1. U3: Force
Newton’s 1st and 2nd Laws

2. 4 Common Types of Forces
Push
Pull
Torque
Tension/Compression

3. Identify the Forces

4. What is Force?
We will study forces that can come either as a push a pull, or a tension.
Measured by the Newton,
Because anytime we accelerate 𝑚 𝑠 2 a mass (kg), we are applying a force.
𝑁=𝑘𝑔 𝑚 𝑠 2

5. Mass (kg)
1kg is the mass of 1L of water.
1kg = 2.2lbs

6. Newton’s 3 laws of motion
Only THREE?!
Newton’s 3 laws of motion

7. 1st Law

8. Newton’s 1st Law of Motion
An object at rest or an object moving with a constant velocity will continue behaving in the same way unless acted on by an unbalanced force.
What’s an unbalanced force?

9. Balanced Forces Balanced forces mean no movement.
Think of two people who kick a soccer ball at the same time in opposite directions.
The ball won’t move!
Why?
The force were balanced!

10. Balanced vs. Unbalanced
Balanced Force
Unbalanced Force
Two oppositely faced forces are exactly the same.
Will “cancel” each other out.
The net force will equal 0N.
Two oppositely faced forces are different.
Will not “cancel” each other out.
The net force will equal 5N to the right. 5N 5N
10N 5N

11. Net Force?
A net force is the overall force that acts on the object we are focusing on.
We can find this net force by adding them together and keeping positive (+) and negative (-) direction in mind.

12. Balanced vs. Unbalanced
Balanced Net Force
Unbalanced Net Force
𝐹 = 𝐹 𝑛𝑒𝑡
𝐹 =5𝑁+(−5𝑁)
𝐹 =0𝑁
 therefore:
The net force will equal 0N.
𝐹 𝑛𝑒𝑡 = 0N 5N 5N
𝐹 = 𝐹 𝑛𝑒𝑡
𝐹 =10𝑁+(−5𝑁)
𝐹 =5𝑁
 therefore:
The net force will equal 5N.
𝐹 𝑛𝑒𝑡 = 5N
10N 5N

13. 1st Law Definition: At rest will stay at rest:
In motion will stay in motion:
If there is no unbalanced force acting upon it.
If there is no unbalanced force acting upon it.

14. 1. A ball is put into the end of the tube indicated by the arrow
1. A ball is put into the end of the tube indicated by the arrow. The ball is then shot out of the other end of the tube at high speed. Pick the path the ball will follow after it exits the tube Note – you are looking down on these tubes, they are not vertical.

15. 1st Law and Inertia
Inertia: The tendency for an object to stay at rest or stay in motion.
Why it takes lots of force to…
Start making a car move
Slow down a moving car

16. 2. Which object has the most inertia?
Baseball
Golf ball
Bowling Ball
Football

17. Challenge: Determine which jar is filled.B
Things you can’t do to the jar:
lift
tap the side
open
drop
Things you can do:
Anything else.

18. Story time

19. Ryan rode on the freeway more than anything else
Ryan rode on the freeway more than anything else. Was he correct to never fill his tank beyond ½?
No.
More mass -> more inertia -> less easy for air resistance and friction to slow down his car.

20. What scenario(s) exist where it would be better to have the less massive car?

21. 2nd Law

22. 2nd Law
Definition: The relationship between an object’s mass and its acceleration is given by:
𝑎 = 𝐹 𝑚
If we use what we learned about net force, the 2nd Law becomes:
𝑎 𝑛𝑒𝑡 = 𝐹 𝑛𝑒𝑡 𝑚

23. Force of gravity! (weight)
Gravity pulls downward with an acceleration of: 𝑎 𝐺 =9.8 𝑚 𝑠 2
The force of gravity will be proportional to the mass being pulled down.
Heavier mass, more force
Lighter mass, less force
Therefore:
𝐹 𝐺 =𝑚 𝑎 𝐺

25. Mass is what weight is in the metric system (kilograms)
What is mass vs. weight?
Mass is what weight is in the metric system (kilograms)
Mass is a measure of how much stuff there is, which is like weight.
Both of the above options

26. What is mass and weight?
Weight is the force of gravity pulling down on it.
Mass is a measure of how much matter there is.
Link

27. Inertia The tendency for an object to stay at rest or stay in motion.
Why it takes lots of force to…
Start making a car move
Slow down a moving car

28. Inertia is based on…
Mass
Weight
A combination of the two

29. 5N 5N It will increase (car go faster!)
Your car’s engine puts force on the car to move forward. Wind resistance of the same magnitude acting against your car pushes back on it. What will happen to your car’s velocity?
It will increase (car go faster!)
It will decrease (car slow down)
It will stay the same
Impossible to tell from the given information. 5N 5N

30. It is some positive value of Newtons.
If the car accelerates to the right, what can we conclude about the net force on the car?
It is 0N.
It is some positive value of Newtons.
It is some negative value of Newtons.
It is equal to the force of the engine minus the force of wind resistance.

31. If gravity pulls down on a middle-weight ball vs. a heavy-weight ball,
They will accelerate at the same rate, because the forces are the same.
They will accelerate at different rates, because one of them experiences more force.
They will accelerate at the same rate because the forces are different (greater on very heavy ball).
Link

32. 3 Incorrect Laws of Motion
Link

33. A truck hits a small car, which experiences more force?
They experience equal force.
The car experiences more force.
The truck experiences more force.

34. 3rd Law

35. Newton’s 3rd Law of Motion
For every force, there is an equal and opposite force.

36. For every force, there is an equal and opposite force.
What does this mean?
Think of the car and the truck.
Which has more inertia?
Truck:
More inertia
Takes more force to accelerate
Force is a cause of…
…Acceleration. (effect)
Car:
Less inertia
Takes less force to accelerate
Force is a cause of…
…Acceleration. (effect)

37. To the Whiteboard! Truck: Car:
Experiences force of car on truck = 90,000N
mT = 3,000kg
Experiences force of truck on car = 90,000N
mC = 500 kg
To the Whiteboard!

38. The cannonball because it has more acceleration.
Let’s analyze a cannon and ball. Which experiences more force when firing, the cannon or the cannonball?
The cannonball because it has more acceleration.
The cannon because it has more mass.
Both experience the same force.

39. CP #13
𝒎 𝑪
𝑭 𝐄𝐂 =
𝒂 𝑪
𝐹 𝐸𝐶
𝐹 𝐸𝐵
𝒂 𝑩
𝑭 𝐄𝐁 =
𝒎 𝑩

40. Free Body Diagraming (FBD)
Diagram forces on an object
Pretend the object is a point
Show all push/pull forces acting on the object
Usually multiple forces

41. Example Drawing and FBD:
𝐹 𝑆𝑅
𝐹 𝑆𝑅
𝐹 𝐺𝑅
𝐹 𝐺𝑅

42. Force of Gravity and Mass
Find the mass of the object or
Find the Force of the Earth on the object

43. Force of Gravity and Mass
Mass of the Rock
𝐹 𝐺
𝐹 𝐺 = 𝑚 𝑅 𝑎 𝐺
Acceleration due to Gravity

44. Let’s Draw a FBD for each object DURING the firing of the gun.
Rifle
Bullet
Boom!
FBD:
FBD:
𝐹 𝐸𝐵
𝐹 𝐸𝑅

45. Yes, it accelerates just as much as I do because the forces are equal.
If you step off a ledge, you accelerate noticeably toward the Earth because of the gravitational interaction between you and earth. Does the Earth accelerate toward you as well?
Yes, it accelerates just as much as I do because the forces are equal.
Yes, it accelerates less than I do because the forces are equal.
Yes, it accelerates more than I do because the forces are equal.
No, it does not accelerate at all, the equal and opposite force exists elsewhere.

46. FBD of Earth and Jumper FBD Earth: FBD Jumper: FBD Air: Force of….
… person’s gravity on earth.
Force of Air Resistance
Force of Earth’s Gravity on person
FBD Air:

47. Yes, it accelerates just as much as I do because the forces are equal.
If you step off a ledge, you accelerate noticeably toward the Earth because of the gravitational interaction between you and earth. Does the Earth accelerate toward you as well?
Yes, it accelerates just as much as I do because the forces are equal.
Yes, it accelerates less than I do because the forces are equal.
Yes, it accelerates more than I do because the forces are equal.
No, it does not accelerate at all, the equal and opposite force exists elsewhere.

48. What is the reaction force to an action force of 1000N exerted by the earth on an orbiting communications satellite?
A 1000N Surface Force on the ground
A 1000N Tension Force on the Earth
A 1000N gravitational force on the Earth

49. The bicycle The truck The same for each
If a bicycle and a massive truck have a head-on collision, upon which vehicle is the impact force greater?
The bicycle
The truck
The same for each

50. Which vehicle undergoes the greater change in motion?
The bicycle
The truck
The same change in motion
Acceleration…

51. No. I’m not exactly sure why.
Since the force that acts on a bullet when a gun is fired is equal and opposite to the force that acts on the gun, does this imply a zero net force and therefore the impossibility of an accelerating bullet?
Yes.
No. I know exactly why.
No. I’m not exactly sure why.

52. Estimate # of people on Earth. Estimate average mass of a person.
If we got all people together in one place on the planet and made everyone jump, we can figure out with what acceleration the Earth will move.
Estimate # of people on Earth.
Estimate average mass of a person.
Estimate the mass of the Earth.
Determine what force each person could jump up with.
Use that force to figure out the total force applied on the Earth.

53. Forces Review
Concept Problems (CP)

54. CP #4 Constant 𝑣 No change in 𝑣 No acceleration ( 𝑎 = 0 𝑚 𝑠 2 )
Based on 𝐹 𝑛𝑒𝑡 = 𝑚 𝑎 , the net force must also be 0, so forces must be balanced.

55. CP #5
Net force is all the forces on JUST ONE OBJECT added together.

56. CP #6
25N 𝒂
15N
𝑵𝒐 𝒂 𝒂
25N
25N
25N
𝐹 𝑛𝑒𝑡 = −25𝑁
𝐹 𝑛𝑒𝑡 = −10𝑁
𝐹 𝑛𝑒𝑡 =0𝑁

57. 𝑷𝒓𝒆𝒕𝒆𝒏𝒅 𝒄𝒂𝒓𝒕 𝒉𝒂𝒔 𝒏𝒐 𝒎𝒂𝒔𝒔.
CP #7
𝑷𝒓𝒆𝒕𝒆𝒏𝒅 𝒄𝒂𝒓𝒕 𝒉𝒂𝒔 𝒏𝒐 𝒎𝒂𝒔𝒔. 𝒎
𝑭 =
𝑭 = 𝒎 𝒂 𝒂

58. CP #8
𝟐𝒌𝒈
𝟏𝒌𝒈
𝐹 𝐸𝑅 =−10𝑁
𝐹 𝐸𝑅 =−20𝑁

59. CP #13
𝒎 𝑪
𝑭 𝐄𝐂 =
𝒂 𝑪
𝐹 𝐸𝐶
𝐹 𝐸𝐵
𝒂 𝑩
𝑭 𝐄𝐁 =
𝒎 𝑩

60. CP #9
𝐹 𝑁𝐻
𝐹 𝐻𝑁

61. CP #10
𝐹 𝐻𝐹
𝐹 𝐹𝐻

62. CP #11
𝐹 𝐻𝑊
𝐹 𝑊𝐻

63. CP #12
𝐹 𝑆𝐻
𝐹 𝐻𝐸

64. CP #14
𝐹 𝐸𝐻
𝐹 𝐻𝐸

65. CP #15
𝐹 𝑆𝐻
𝐹 𝐻𝐸