Transcript The Force - Belton ISD

When Making a sudden stop in
your car, what pushes you into
the seat belts?
A.
B.
C.
D.
Nothing, your own motion does it
Your seat
Nothing, your seat belts just
tighten
The Force
“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




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
[Enter
question
here] 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



Fweight = mass x gravity
As gravity changes, WEIGHT changes, NOT
MASS
So I’m cruising around Belton in my awesome
[Enter
question
here]
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. Gravitational
Force
B. Electric Force
C. Frictional Force
D. Magnetic Force
E. 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
net force.
An object in motion will stay in motion
unless acted upon by an outside net 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!
question
here]
Which[Enter
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.
You pay a large amount of money to be sent
to outer[Enter
space. question
You finallyhere]
reach a point
where all the gravitational forces acting on
you are close to zero. If you are traveling
at the speed of light and then run out of
fuel you will…..
A.
B.
C.
D.
E.
F.
G.
Slow to a stop
Travel in a parabola
Continue in a straight line.
Maintain constant speed.
Both A and B
Both C and D
All of the above
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… here]
[Enter
question
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.
I have a mass attached to the
ceiling by a thread. There is
also a thread attached at the
bottom. If you pull the bottom
string which will break first?
A.
B.
C.
D.
Both at the same
time
The top one
The bottom one
Depends on how
hard you pull it

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 places!


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






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 [Enter
is the question
weight of
a 60 kg
here]
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
[Enter
here]
person question
on the moon?
(gm= 1.67 m/s2)
A.
B.
C.
D.
E.
1 N
10 N
100 N
1000 N
1.67 N
Constant acceleration from gravity




Galileo found that balls of
different masses fell at the
same rate.
The force of gravity is stronger
for more massive objects.
But it requires more force to
accelerate a more massive
object (inertia).
These two effects cancel each
other out.
Force of Normal




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



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



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
Force of Friction






Static Friction is the force which
keeps an object stationary
Kinetic Friction slows an object
in motion
Force is dependent on surface
type and pressure pushing the
two objects together
F f = m FN
m is the coefficient of friction,
and describes the “roughness” of
a surface.
Force is directed opposite motion
or expected motion
Find the Force of Friction…

For a 5 kg box that is being
pushed across a table
which has a µ of 0.655. :)
Find the Force of Friction…

For the shoes of a 60 kg
student as they walk
across a floor with a µ of
0.654.
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
What is the mass of
a 294 N box
A.
B.
C.
D.
30 kg
2880 kg
294 N
66 lbs
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




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


Will accelerate!
Objects in equilibrium will not accelerate (inertia)

Net force required to speed up, slow down, turn, etc.
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?
2.
A 5 kg box slides across a table that has a coefficient of
friction of 0.236. If the force applied to the box is 25 N,
what is the net force acting on the box 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)
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?
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.
The mass of a refrigerator that
slides across a kitchen floor that
has a µ of 0.245 and an applied
frictional force of 300 N.
A.
B.
C.
D.
125 kg
1220 kg
73.5 kg
7.50 kg
Basic Info: balanced Forces

Objects are balanced only if their net force is zero in
both the vertical and horizontal directions


Meaning all the forces in the x direction add up to zero AND
all the forces in the y direction add up to zero
All forces that act in a angle needs to be broken into
components using trig.

Meaning using a right triangle with x and y components.
Vector Components

In order to find the
components of a
vector (like force) you
will need to use those
timeless Trigonometric
Functions.
Vector Components


So we have a person
pulling a sled 30o with
respect to the
horizontal at a force of
50 N.
We need to think of it
like the sled being
pulled vertically and
horizontally at the same
time, giving it both
components.
Fy
Θ=30o
Fx
Vector Components



In order to calculate the
components, we need to
shift Fy to make a right
triangle.
Then we can use trig
functions to solve for Fy
and Fx like they are sides of
a right triangle.
To solve for Fx, we will use
cosine because it is adjacent
and we have the
hypotenuse.
Fy
Θ=30o
Fx
Vector Components

To solve for vx, we will use cosine
because it is the adjacent side and we
have the hypotenuse.
adj
cos  
hyp
F
cos   x
F
Fx  F cos 

To solve for vy, use the same process but
with sine.
sin θ 
sin θ 
opp
hyp
Fy
F
Fy  F sin θ
Fy
Θ=30o
Fx
Fy equals?
A.
B.
C.
D.
43.3 N
25 N
28.9 N
50 N
Fy
Θ=30o
sin θ 
sin θ 
opp
hyp
Fy
F
Fy  F sin θ
adj
hyp
F
cos   x
F
Fx  F cos 
cos  
FBD Example 1

A 50 kg mass is
suspended from two
wires, as in the
diagram below. What
is the tension in the
wires?
FBD Example 2
A 25.0 N picture is hanging from two wires.
The wires make a 30˚ angle with the top of
the picture. Calculate the tensional force on
each wire.
A force of 60 N is applied to a rope to pull a sled
across a horizontal surface at a constant
velocity. The rope is at an angle of 30 above the
horizontal.
Calculate the magnitude of the component of
the 60N force that is parallel to the horizontal
surface.
A.
B.
C.
D.
60 N
30 N
52 N
35 N
A force of 60 N is applied to a rope to pull a
sled across a horizontal surface at a constant
velocity. The rope is at an angle of 30 above
the horizontal.
Determine the magnitude of the frictional
force.
A.
B.
C.
D.
60 N
30 N
52 N
18 N
How to draw a free-body diagram for
an object on a ramp?
FBD example 3
A 20.0 kg block rests on
a frictionless ramp, as in
the diagram below.
What is the tension in
the string holding the
block in place?
Formulas!




W = mg
Ff = μFN
Fnet = FA – Ff
Object on a flat surface:


FN = Weight
Object on an incline:


FN = Wcosθ
FA = Wsinθ
What is the coefficient of
friction of a 30 kg box being
pushed across a table in a
constant speed with an
applied force of 250 N.
A.
B.
C.
D.
1.176
.12
.850
8.3
Even though the force of
gravity on a less massive
object is smaller, it still fall at
the same rate as more massive
objects because:
A.
B.
C.
D.
Air is pushing it down more.
Because it has less resistance to
changing it motion (inertia).
Because it has more resistance to
changing it motion (inertia).
Magic