1. What are forces? Inertia Gravity Newton’s Laws

2. Key Points of the Lesson
-Understand that unbalanced forces cause changes in motion (acceleration)
-Understand Newton’s Laws of Motion
- Understand how to draw and read a free body diagram
-Use Newton’s Laws to calculate problems involving forces in one or two dimensions

3. Force Inertia Gravity Orbit Mass Newton’s Laws Free Body Diagram
Forces Vocabulary
Force
Inertia
Gravity
Orbit
Mass
Newton’s Laws
Free Body Diagram

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. Forces are represented by vectors
motion of
book
friction

10. Forces are added with vectors

11. Friction sliding fluid rolling
A force which opposes motion, bringing objects to rest
Types:
sliding
fluid
rolling

12. 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

13. Inertia keeps astronauts in motion

14. Seat belts protect against inertia

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

17. 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

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

19. Gravity

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

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

22. 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.

23. Inertia keeps satellites in orbit

24. 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.

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

26. 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?

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

28. 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.

29. 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?

30. 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

31. 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

32. 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

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

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

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

37. 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.

40. 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.