1. Forces
-Review-
IS3

2. What can you infer from the image?
• Suppose you observe a proton moving
through a region of outer space, far
from almost all other objects.
• The proton moves along a path like
the one shown on the left. The arrow
indicates the initial direction of the
proton’s motion, and the “x’s” in the
diagram indicate the position of the
proton at equal time intervals.
• Do you see evidence that the proton
is interacting with another object?

3. What keeps the atom’s particles
together?

4. Describe the motion

5. What forces are acting on you?

6. What forces are acting on the cart?

7. What is Force?
• Force is an interaction that causes a change in motion
• Described by vectors
• Vectors have magnitude and direction
• Unit: Newton (N)
– 1N: force required to give a 1kg object an acceleration of 1m/s2
• Contact forces: friction, normal, air resistance
• Non-contact forces: gravity, magnetism, electrical force

8. Free Body Diagrams
• Free-body diagrams = used to show the relative
magnitude and direction of all forces acting upon an
object in a given situation.

9. Determining the net force…
• Net force: vector sum of all the forces that act upon an
object

10. Balanced or Unbalanced?

11. What forces are acting on the football in
the air?

12. Friction
• Force that 2 materials exert on each other
• Causes: type of surface involved + how hard
surfaces push together
• Acts in a direction opposite to the direction of
the object’s motion
• It depends on an object’s mass

13. Draw the free-body diagrams…
1. A book is at rest on a tabletop. Diagram the forces acting on the
book.
2. A girl is suspended motionless from the ceiling by two ropes.
Diagram the forces acting on the combination of girl and bar.
3. An egg is free-falling from a nest in a tree. Neglect air
resistance. Diagram the forces acting on the egg as it is falling.
4. A flying squirrel is gliding (no wing flaps) from a tree to the
ground at constant velocity. Consider air resistance. Diagram the
forces acting on the squirrel.
5. A rightward force is applied to a book in order to move it across
a desk with a rightward acceleration. Consider frictional forces.
Neglect air resistance. Diagram the forces acting on the book.

14. Gravity
• Attraction between all matter
• Downward force of gravity on an object is called weight
• Earth: acceleration due to gravity = 9.8m/s2
• Force: mass x acceleration (F=ma → W=ma)
• Newton realized it is universal (Law of Universal Gravitation)
• Factors affecting: mass and distance

15. Mass x Weight:
- Mass: amount of matter => measure of body’s inertia
- Weight: effect of gravity on mass
 Force = mass x acceleration
 Weight = mass x acceleration
Example 1: Earth
Weight = 50Kg x 9.8m/s2
Weight = 490 kg . m = 490N
s2
Example 2: Jupiter
Weight = 50Kg x 24.8m/s2
Weight = 1240 kg . m = 1240N
s2

16. Moon: Apollo 15

17. NEWTON’S LAWS
The Three Laws of Motion by Sir
Isaac Newton explain all aspects
of motion.

18. Galileo’s experiment

19. Newton's First Law:
“An object at rest will remain at rest, and a moving
object will remain at a constant speed unless
unbalanced forces act on it”.
• Inertia: ability to resist change in motion
• Egg drop inertia

20. Newton's Second Law:
“An object that has an unbalanced force acting on it will accelerate
in the direction of the force”.
· Force = mass X acceleration (F = ma)
· change in motion proportional to force applied, inversely
proportional to body’s mass

21. Newton's Third Law:
• “For every action, there is an equal and opposite reaction”
• Forces act in pairs
• Mission to Mars movie

22. Exit Card
Relate each Simpsons video with one Law of Motion and explain. Use the words
associated with the specific law in your answer
a) Krusty is driving a car, the car hits a tree and Krusty is ejected from the car through
the windshield while the car stops.
b) Bart is inside a warehouse. He finds a chair with wheels and a fire extinguisher. He
decides to sit on the chair and pull the fire extinguisher’s handle. Air comes out of
it. Bart’s chair moves to the back of the room.