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:acceleration and force
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Physics
Physics
:acceleration and force
Physics
:acceleration and force
Warm-up
Physics
:acceleration and force
Warm-up
Explanation
Physics
:acceleration and force
Free Body Diagram HO 3.4

A free body diagram is a pictorial
representation of all forces acting on
an object.

We consider the object to be isolated
from the physical system and choose
to examine only the forces directly
acting ON the object, not forces
applied BY the object.
Explanation
Physics
:acceleration and force
Free Body Diagram Example
Label the forces acting on the block.
Explanation
Free Body Diagram Example
Physics
:acceleration and force
Force on the block by
the ground: FNormal
Force on the
block by Antonio
Force on the block
by the ground:
Ffriction
Force on the block
by the rope.
Force on the block by
the earth: Fgravity
Engagement
Galileo’s Investigation
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:acceleration and force

What are the
similarities and
differences between
a basketball and a
tennis ball?
Engagement
Physics
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Galileo’s Investigation

What will happen if
the balls are
released from the
same height above
the floor?

Try to use Physics
vocabulary in your
response.
Engagement
Physics
:acceleration and force
Galileo’s Investigation

What’s going on here,
why did they move?

How would you
describe the motion
of the balls?

–
Path
–
Speed
Again, try to use
Physics vocabulary in
your response.
Explanation
Physics
:acceleration and force
Force of Gravity: Pull or Push?

Force can be described as a
push or pull one object
exerts on another.

One specific force is weight.
Weight = the force of gravity
on an object.

But what causes gravity?
–
The presence of mass
Explanation
Newton’s Universal Law of Gravitation HO 3.2
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
Masses attract one another
about their centers with a
force proportional to the
product of their masses and
inversely proportional to the
square of the distance
between them (whew!).
F =G m1 m2 ÷
Fweight = mg
2
d
Explanation
Newton’s First Law

Objects require a force to change their velocity.
Physics
:acceleration and force
–

Another term for this property of matter is
inertia
What is the principle property of an object that
determines its inertia?
–
mass
Explanation
Newton’s First Law Example
Physics
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
If we apply the same force (push) to a bus and a
bicycle, which object will experience a greater
change in velocity?
Explanation
Newton’s Second Law
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
Force causes a change in motion
–
Change in motion means
change in velocity
–
Change in velocity means???

A change in velocity occurs in a
certain amount of time.

AHA! That’s a rate.
SF = ma
Explanation
Physics
:acceleration and force
Acceleration is a Rate

The speedometer in your car
indicates what property of
motion?

What happens to the needle if
you speed up or slow down?

The speed of the needle is a
measure of your acceleration
(in a straight line).

Acceleration is the rate of
change in velocity.
Explanation
Newton’s Second Law Example #1
Physics
:acceleration and force
SF = ma

Important note: the acceleration vector is always
in the same direction as the force vector.

If we apply a 100N force to a wagon full of rocks
initially at rest with a mass of 50 kg, what is its
acceleration?

What does this answer tell us?

What is the velocity of the object after 1 second?
–
2 seconds?
–
5 seconds?
2 m/s2
100 N
Explanation
Newton’s Second Law Example #2
Physics
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
What happens when the acceleration vector points
opposite to the velocity vector?
30 m/s
F
-5m/s2
How long will it take Mighty Mikey to stop the train?
Explanation
Physics
:acceleration and force
Newton’s Third Law

No single isolated force can exist

Equal and Opposite forces exist only between two
objects
WRONG!
Push on
water
Explanation
Physics
:acceleration and force
Newton’s Third Law Example

You push on the water

The water pushes back on you with an equal force in
the opposite direction

So why do you move?
Push on
swimmer
Push on
water
CORRECT!
Explanation
Physics
:acceleration and force
Acceleration Due to Gravity

The Tennis Ball and Basketball hit the ground at the
same time because they accelerated at the same
rate (~10 m/s2).

What force changed their velocities?...Weight.

Weight = mass X gravity (FW=mg).
Wbb>W
tb
Explanation
Acceleration Due to Gravity
F
m
F/m a
F = weight
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:acceleration and force
= weight
m= mass
= mass
=
F/m
a=g
regardless of mass!
=a
Explanation
Free Body Diagram Example 2
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
Forces on coffee filter before release (balanced
forces)
Support
from my
hand
Weight
Explanation
Free Body Diagram Example 2
Forces on coffee filter immediately after release
(unbalanced force  acceleration)
Physics
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
Weight
Explanation
Free Body Diagram Example 2
Physics
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
Forces on coffee filter while falling
What factors
influence the
magnitude of
the force of air
resistance?
Air
resistance
Weight
Explanation
Physics
:acceleration and force
So Why Different Times?

Force due to gravity is different for the two
objects

Their masses are different and therefore they
have different inertias

They do not accelerate at the same rate
…WHY???
–
We must examine ALL the forces acting on the
objects in order to determine the Net Force.
Explanation
Net Force
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
SF
If you earn $2000/mo but taxes, “free” health care, social
security, and other reductions total $1350/mo then your:
–
Gross Pay = $2000/mo
–
Net Pay = $650/mo
Net Force
Explanation
Physics
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Net Force

The force of air
resistance is greater for
the basketball than the
coffee filter.

However, the force of air
resistance is negligible
compared to the weight
of the basketball,
therefore the Net Force
is toward the floor.
Air
resistance
Weight
Explanation
Physics
:acceleration and force
So Why Different Times?

The force of air resistance is also acting on the coffee
filter and is comparable in size to the weight.

In fact, at some point, the force of air resistance is
equal to the weight of the coffee filter.

When this is the case:

–
There is NO net force
–
Therefore there is no acceleration
–
Therefore the object moves at a constant speed
This equilibrium condition is called terminal velocity
Evaluation
Sky Diver Physics HO 3.5

A skydiver jumped out of a plane 1 sec ago.
–
Physics
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
Label the forces on the skydiver
After some time, the skydiver has zero acceleration
–
What does this indicate about her speed?
–
What is the name for this equilibrium condition of a
falling object?
–
Label the forces on the skydiver

What factors influence the force of air resistance on the
skydiver?

BONUS: What is the terminal velocity of a human?
Summary
Physics
:acceleration and force
Summary

Force

Gravity & Weight

Newton’s Laws

Air resistance

Free Body Diagrams

Net Force

So what?
–
This applies to transportation, moving objects,
sports, etc.