposes
the motion of an object
This contact force acts
whenever an object in motion
rubs against a surface
Friction opposes motion when
two surfaces tough

21. Mass and Inertia The contact reduces the speed of
the moving object and releases
heat
The quantity of friction depends on
the push on the object, the
bumpiness of the surfaces, and the weight of the object being pushed

22. Gravity Gravity is a force of attraction between all objects
Newton stated that the force of
gravity between two objects
depends on their masses and the
distance between them

23. Gravity Since Earth is so large, all objects are attracted to it
Gravity acts on the motion of
objects that are not touching the
planet and causes objects to stop their forward movement in the atmosphere and fall to the ground

24. Gravity Gravity acts at a distance A simple example of the
gravitational force on motion is
a person throwing a ball
* The ball is pulled to Earth’s surface by gravity unless it is stopped by an outside force, such as someone catching it