1. Forces
Standard: S8P3
b. Demonstrate the effect of balanced and unbalanced forces on an object in terms of gravity, inertia, and friction.

2. What are forces? Inertia Gravity Newton’s Laws Projectile motion

3. FORCES

4. What is a force?
A push or a pull
May be unbalanced or balanced

5. What does a force do? Gives energy to an object
Can set an object in motion, make it stop, or change its motion

6. Unbalanced forces change motion

7. Unbalanced forces change motion
The truck motion is changed,
but the ladder keeps going

8. Unbalanced forces change motion
crosswind
tailwind
headwind

9. What are forces represented by? Forces are represented by vectors
motion of
book
friction

10. Forces are added with vectors

11. What is friction? sliding fluid rolling
A force which opposes motion, bringing objects to rest
Types:
sliding
fluid
rolling

12. Inertia

13. What is inertia? I have more inertia but we fall at the same rate
The tendency of an object to stay at rest or in motion.
Objects with greater mass have greater inertia
but we fall
at the same rate

14. Inertia keeps astronauts in motion

15. Seat belts protect against inertia

16. Gravity

17. Gravity
The attractive
force between
all objects in the
universe

19. F = M1 M2 d2 More mass means greater gravity
More distance means less gravity
My gravitational
attraction is small
no matter what the
distance!
Massive objects have
lots of gravity

20. The earth exerts a force on the student
and the student
exerts a force
on the earth
because
of GRAVITY!

21. Gravity warps the fabric
of space-time

22. Gravity

23. gravity pulls it in
inertia keeps it going
What forces are
acting on the moon?
Inertia

24. Newton’s Laws of Motion
1. Inertia
2. F=ma
3. Action-reaction

25. Newton’s First Law of Motion
The law of inertia:
An object at rest stays at rest
an object in motion stays in motion
unless acted upon by an outside force.

26. Inertia keeps astronauts in motion

27. Inertia keeps satellites in orbit

28. Inertia keeps you at rest

29. Newton’s Second Law of Motion
Force = Mass x acceleration
A force is needed for acceleration
Acceleration will occur in the direction of the force.

30. Which cars are accelerating?
If you watch carefully, the green and blue cars are accelerating. The red car is traveling at constant speed.

31. A force is needed for acceleration
Will these accelerate?
A skier going downhill?
What force is acting on
the skier?
An astronaut on a
spacewalk?
Are any forces acting
on the astronaut?

32. Which of the following will accelerate?
A bullet shot from
a gun?
What forces are
acting on the
bullet?
A hammer and
feather dropped on
the moon?
What force is acting?

33. Newton’s 2nd Law F_____ M A
Force = mass x acceleration
F_____
M A
Force is measured in newtons. A newton is a kg m s2
Example: A car with mass 1000kg hits a tree and decelerates at 50m/s/s. What force does it exert on the tree?

34. Answer:
F_____
M A
1000kg * 50m/s/s =
50,000 kgm or 50,000N s2

35. F = ma A car is traveling at constant speed. Is a force acting on it?
Answer: although there are forces acting on the car (gravity, friction, air resistance,etc…) since there is no acceleration, the net force in the direction of motion must be zero.

36. More 2nd Law Problems:
1. What is the acceleration on a mass of 50kg if a force of 10N is applied?
2. An object accelerates due to gravity at a rate of 10m/s/s. If its mass is 15kg, what force is acting on the mass?

37. Answers: _F_ M A 1. _10N_ 0.2 n/kg= 0.2 kg m = 0.2 m/s2 50kg A s2/kg
15kg 10m/s2
150 kgm/s2 or 150N

38. Gravity is a force that causes an acceleration
On earth, ALL objects accelerate at 9.8m/s2 (ignoring air resistance) because of gravity.
No matter what the mass, ALL objects on earth accelerate at 9.8 m/s2

40. Newton’s 2nd Law
F__
m a
An object of mass 3kg falls off a cliff. How much force is acting on the object?
Answer: F_______
3kg m/s2
29.4N

41. Free fall without air resistance
Both the elephant and
the feather land at
the same time!

42. Free fall with air resistance
The feather takes
lots longer to hit
the ground!

43. Newton’s Third Law
For every action, there is an equal and opposite reaction

45. For every action, there is an equal and opposite reaction
In rocketry……
The gases
being expelled
is the action.
The rocket
propelling
forward is the
reaction.

48. How do Newton’s laws describe the motorcycle rider?
1st: Inertia keeps the rider moving forward.
2nd: The force with which the rider hits the ground will be equal to his mass times his deceleration.
3rd: The action of hitting the ground will equal the reaction of the dirt.

49. Projectile motion

50. Projectile motion
An object moves forward due to inertia and accelerates downward due to gravity.

51. In this video, the ball is dropped straight down and moves forward due to inertia

52. Projectile motion of a ball fired vertically
The motion
of the truck
causes the
ball to project
forward.
What would
happen if
the truck
stopped?

53. Package dropped from plane
The forward motion
of the plane causes
the package to land in front
of where it’s dropped.

54. Monkey in gravity-free
environment

55. With gravity-
monkey lets go when
banana is fired

56. with gravity-
cannon shoots at
slow speed

57. Satellites Projectile with no gravity with gravity- launched
at < 8000m/s
with gravity- launched
at > 8000 m/s

58. The End