Newton

1
Newtons Laws of Motion
Forces change the motion of an object in very
specific ways Sir Isaac Newton (1642-1727) was
able to state the Laws that describe the effects
of forces.
2
Newtons 1st Law of Motion
Law of Inertia
INERTIA is the tendency for an object to resist
its change in motion. It wants to stay where it
is.
First Law basically states that An object in
motion tends to stay in motion, while an object
at rest tends to stay at rest until a force acts
upon the object.
3
Newtons 1st Law of Motion - Continued
The greater the mass of the object the greater
its inertia.
Think about it like this The book is sitting on
the table is at rest.
The book will stay there (remain at rest) until
someone moves it (the outside force).
4
Newtons 1st Law of Motion - Continued
Another example If you were standing the same
distance from a softball being hit or a baseball
being hit which one would hurt more if it hit you?
Which has more inertia?
5
Newtons 2nd Law of Motion
Law of Acceleration
Second Law basically states thata force acting
on an object causes the object to accelerate in
the direction of that force.
Force causes acceleration. Greater the mass the
greater the resistance to acceleration.
6
Newtons 2nd Law of Motion - Continued
Lets think about this oneif the soccer player
kicks the soccer ball which direction will the
soccer ball go?
Force causes acceleration of the object to
accelerate in the direction of that force (the
kicking of the ball).
7
Newtons 2nd Law of Motion - Continued
Think about it like this If the blue truck has a
mass of 25,000kg and the red truck has a mass of
30,000kg which truck is going to take longer to
speed up?
The RED Truck 30,000kg
It goes back to the greater the mass the
greater the resistance to acceleration.
8
Newtons 2nd Law of Motion - Continued
Newtons 2nd Law of Motion can be expressed in
an equation
Force Mass X Acceleration
F ma
divide
multiply
Force units kg.m/s2 OR Newton Mass units
kilograms (kg) Acceleration units m/s2
9
Newtons 2nd Law of Motion - Continued
Problem What force is exerted on a 1000kg car
accelerating at a speed of 15m/s2?
Step 1 Read the problem. Draw a picture.
1000kg
15m/s2
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What force is exerted on a 1000kg car
accelerating at a speed of 15m/s2?
Step 2 Write down what you know. What are you
trying to find?
?
F
M
1000kg
A
15m/s2
11
What force is exerted on a 1000kg car
accelerating at a speed of 15m/s2?
Step 3 Set up the formula.
?
F
F ma
M
1000kg
A
15m/s2
12
What force is exerted on a 1000kg car
accelerating at a speed of 15m/s2?
Step 4 Substitute the numbers. Solve. Box
Answer with Units.
F
15,000N
F 15,000 kg.m/s2 OR F 15,000 N
F ma
1000kg
M
F 1000kg x 15m/s2
15m/s2
A
13
Do problems 1-3 in your notes.
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Newtons 2nd Law of Motion - Continued
The acceleration for any object moving under the
sole influence of gravity.
Any moving object being acted upon only by the
force of gravity is said to be "in a state of
free fall."
Characteristics of Free-Falling Objects

• do not encounter air resistance.
• (on Earth) accelerate downwards at a rate of 9.8
  m/s2.

15
Lets Look at a Diagram
16
Remember the formula to find acceleration?
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Lets Look at a Problem
What is the velocity of a rubber ball dropped
from a building roof after 5 seconds?
Step 1 Read the problem. Draw a picture.
Step 2 Write down what you know. What are
you trying to find?
Vf
?
Vi
0 m/s
t
5 s
a
9.8 m/s2
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What is the velocity of a rubber ball dripped
from a building roof after 5 seconds?
Step 3 Set up the formula.
Step 4 Substitute the numbers. Solve. Box
Answer with Units.
Formula
Substitute
Answer
49m/s Vf
19
Do problems 4 5 in your notes.
20
Newtons 3rd Law of Motion
Law of Action / Reaction
Third Law basically states thatto every ACTION
there is an equal and opposite REACTION.
Action/Reaction forces act in PAIRS.
21
Newtons 3rd Law of Motion - Continued
In this example, when the lady is walking, her
feet push against the ground while the ground
pushes against her feet.
Thus action/reaction pair is the feet and
ground pushing off of one another.
22
Newtons 3rd Law of Motion - Continued
Another example Lets think about this onewhat
are the pairs of forces that are acting against
each other in order for the shuttle to launch?
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MOMENTUM
Property of a moving object because of its mass
and velocity.
An increase in mass or velocity causes an
increase in momentum.
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Momentum can be calculated with the following
formula
Momentum Mass X Velocity
P mv
Units for Momentum are kg.m/s
25
Problem Find the momentum of an 7.2kg rock that
is rolling down a hill with a velocity of
3.0m/sec.
Step 3 Set up the formula.
Step 1 Read the problem. Draw a picture.
Step 4 Substitute the numbers. Solve. Box
Answer with units.
Step 2 Write down what you know. What are you
trying to find?
P
21.6kg.m/s
7.2kg
m
P mv
P 7.2kg x 3.0m/s
P 21.6 kg.m/s
v
3.0 m/s
Units, units, units!
26
Do problems 6 7 in your notes.
27
Law of Conservation of Momentum

• In the absence of external forces applied to a
  system, the total momentum cannot change.
• m1 v1 m2v2

28
Do problems 8 9 in your notes.