Download Forces and the Laws of Motion

“The Force”

“An energy field created by all living things. It
surrounds us, penetrates us, and binds the galaxy
together.”

The Force has two components:
Light side
 Dark side

The Real Force

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
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Something that causes an object’s motion to
change (causes acceleration).
A “push” or a “pull.”
Common Examples of forces: Gravity fields,
pushing on something, compressing a spring, a
magnetic field, tension, friction, and the
“normal” force.
Units are Newtons (N)
Which of the following is an
example of a force?
a)
b)
c)
d)
e)
f)
Your weight
A magnet pulling on another magnet
A bully pushing you down the stairs
A rope pulling a cartful of turnips
All of the above
None of the above
Types of
Forces

Contact


Me slapping you
upside the head
Baseball bat on a
baseball
•Field
•Balloon bending
stream of water
•Projectile Motion
•Magnetic fields
Easily forgotten Forces!!
Important contact force: FRICTION
Acts in the opposite direction of
motion
 Produces HEAT

AIR RESISTANCE is a form of FRICTION!!
 SLOWS MOTION
 Can be desired (tires on road) or undesired
(skates on ice)

Easily forgotten forces!!
Important field forces: GRAVITY and Magnetic
Fields

Opperate at a distance (don’t need to “touch”)
Decreases with the distance between objects.

WEIGHT is a FORCE caused by gravity


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Fweight = mass x gravity
As gravity changes, WEIGHT changes, NOT
MASS
As a meteor passes the earth our gravity
causes its path to bend. Another meteor
passes us and its path is not bent nearly as
much. Which meteor is closer to the
earth?
a)
b)
c)
d)
The first meteor
The second meteor
What meteor?
Yes
So I’m cruising around Belton in my awesome
car when I see a dog in the middle of the
road! What is the main force that lets me
stop my car so the doggy can live?
a)
b)
c)
d)
e)
Gravitational Force
Electric Force
Frictional Force
Magnetic Force
THE Force
Force is a vector

Has magnitude and direction

Amount of force makes a difference
Little push vs. big shove
 Earth gravity vs. moon gravity


Direction makes a difference
Push vs. Pull
 Thrust vs. Drag


Forces “add” to give a net force.

Net force determines what an object will do.
The law of inertia: An object at rest will
stay at rest unless acted upon by an outside
force.
An object in motion will stay in motion
unless acted upon by an outside force.
Galileo’s Unique Idea

Objects don’t need a force to keep moving!

Every object naturally wants to maintain its state of
motion or rest


INERTIA! (resistance to change in motion)
Refined by Newton in 1800’s:
Basic Info: Inertia

Inertia depends on:
Mass
 Shape/Mass Distribution of object- rotational inertia

Solid Cylinder (like a wheel of cheese,)
 Hoop (like a bicycle tire)


Inertia does NOT depend on:

Velocity/Speed of object

It takes the same amount of force to speed a bus up as to slow it
down!
Which of these objects has the
greatest amount of inertia?
a)
b)
c)
d)
e)
A 0.5 mg cockroach.
A 2.0 g lump of cheese.
A 35 g ball of goo.
A 15 kg baby goat
A 300 kg circus freak on roller
skates.
Net Force

Net Income: How much money did you make
after you counted up all profit and expense?

Net Force: How much force is there after all
individual forces are added up?
What if…
The net force acting on an
object is zero?
a)
b)
c)
d)
e)
The object will slow to a stop.
The object will begin to fall.
The object’s motion will not change.
The universe will open up and swallow
the object.
Time will stop for us but the object will
grow old and smelly.
Fun Stuff



Wear your seatbelt!
Fun stuff
Other one
In other words…
You already know this! the important part
is the Σ. This means “Sum” or “Net”
Net Force

Net Income: How much money did you make
after you counted up all profit and expense?

Net Force: How much force is there after all
individual forces are added up?
Mass
X
Acceleration
Typical Situations

Which is easier to accelerate?


A tiny sports car? Or a ridiculously large and
completely unnecessary SUV?
Your car has stalled in the middle of the road!

Easier to push it to the side by yourself? Or with
friends?
More mass requires more Force!
Don’t
forget the
Force

Causes accelerations
Gravity leads to: Projectile Motion, and more projectile
motion
 “Pushes and Pulls” can also cause accelerations (like this).


Measured in Newtons (N)

The amount of force that causes a 1 kg mass to accelerate
at 1 m/s2
Force (N) = mass (kg) x acceleration (m/s2)
Always make sure your mass is
in kilograms (kg)!
Calculate the following
1.
What force is required
to push your 1,100 kg
car with an acceleration
of 0.5 m/s2?
Calculate the following
2.
If a 10 N force is applied to an
object that causes it to
accelerate at 22 m/s2, what is
the mass of the object?
Calculate the following
3.
The net force on the
propeller of a 3.2 kg
model airplane is 7.0 N
forward. What is the
acceleration of the
airplane?
Calculate the following
4.
A force is applied to a 1.5 kg ball that
causes it to accelerate from rest to a
velocity of 4.5 m/s in 0.5 seconds.
What is the magnitude of the force?
Remember: Fnet = ma
and a = (Vf-Vi)/t
Calculate the following
5.
A 12 kg box starts from rest and slides
down a frictionless ramp. If the
gravitational force pulling it down the
ramp is 35 N, what is the final velocity
of the box after 3.4 seconds?
Gravity
“Gravitational Force”
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Force which is exerted by any object with mass.
Earth’s gravitational force is measured as
Fg = mg, g = 9.8m/s2
Direction is always down
Designated Fg
sometimes called weight
Gravitational Force
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Newton’s Law of Universal Gravitation
The amount of force is directly proportional to
the mass of each of the objects.
The gravitational force between two objects
decreases as the distance between them (r)
increases! This is an inversely proportional
relationship!
Don’t worry, we wont be
using this equation yet!

The amount of gravitational force exerted on an object

1 lb = 4.448 N
1 N = 0.225 lb
FW = mg
FW (also W or Fg):
weight (N)
m: mass (kg)
g: acceleration due to
gravity (m/s2)
MASS
WEIGHT
Weight and mass are two
always the completely
same
depends on gravity
different
(kg)
(N)
things!!!

Would you weigh more on Earth or Jupiter?

Jupiter because...
greater mass
greater gravity
greater weight
Weight

Find your weight on different planets!


First, find your mass in kilograms: take your
weight and divide it by 2.2 (because 1 kg
weighs 2.2 lbs)
Now, to find how many Newtons you weigh,
take your mass (m) and multiply it by the
gravitational acceleration (g). Remember:
F=ma, or FW = mg
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gearth: 9.8 m/s2
gmoon: 1.67 m/s2
gmars: 3.7 m/s2
gsun: 274.13 m/s2
gpluto: 0.42m/s2
To convert Newtons to Pounds, remember:
1 lb = 4.448 N
What is the weight of a 60 kg
person on earth?
a)
b)
c)
d)
e)
f)
5.8 N
58 N
588 N
5880 N
60 kg
None of the above
What is the weight of a 60 kg
person on the moon?
(gm= 1.67 m/s2)
a)
b)
c)
d)
e)
1 N
10 N
100 N
1000 N
1.67 N
Force of Normal
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The force that a surface exerts on an object.
Force is always in a direction perpendicular to the
surface the object or system is in contact with
Usually a balancing force when an object is in contact
with another object
Designated FN or N
Force of Normal - Example

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Let’s say the box in this diagram has a mass of
25 kg.
That means it has a weight of 245 N.
Since the ground is level, the Normal Force is
going to be equal to the box’s weight, but the
force is exerted in a different direction (up).
Force of Normal - Example 2
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That last one was easy! Let’s say this box is seated on an
incline and still has a mass of 25 kg.
The weight of the box (mg) is still directed straight
down, but the Normal Force is at an angle, which
makes it a little harder to find…
FN = wcosθ
Tension



Force applied by a string
or cable when the object
or system is hanging
Direction can be variable
A force of tension is
described as FT or T
Basic Info: Force Diagrams

Definition: A Diagram that shows all the forces
acting on a body

Does NOT include forces exerted by the body!

Forces are drawn as vectors.
Free Body Diagram
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Simple drawing of all forces
working on an object or
system
Use a box to represent the
object or system
All forces move away from
the box.
Remember: gravity will
always affect an object and
so Fg will always be in a
F.B.D!

Diagram the forces
acting on a car stuck in
a traffic jam.

Diagram the forces
acting on a car driving
on IH-35.
Basic Info: Unbalanced Forces

Objects that are NOT in equilibrium


Objects in equilibrium will not accelerate (inertia)


Will accelerate!
Net force required to speed up, slow down, turn, etc.
Demos:

Frictionless Track at an angle- Remember
Forces are Vectors and can have 2 components!!
Which of
these
situations
shows
balanced
forces?
A
B
Which will
accelerate?
D: all of
them
E: none of
them
C
1.
As a 1150 kg car is driving on the highway the engine is
supplying a force of 900 N. The total force of friction
(including air resistance and the internal friction of the engine)
is 460 N. What is the net force on the car in the direction of
motion?
A force of 150 N is applied to a 15 kg wood block to make
it slide across a piece of sandpaper.
3.
a)
b)
c)
d)
What is the force of normal acting on the wood block?
What is the force of friction acting on the wood block if the μ of
the sandpaper is 0.995?
What is the net force acting on the wood block?
What is the acceleration of the block?
4.
A 8 kg block of cheese is sitting on a ramp with a 30o
incline and a µ of 0.745. Draw a force diagram showing all
the forces acting on the cheese.
Formulas!
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Fnet = ma
W = mg
Ff = μFN
Fnet = FA – Ff
Object on a flat surface:

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FN = Weight
Object on an incline:
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FN = Wcosθ
FA = Wsinθ