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## Today: Re-Teaching Newton

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### Today: Re-Teaching Newton s First and Second Laws of Motion * Net force acting on object (N) Object s mass (kg) Object s acceleration (m/s2) F = ma F m a A car ... – PowerPoint PPT presentation

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Title: Today: Re-Teaching Newton

1
Today Re-Teaching Newtons First and Second Laws
of Motion
2
Balanced vs. Unbalanced Forces
• Balanced Forces
• 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 gt 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.

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6
What is the net force acting on this box?
Fnet12 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
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
Newtons 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

16
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19
FR
F
f
F FR f
20
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21
Newtons 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, thats when objects
accelerate
24
Forces are either balanced, or unbalanced
But when they are unbalanced, thats when objects
accelerate
25
Forces are either balanced, or unbalanced
But when they are unbalanced, thats when objects
accelerate
26
Forces are either balanced, or unbalanced
But when they are unbalanced, thats when objects
accelerate
27
Forces dont cause motion, forces cause
ACCELERATIONS!
Accelerations cause MOTION!
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FR
F
f
F FR f
32
FR
f
F
The car will accelerate if F gt FR f
33
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Objects mass (kg)
Objects acceleration (m/s2)
Net force acting on object (N)
35
F ma
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. Ds 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)
20 N
a
5 kg
46
? mass (m) ? ? acceleration (a)
20 N
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?
F
200 kg
2 m/s2
52
How much force is needed to accelerate this
wrestler at a rate of 2 m/s2?
F
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
• Balanced Forces
• 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 gt 0 N)
• YES acceleration (a gt 0 m/s2 or a lt 0 m/s2)
• Object changes speed

57
Balanced Forces Unbalanced Forces
Fnet is
Acceleration is
Object will
Described by which of Newtons Laws?
58
Balanced Forces Unbalanced Forces
Fnet is Fnet 0 N Fnet gt 0 N
Acceleration is
Object will
Described by which of Newtons Laws?
59
Balanced Forces Unbalanced Forces
Fnet is Fnet 0 N Fnet gt 0 N
Acceleration is a 0 m/s2 a gt 0 m/s2 or a lt 0 m/s2
Object will
Described by which of Newtons Laws?
60
Balanced Forces Unbalanced Forces
Fnet is Fnet 0 N Fnet gt 0 N
Acceleration is a 0 m/s2 a gt 0 m/s2 or a lt 0 m/s2
Object will remain at rest or maintain constant speed accelerate (change speed)
Described by which of Newtons Laws?
61
Balanced Forces Unbalanced Forces
Fnet is Fnet 0 N Fnet gt 0 N
Acceleration is a 0 m/s2 a gt 0 m/s2 or a lt 0 m/s2
Object will remain at rest or maintain constant speed accelerate (change speed)
Described by which of Newtons 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