1. Today: Re-Teaching Newton’s First and Second Laws of Motion

2. Balanced vs. Unbalanced Forces
Cancel each other out
NO net force acting on object (Fnet= 0 N)
Do not cancel out
YES net force acting on object (Fnet > 0 N)

3. Check For Understanding
Which statement below is true?
When two unbalanced forces are applied to an object, the net force is 0 N.
When two balanced forces are applied to an object, the net force is 0 N.
Two unbalanced forces cancel each other out.
When balanced forces act on an object, net force is negative.

6. What is the net force acting on this box?
Fnet=12 N – 8 N = 4 N to the right

7. Practice Finding the Net Force

8. Check For Understanding
You must have a magnitude and a direction for Fnet.
Balanced or unbalanced?
Fnet =
Balanced
0 N

9. Check For Understanding
Balanced or unbalanced?
Fnet =
Unbalanced
5 N to the left

10. Check For Understanding
Balanced or unbalanced?
Fnet =
Balanced
0 N

11. Check For Understanding
Balanced or unbalanced?
Fnet =
Unbalanced
15 N up

12. Answer “Balanced and Unbalanced Forces” questions on your sheet
Answer “Balanced and Unbalanced Forces” questions on your sheet. 5 minutes!

13. Check For Understanding
When balanced forces act on an object, what is the net force acting on the object?

14. Check For Understanding
If the net force on an object is greater than zero, what can you conclude about the forces acting on the object?
It is being acted on by unbalanced forces.
It is being acted on by balanced forces.
It is being acted on by no forces at all.
It is moving at a constant speed.

15. Newton’s First Law
Objects at rest remain at rest unless acted upon by a net force.
Objects in motion remain in motion at a constant speed unless acted upon by a net force.
“Law of Inertia”

19. FR f F
F = FR + f

20. AHH!
Ouch

21. Newton’s Second Law
When a net force acts on an object, the object accelerates in the direction of the force.
Acceleration is directly proportional to net force and inversely proportional to mass.

22. Forces are either balanced, or unbalanced
When forces are equal, no acceleration occurs.

23. Forces are either balanced, or unbalanced
But when they are unbalanced, that’s when objects accelerate

24. Forces are either balanced, or unbalanced
But when they are unbalanced, that’s when objects accelerate

25. Forces are either balanced, or unbalanced
But when they are unbalanced, that’s when objects accelerate

26. Forces are either balanced, or unbalanced
But when they are unbalanced, that’s when objects accelerate

27. Forces don’t cause motion, forces cause ACCELERATIONS!
Accelerations cause MOTION!

31. FR f F
F = FR + f

32. The car will accelerate if F > FR + f

34. Object’s acceleration (m/s2) Net force acting on object (N)
Object’s mass
(kg)
Object’s acceleration
(m/s2)
Net force acting on object
(N)

35. F = ma F m a

36. Check For Understanding
A car is accelerating at 5 m/s2. The car has a mass of 1000 kg. What is the force that is pushing the car? F
1000 kg
5 m/s2

37. Check For Understanding
Kristopher pushes with 150 N of force on a box with a mass of 50 kg. At what rate does the box acclerate?
150 N a
50 kg

38. Check For Understanding
Tylesia pushes Mr. D with a force of 240 N. Mr. D accelerates forward at a rate of 3 m/s2. What is Mr. D’s mass?
240 N m
3 m/s2

39. Relationships in F = ma  net force (Fnet)   acceleration (a)
 mass (m)   acceleration (a)
 mass (m)   net force (Fnet)

40.  net force (Fnet)   acceleration (a)

41. How fast will this rock accelerate?
100 N a
50 kg

42. How fast will this rock accelerate?
200 N a
50 kg

43.  net force (Fnet)   acceleration (a)

44.  mass (m)   acceleration (a)

45.  mass (m)   acceleration (a)
5 kg

46.  mass (m)   acceleration (a)
20 kg

47.  mass (m)   acceleration (a)

48.  mass (m)   acceleration (a)
Kristopher pushes with 150 N of force on a box with a mass of 50 kg. At what rate does the box acclerate?
150 N a
50 kg

49.  mass (m)   acceleration (a)
Kristopher pushes with 150 N of force on a box with a mass of 100 kg. At what rate does the box acclerate?
150 N a
100 kg

50. Who will be harder to move?

51. How much force is needed to accelerate this wrestler at a rate of 2 m/s2?
200 kg
2 m/s2

52. How much force is needed to accelerate this wrestler at a rate of 2 m/s2?
20 kg
2 m/s2

53.  mass (m)   net force (Fnet)

54. Check For Understanding
A car is accelerating at 5 m/s2. The car has a mass of 2000 kg. What is the force that is pushing the car? F
2000 kg
5 m/s2

55. Check For Understanding
A car is accelerating at 5 m/s2. The car has a mass of 5000 kg. What is the force that has to be pushing the car? F
5000 kg
5 m/s2

56. Balanced vs. Unbalanced Forces
Cancel each other out
NO net force acting on object (Fnet= 0 N)
NO acceleration (a = 0 m/s2)
Object keeps same speed
Do not cancel out
YES net force acting on object (Fnet > 0 N)
YES acceleration (a > 0 m/s2 or a < 0 m/s2)
Object changes speed

57. Balanced Forces
Unbalanced Forces
Fnet is…
Acceleration is…
Object will…
Described by which of Newton’s Laws?

58. Balanced Forces
Unbalanced Forces
Fnet is…
Fnet = 0 N
Fnet > 0 N
Acceleration is…
Object will…
Described by which of Newton’s Laws?

59. Balanced Forces
Unbalanced Forces
Fnet is…
Fnet = 0 N
Fnet > 0 N
Acceleration is…
a = 0 m/s2
a > 0 m/s2 or a < 0 m/s2
Object will…
Described by which of Newton’s Laws?

60. Balanced Forces Unbalanced Forces
Fnet is…
Fnet = 0 N
Fnet > 0 N
Acceleration is…
a = 0 m/s2
a > 0 m/s2 or a < 0 m/s2
Object will…
remain at rest or maintain constant speed
accelerate (change speed)
Described by which of Newton’s Laws?

61. Balanced Forces Unbalanced Forces
Fnet is…
Fnet = 0 N
Fnet > 0 N
Acceleration is…
a = 0 m/s2
a > 0 m/s2 or a < 0 m/s2
Object will…
remain at rest or maintain constant speed
accelerate (change speed)
Described by which of Newton’s Laws?
First Law
Second Law

62. Challenge Question
A car moves along a road at a high speed. The driver pushes the gas pedal down farther, generating a forward force of 3000 N on the car. A strong gust of wind hits at the exact moment the driver pushes the gas pedal, exerting a backward force of 2500 N on the car. The road also applies a backward force of 500 N due to friction.
What is the acceleration of the car at the exact moment the gust of wind hits the car?
0 m/s2
500 m/s2
2500 m/s2
3000 m/s2