Physics of Forces

1. Unit 3 Forces

2. Forces
 A force is a push or a pull
 A force affects how an object moves
Can make objects start or stop
moving
Can make objects move faster or
slower
Can make objects change direction

3. Push/Pull Activity
 Move around the classroom and find
at least 10 things that you push, pull,
or push and pull (3 minutes)
 Complete the Venn diagram by
placing the items in the appropriate
categories (2 minutes)

4. Forces
 Remember acceleration?
change in speed or direction
 Forces cause a change in the speed or
direction of an object
 F=ma
 Forces are measured in Newtons (N)
1N=1kg·m/s2

5. Combining Forces
 Forces usually act in pairs
 Forces act in a particular direction
 Forces usually cannot be seen, but
their effects can

6. Forces
 More than one force can act on an
object at a time
 Forces can push or pull in any direction
 What happens when more than one
force acts on an object depends on:
How strong the forces are
The direction of the forces

7. Combining Forces
 When more than one force acts on an
object, the forces combine to form a
net force
 The net force is the combination of all
forces acting on an object
 Forces may work together or they may
be opposite forces

8. Balanced Forces
The total or net force is the difference
between the individual forces
 Total/Net force= 5N - 5N=0N
 When the forces are balanced, the
object is at equilibrium and does not
move

9. Unbalanced Forces
 When 2 or more forces act in the
opposite direction and are not equal in
magnitude (number), the forces will be
combined by subtracting to produce a
net force
 The total or net force is the difference
between the individual forces
 The object will move in the direction of
the greater force

10.  The total or net force is the difference
between the individual forces
 Total/Net force= 10N - 5N=5N right
Unbalanced Forces

11. Practice
 You are pulling a wagon with your
niece in it up a hill with a force of 18N.
She is holding your dog’s leash, and
the dog is pulling in the opposite
direction with a force of 12N. What is
the force on the wagon?
 Total/net Force = 18N – 12N = 6N

12. Unbalanced Forces
 When 2 or more forces act in the same
direction, the forces are added to
produce the total force
 Unbalanced forces result in motion

13. Unbalanced Forces
 The total force is the sum of the
individual forces
 Total/Net force= 25N + 30N = 55N
 Will the box move?

14. Practice
 Example:
You are pushing your twin brothers up
a hill in a stroller with a force of 18N.
Your best friend sees you and comes
to help. He or she pushes with a force
of 12N. What is the force on the
stroller?
Total/Net Force = 18N + 12N = 30N

15. Friction
 Dragon Racers Activity

16. Friction
 Friction exists when two surfaces are
touching and comes from the fact that
no two surfaces are perfectly smooth
 Friction acts in the opposite direction of
motion and will cause a moving object
to slow down and eventually stop

17. Friction
 To make an object move, the force you
exert on an object must be greater than
the force opposing it
 What did you observe about the friction
of the surfaces that were rougher or
softer in the Dragon Racer Activity?

18. Friction
 Four types
Static friction – occurs between
stationary objects
Books on table, box on floor
Sliding friction – occurs when solid
objects slide over each other
Books or box being pushed

19. Friction
 Four types
Rolling friction – occurs when a solid
object rolls over another solid object
Tires on a road
Fluid friction – occurs when an object
moves through a fluid (liquid or gas)
Fish swimming, airplane flying

20. Friction
 Identify the type(s) of friction involved
in walking or running.

21. Friction
 Identify the type(s)
of friction involved
in stopping a car
with brakes.
 What would be an
example of fluid
friction?
 Rolling friction?

22. Friction
 How can we reduce friction?
Use oil in an engine between moving
parts
Grease the bearings in a car wheel
Have you ever used candle wax on a
sled?
Use a lubricant (oil, grease, etc.)

23. Friction Practice
 Consider a book on a table
 If I push the book with a force of
32N and the table applies 4N of
friction, what is the net force on the
book?

24. Friction Practice
 Consider a book on a table
32N 4N
The net force on the book is
F= 32N - 4N = 28N

25. Force Calculations
 Force = mass x acceleration
 F=ma
 The triangle:
Force
Mass Acceleration

26. Friction Practice
 Your mass is 70kg. Your friend
pushes you down a hill on a sled
with a force of 66N. The snow has
a friction of 2N. What is your
acceleration?

27. Friction Practice
66N
2N

28. Friction Practice
 Calculate net force:
F = 66N – 2N = 64N
Then find acceleration:
a = F/m
a = 64N/70kg = 0.91m/s2

29. Practice
 A 50kg skater pushed by a friend
accelerates at 5m/s2. How much force
did the friend apply?
 How much force is required to push a
16kg child in a 22kg stroller with an
acceleration of 2.5m/s2?

30. Practice
 If a 90kg water skier is being pulled
behind a boat with a force of 360N,
what is the skier’s acceleration?
 If a 60kg person on a 15kg sled is
pushed with a force of 300N, what will
be the person’s acceleration?

31. Practice
 A bowling ball rolled with a force of 15N
accelerates at a rate of 3m/s2. A
second ball rolled with the same force
accelerates at 4m/s2. What are the
masses of the two bowling balls?