Download Normal force

Mental note for next year.
Do Hooke’s law here with forces (or later with energy)
If an object is stationary,
what will cause it to start to move
A FORCE
But what is a force??
A force is a push or a pull
on something by something
Does a push have a direction?
FORCES ARE VECTORS,
Magnitude and Direction
The force on the truck is a push by the man
Since the truck increased its speed we say
that it has…..
Accelerated
Forces cause objects to accelerate in the
direction the force is applied.
What are the 3 ways an object can accelerate?
What is the direction of the force?
Force
The car accelerates to the right (+X)
Which way would you apply a force to cause the truck to slow down?
Force
The car accelerates to the left (-X) while slowing
In what direction should
a force be applied in
order for the truck to
make a right turn?
Force
What if two people push equally hard on both sides of
the truck. In what direction will it accelerate?
Force
Force
FORCES ARE BALANCED IF THEY ARE EQUAL AND OPPOSITE.
But… EQUAL what? and OPPOSITE what?
Equal Magnitude but Opposite Direction
What if two people push equally hard on both sides of
the truck. In what direction will it accelerate?
Force
Force
FORCES ARE BALANCED IF THEY ARE EQUAL AND OPPOSITE.
But… EQUAL what? and OPPOSITE what?
Equal Magnitude but Opposite Direction
Force
Force
Forces are vectors if each what would you get if you added to
two vectors.
Balanced Forces cancel out in both
magnitude and direction
Consider the “Forces” acting on the balls below
balanced
balanced
NOT
balanced
balanced
NOT
If you shove a book across a table your hand
accelerates it.
After it leaves your hand what will happen to
the books speed.
WHY does it slow down?
Friction
Galileo experimented with Spheres on a level
surface and found that they slowed very
little.
Because they have very little friction
Galileo concluded if there
was absolutely no friction
the ball would…
JUST KEEP ROLLING AND NEVER STOP
(Or even slow down)
This is opposite what everyone thought (at the time)
An object’s
natural state
is at rest!
This was common sense
regarding objects around them.
(planets had their own natural states)
Sir Isaac Newton studied how
objects move too.
Developed calculus
About the age of 20
in order to describe motion
He lays the foundation for modern physics in
his book Principia when he was 24.
The physics we are learning now is called
Newtonian Physics which is modified by
Einstein later.
Newtonian Physics is an approximation which
works GREAT unless you are dealing with
EXTREME circumstances. (moving close to
the speed of light etc…)
In Newtons 1st of 3 laws of motion he restates
Galileo
NEWTON’s 1st LAW
An object will move at a constant velocity
unless an unbalanced force acts upon it
A force not cancelled out by another one
Which ball has balanced
a constantforces?
velocity?
Newton’s first law is often stated as:
An object at rest tends to stay at rest and an object in
motion tends to stay in motion UNLESS an
unbalanced force acts on the object.
If an object is stationary or moving in a straight
line at a constant speed its velocity is constant.
Forces must be balanced.
Two ping pong balls are launched into space and experience no
wind resistance or any other forces.
The first is launched at 25 m/s and the other at 3 m/s.
Which will come to a stop FIRST?
NEITHER--- THEY WILL BOTH KEEP
MOVING FOREVER
Soccer ball hover disk demo
INERTIA– the property of matter that
resists changing velocity.
It is the laziness of matter to just keep
doing what it was doing.
Which way will tighten the hammer head to the shaft.
Why isn’t the top of the card flying forward?
If you are driving too fast and hit a tree head on,
What “force” would send you through the windshield if
you weren’t wearing your seat belt?
Crash Test - No seat belt
Crash Test - With seat belt
When people are hit from behind,
sometimes they experience whiplash.
What is it and what causes it?
You put your cell phone down in the seat next to you
while driving 60 mph. What happens?
What keeps a ball in motion after it is thrown?
Is inertia a force?
Does air exert a force?
Does wind resistance speed up or slow down a projectile?
A tennis ball has inertia, and you would have to
exert a force on it to slow it down from 25 mph
to a stop.
An 18 wheeler also has inertia, would you want
to slow it down from 25 mph?
HOW MUCH INERTIA an object has
depends on its Mass.
The more mass an object has, the harder it
is to change its velocity.
More massive objects are lazier to speed
up, slow down, or change direction.
INERTIA is NOT a force.
BOWLING BALL GRAND PRIX
Notation for Forces
F
The object of interest
being pushed or pulled
Type, Feeler, Dealer
The thing doing the
pushing or pulling
Types of Forces
Gravity Fg
Always “down” or
toward the center of the earth
Depends on the mass of an object
Fg, diver, earth
Fg
Types of Forces
Normal
FN
A push resulting from two
objects contacting
Always perpendicular to the
surface and away from it
FN
FN, elephant, pedestal
FN
Surface
The normal force is always perpendicular to the surface
What is the direction of the normal force on the marble?
FN,marble,ground
Is the elephant accelerating?
No, then all forces cancel.
What is cancelling out FN?
FN
Fg
If Forces do not cancel, then what’s leftover is
called the NET force
F1
F2
FNET
$20.00 made – $15.00 spent = $5.00 NET
(what’s leftover)
If Forces do not cancel, then what’s leftover is
called the NET force
F1
5N
F2
-9 N
FNET = 4 N down
Realize that to find the Net force you have added the two
vectors and found the .....
SF= RESULTANT = FNET
IF forces DO cancel (balance) then FNET =
F1
9N
F2
9N
SF or FNET = 0
If SF = 0, what is the
motion like for the object
A Find the net force on each “object”
10 N
10 N
10 N
15 N
10 N
15 N
15 N
5N
10 N
5N
25 N
Types of Forces
Tension
FT
Always a pull
in the direction
of the cable
FT, block, string
FT
What is the direction of the tension force on her hands?
Types of Forces
Friction
FF
Contact Force
opposite direction of
motion (or impending
motion)
FF, player, dirt
FF
A crate sits motionless on a hill,
Which direction is the FF acting?
What happens to the following rolling balls when a force is applied,
The direction of the movement of the ball is shown on the ball in blue.
Force
Force
Slow down
Speed UP
Change Direction
Force
What happens to the following rolling balls when a force is applied,
The direction of the movement of the ball is shown on the ball in blue.
Force
Force
Slow down
Speed UP
Change Direction
Force
What will happen to the ball at rest, when
balanced forces are applied?
Force
still
Force
IT REMAINS STILL
What will happen to the moving ball, when
balanced forces are applied?
Force
Force
It keeps moving at the same
speed in a straight line.
A Freebody diagram replaces the
object of interest with a dot
(to simplify the scenario)
Then forces acting on the object are
represented by arrows (vectors) extending out
from the dot in the proper direction. It doesn’t
matter where the forces act on the boy just the
direction of the force.
FN,boy,pillow
Fg,boy,earth
If the object doesn’t accelerate then
forces must balance (SF=0)
FN,boy,pillow
Fg,boy,earth
Draw a free body diagram for a motionless
wrecking ball.
Are the forces balanced?
SF=0
FT,ball,cable
Fg,ball,earth
What would be different if the ball was moving
upward at a constant speed?
Are the forces balanced?
SF=0
FT,ball,cable
Fg,ball,earth
What would be different if the ball was moving
downward at a constant speed?
Are the forces balanced?
SF=0
FT,ball,cable
Fg,ball,earth
What would be different if the ball was moving
upward faster and faster?
Are the forces balanced?
SF=0
Fg,ball,earth
Force diagram practice sheet
in class
Wind resistance
A force exerted by air which is opposite
the direction of motion. (like friction)
Does the magnitude of the force change
based on how fast an object is moving.
(Think about driving with your hand out the window)
Terminal Velocity
When a sky diver First jumps out of a plane they are falling
slowly so the force of air resistance is low (at least upward)
Fg, diver, earth
Falling slowly
Falling Fast
FF, diver, air
Fg, diver, earth
FNET
FNET
FNET = 0
Is there a net force?
When the forces are equal does the diver stop falling?
The diver is falling fast and must slow down, what does he do?
Opens a parachute
(how will forces change)
FNET = 0
FNET
Does he start going UPWARDS???
Equal weights on pulley demo
Bungee jumping when he first jumps off what forces are acting on him
(let’s ignore wind resistance on this one)
Fg, jumper, earth
When the slack is out of the cord it starts to pull but very weakly.
Net FORCE? What happens to the jumper?
FT, jumper, cord
FNET
Fg, jumper, earth
As the cord stretches it pulls harder
Net FORCE? What happens to the jumper?
FT, jumper, cord
FNET
Fg, jumper, earth
Eventually it is pulling equally hard as gravity
Net FORCE? What happens to the jumper?
FT, jumper, cord
Fg, jumper, earth
The jumper keeps falling and stretching the cord further.
Net FORCE? What happens to the jumper?
FT, jumper, cord
FNET
Fg, jumper, earth
free body diagrams WS 2
Hovercraft analysis WS
In order for the
SF =0
then
Forces Up = Forces Down
Forces Left = Forces Right
In what direction would the object below
accelerate?
SF =0
In what direction would the object below
accelerate?
SF =0
In what direction would the object below
accelerate?
SF =0
In what direction would the object below
accelerate?
Create a free-body diagram for the ball player who is sliding
(and slowing) into 2nd base.
Are the forces balanced?
SF=0
FN, player, dirt
FF, player,
dirt
Fg, player, earth
We have left a few forces out of the picture, let’s fill them in now.
Which forces cancel?
Can friction cancel any of the other two forces?
Make free body diagrams for the hover puck that is
being
slowed
by a push
coasting
pushed
off
and
stationary
on and
stationary
Make a free-body diagram for the person who is not moving
Forces that act at an angle can be thought of as made
up of an X component and a Y component.
F
F
y
Fx
For example if the object below is not accelerating
with 3 forces acting on it then....
C
CY
A
CX
B
If the force of gravity on each weight is 50 N.
What is the tension in each cable?
SF=0
Make a freebody diagram
For the stationary ring
cable
rod
Make a freebody diagram for the
ring supported by a rod and cable. If
the everything is stationary.
Make a free body diagram for the basketball,
Are the forces balanced?
SF=0
F
F, ball, air
F
g, ball, earth
Create a free-body diagram for a cannonball,
moving at a constant 2,000 m/s through
empty space.
Draw a freebody diagram for the pendulum below.
Create a freebody diagram for the crate which is pulled by a rope
at a constant speed along level ground
Draw a freebody diagram for a box which is
stationary on a ramp.
Is there a Net Force?
SF = 0, What’s Missing?
FF
FN
Fg
q
Turn the whole page so that it looks flat
Break gravity into its components
The object is at equilibrium so….
FUP = FDown
FLeft = FRight
FN
FF
Fg
Draw a freebody diagram for a box sliding down a
frictionless ramp
Will it accelerate?
SF=0
Which way?
What is the direction
of the net force?
A trick is to rotate the page until the slope is flat
FN
Fg
Use your turned page to define up/down & left right.
Now break gravity into its two components.
Consider the direction of acceleration,
Which forces must cancel?
FN
What is the Fnet
Fgy
Fgx
Freebody diagram WS #2
Gravity and page tilting
Mass = Weight
What is matter?
I will summarize this at
the end so don’t take
notes on all this
MATTER anything that has mass
and volume
Is everything matter?
What is matter and what is not?
Car
1 liter of nothing
(a vacuum)
thought
Earth
feeling
water
air
light bulb
light ?
Volume
how much space an object takes up
Mass indicates:
how much matter is in an object
On earth the more mass an
object has the more it weighs.
The more mass an object has the harder it is
to get moving or stop moving.(inertia)
Weight:
The force of gravity on an object
If you go into space, there is no gravity.
So you have no weight.
Did you lose any matter?
You still have the same mass but no weight.
Kicking cans:
1 empty
1 filled with sand
1 filled with lead.
The more mass an object has the more inertia it has
And the harder it is to get going.
If you and a huge fully loaded dump truck took a trip to space
Could you grab it and shake it up and down
easily?
Mass is measured in grams or kilograms
or slugs
Weight is measured in pounds or newtons
How do we measure mass on earth?
With a scale that basically measures its weight.
If you took a triple beam balance into space,
even though it says it measures mass it really measure weight.
SUMMARY – write this down
Mass: how much matter is in an object
how much inertia an object has
How lazy an object is to change
what it is doing
Units of Kilograms
Weight: the force of gravity on an object
Units of Newtons (N)
(weight)
Fg = m g
Mass (kg)
9.8 m/s2
(on earth)
What is the force of gravity on 1 kg object (on earth)
The force of gravity (weight) on a 1 kg object is:
9.8 N
1N =
kg m
1 s2
An object weighs 250 N on earth, what is its mass?
Draw a freebody diagram for a 3 kg book
sitting motionless on a desk. Then give
the magnitudes of all forces.
A spring scale can be used to determine the
mass of an object.
Demo with a mass…
Does it measure mass or force?
A spring scale measures force
It has a Newton scale also.
How can it tell us the mass of an object?
(show with finger)
But the more mass an object has, the
more weight (Force) it has.
A spring scale reads 10 N when holding a 10 N mass.
What will 2 scales read when holding up the same mass?
0N
10 N
10 N
10 N
A girl sitting in a swing has
weight of 100 N.
What is the tension force in
each chain?
How strong does the rope need to be in each case if
the person and seat have a mass of 72 kg?
Is it the same?
A
B
If unbalanced forces are applied to an object,
there is always a SINGLE force will cancel out the
other forces.
It is called the EQUILIBRANT,
because it establishes equilibrium
What would the equilibrant be in the following cases
10 N
10 N
15 N
20 N
30o
15 N
20 N
30o
What is the equilibrant in the following cases with 2 or more forces.
The length of the vector indicates the magnitude of the force
10 N
10 N
10 N
10 N
What is the equilibrant in the case below
10 N
15 N
What single force would cancel out these two forces?
First add them together and find their resultant, then find the opposite
25 N
10 N
Let’s try one on the board.
What single force will cancel a 50 N force at 30o N of E
and a 25 N force at 40o S of E?
30o
40o
25 N
50 N
Equilibrant WS
For an object at equilibrium, all forces must cancel.
So one of the forces must be the equilibrant for all of
the other forces?
A+B+C=0
B
A
C
The resultant of A + B is equal and opposite of C
A+B=-C
B
A
C
The resultant of B + C is equal and opposite of A
B + C = -A
B
A
C
The resultant of A + C is equal and opposite of B
A + C = -B
B
C
A
Equilibrant WS
A 120 N girls sits in a swing
The FT in each chain is 60 N
60 N
60 N
120 N
If the chains are slanted out at 30o, will
the force in each go:
Up, down, or stay the same
?
?
Consider an example
with 2 spring scales
and a mass
120 N
If the chains are slanted out, will the
force in each chain go:
Up, down, or stay the same
?
?
Draw a free body diagram
120 N
The length of the cable does not tell us its force,
just the direction of the force.
To find how much we need to draw to scale
30o
120 N
Fg = 120 N
The two tension forces must cancel out the
force of gravity which means, that their resultant is..
30o
120 N
Fg = 120 N
Now to finish off the parallelogram, but how??
I only know the directions of the forces?
30o
120 N
Fg = 120 N
Now draw the tension vectors…
30o
120 N
FT1
FT2
Fg = 120 N
How
we tension
find theinnumber
Newtons
In thiswould
case the
EACHofrope
is 120 N.
of tension?
Why is the tension in the cable twice what it would
be if they were both straight up?
FT1 = 120 N
FT2 = 120 N
30o
120 N
Fg = 120 N
Just to connect with to what we have done before
in free body diagrams. What do we know about
the component vectors?
FT1
FT2
30o
120 N
Fg =
Just to show you with all of the numbers
120
120
60 N
60 N
30o
104 N
104 N
120 N
120 N
Much of the tension in the rope just
cancels horizontally
120 N
Fg
Which cables will have more tension in them?
120 N
Fg
60o
145 kg
Fg
30o
145 kg
Fg
60o
FTY
145 kg
Fg
FTY
30o
145 kg
FTY
FTY
Fg
FTY
60o
Fg
30o
FTY
FTX
Fg
Which rope is more likely to break?
Which rope is more likely to break?
FT
Two cables both at 40o from the horizontal are holding up a 15 kg mass.
Find the force of tension in each cable.
What if 1 cable is at 45o and the other at 20o. Is
the tension in each the same?
5.0 kg
Fg
Fg
Book problems page 57-58
21, 25, 36
Concept WS
Force Mass and Acceleration Lab
So if a net force is on an object it accelerates
Is that all there is too this?
What if I blew the yard blower on a tennis ball,
then a bowling ball?
IF the SAME Force is applied to different objects,
How quickly they speed up depends on their:
MASS
Newton’s 2nd law
Force is proportional to mass x acceleration
F=ma
NET
Note that Force and acceleration are vectors
An object will accelerate in the direction of NET force,
UNITS???
F=ma
Kg
Kg m
2
s
Newton
m
2
s
Kg m
s2
Newton
1 Newton of force is equivalent to the weight of 100 mL of water
or about 1/5 of a pound.
F=ma
The Force increases?
What happens to Acceleration if
The Mass increases?
If a force of 800 N is applied to a 1200 Kg car, what is its
acceleration?
What force is needed to accelerate a 1300 Kg car at 2.5 m/s2?
A 3.0 Kg object has a force of 250 N pushing it forward and
650 N pushing it backwards. What is it acceleration?
A 3.0 Kg object has a force of 250 N pushing it forward and
250 N pushing it backwards. What is it acceleration?
What force would be required to accelerate a 1200 kg car
from rest to 15 m/s in 6 seconds?
A 1 kg mass and a 10 kg mass are dropped which hits the ground first?
BUT WHY?????
Does the larger mass have a bigger Force
(of gravity) accelerating it ???
1
10
force
force
YES 10 x as much
But the larger mass also has more inertia!!!!
1
force =
mass
a
The 10 kg has:
10 x as much Force on it
and
10 times as much inertia
10
force a
=
mass
If the same force is applied to a
1 kg mass and a 2 kg mass.
Compare their accelerations
Compared to the 2 kg object,
After 2 seconds will the 1 kg object have gone:
the same distance
twice as far
½ as far
4 times as far?
A truck can accelerate at 5 m/s2 fully loaded.
At what rate can it accelerate after it drops off its load
And decreases its mass to ¼ of its original mass?
Two forces are applied to boats,
What is the acceleration?
100 N
100 N
100 kg
100 N
100 N
100 kg
100 N
100 kg
200 N
100 N
100 N
100 kg
NET FORCE determines acceleration!!!!!!
A rope pulls on a 5 kg bucket with a a force of 120 N. What is the
buckets acceleration? Don’t forget about gravity here.
How much force would need to be applied to move it upward
At a constant 2.5 m/s?
F = ma WS
One student sits down in a wheeled chair.
And another student stands and pushes them.
Two students on wheeled chairs push off of each other
Two students on wheeled chairs, one student pushes the other
What force accelerates
the car forward
FF tire, road
But how does the
shuttle take off?
The answer can be found by simply jumping off an untethered boat.
In order for you to make it to the dock the
boat must exert a force on you
FN, person, boat
The boat pushes you forward but….
You exert a force on the boat which is
Equal in magnitude
Opposite in Direction
FN, boat,
person
FN, person,
boat
Forces come in pairs
-action reaction pairs
For every action there is an equal and opposite
reaction (force)
Newtons 3rd Law
Or restated
Whenever two objects are in contact they
exert equal and opposite forces on each other
called action – reaction.
this is NOT a conscious push or pull, it is an
inescapable law of nature.
Newtons 3rd Law
Reminderthe equal and opposite forces are
on DIFFERENT objects.
Activity:
Pair up and everyone take a spring scale.
Hook the ends together and
have 1 person pull with a force of 4 newtons and
the other with a force of 8 N
While the ball is in contact with the face…
There is a force on the player
HOW DO WE KNOW THIS?
While the ball is in contact with the face…
There is a force on the player
AND
ON THE BALL
HOW DO WE KNOW THIS?
These forces are equal and opposite
They are on different objects.
FN, Person, Ball
FN,Ball,Person
This is not a conscious
push.
These forces are equal and opposite
They are on different objects.
FN, Person, Ball
FN,Ball,Person
Do the forces cancel
each other out?
No, they are on
different objects.
Action – Reaction forces don’t cancel each
other because they are on different objects.
5N
5N
5N
5N
Common Sceniarios
Catching a ball
Hammering a nail
Driving a car
A falling ball
What are the action reaction forces
When a gun or cannon is fired, what are the action reaction pair?
Compare the forces each object feels
They will both accelerate, do they both speed up the same?
a=
F
m
a
=
F
m
Your jet pack stops
working & you are
drifting away. All you
have is a hammer. What
do you do?
What makes the shuttle go?
Think back to firing a gun.
What are the action reaction forces.
Fuel
Oxygen
What is direction of the net
force on the exhaust
FN,exhaust,shuttle
What is the reaction force?
FN,shuttle,exhaust
FORCES ON AN AIRPLANE
How does an airplane wing generate lift?
CONSIDER A WING MOVING THROUGH THE AIR
What direction is the force applied to the air molecule,
think about making El Nino perform a sharp turn
F N,wing, air
F N,air, wing
Wings are usually at shallow angles
F N,wing, air
F N,wing, air
lift
drag
drag
lift
A person is stationary on a floor what is an
action – reaction pair of forces?
Are these forces a action reaction pair?
No they are both parts of two interaction pairs
What is an action reaction pair for:
A person standing on a scale
If the force of gravity on the person is 350 N. What does the scale read?
A car running into a tree
A big kid pushes a little kid on frictionless ice, who
feels the bigger force?
A little kid and a big kid are holding a rope (on frictionless ice)
If the big kid yanks on the rope, who feels the greater force?
They both feel the same amount of force!! Again a consequence
of Newton’s 3rd law. A rope just transfers that force.
Both objects connected to a rope feel same amount of force.
Think about what happens if one side lets go.
Newton’s Third Law Competition
There are 6 questions, work in groups of 4
Write your answers on a piece of paper with your group names
Don’t write on the hand outs. OK
The group that gets the most correct will get 2 BP per person
A tie will result in 1 BP for all members involved.
You have 10 minutes to come up with your answers.
Then we will go over them
Book problems:
Page 84 – 85:
24, 25, 28, 32, 33, 35, 36, 37, 38, 39
due Fri
Friction
What would life be like without it?
Friction
If an object slows down, where does its
Kinetic Energy go?
What causes it?
•all surfaces are rough on a microscopic scale.
Frictional force can also be thought of as a lot of
teeny weeny normal forces resulting from points
of contact
also may be some attraction between the
atoms and molecules between two surfaces
This is definitely the case when it comes to many adhesives
Two types of Friction
• Force that opposes the motion or
impending motion of two surfaces
in contact
FF,box,ground
FF,box,ground
Static Frictionobjects are not sliding
Kinetic or Sliding Frictionobjects are sliding
FF,box,ground
FF,box,ground
Friction Lab
Friction Lab Recap
Factor
Effect on FF
Weight pushing down
Weight FF
Area of Contact
No Effect
Materials in Contact
Depends on Materials
Static vs Kinetic
Static > Kinetic
Velocity of Object
No effect
DOES depends on:
• Types of material in contact (rougher = more)
FF
• Greater Normal force = more friction
• Whether the object is moving or not
(static is more than kinetic)
It does NOT depend on:
•the area of contact
•the speed of movement
FF = mFN
Friction Force
in N
What are the
units on m?
Normal Force
in N
Coefficient of Friction
Fudge Factor based on:
Materials in contact
&
Moving (kinetic, mK) or Stationary (static, mS )
Why FN & not just Fg
(sometimes they are the same)
consider a stationary box
FN
Fg
Why FN & not just Fg
(sometimes they are different)
If another force is pushing down or up
FN, box, floor
Fg, box, earth
Frocket
Why FN & not just Fg
(sometimes they are different)
If another force is pushing down or up
FT, box, rope
FN, box, floor
Fg, box, earth
Why FN & not just Fg
(sometimes they are different)
If the surface is not level
FN, box, floor
Fg, box, earth
Some examples of coefficients of friction
ms
rubber/dry concrete
rubber/wet concrete
steel / steel (dry)
steel / steel (oiled)
dry auto brakes
wet auto brakes
.8
.60
.78
.15
mk
.65
.40
.58
.06
.4
.2
Why is it easier to spin your tires when it is wet?
Smooth Surface
Rougher Surface
Block is stationary - STATIC FRICTION
(the harder you pull, the greater Friction resists)
FF = ms FN
FF
(N)
Time
When the applied force exceeds the maximum FF,
the object slips and begins to move
kinetic friction
FF = ms FN
FF
(N)
FF = mk FN
Time
The wheel of a tire is rolling down the road,
the driver hits the gas and friction drives the
wheel forward.
Is this
Static Friction
or
Kinetic Friction?
Why do you accelerate poorly
when you are “peeling out”
People have been known to let some air out of
their tires when street drag racing to get more
contact with the road.
Does this help acceleration?
What is weight?
What is a situation that makes
you feel heavier or lighter?
What does your bathroom scale measure?
Mass or Force
Is mass a force?
NO, but it can exert a force,
What is causing the person to exert a force
This force is
usually referred to
as weight
Fg = m g
Mass of the
person
gravity,
9.80 m/s2
Weight is a force
It changes depending on where you are
earth vs. the moon
the acceleration on object would experience on the moon
is 1.6 m/s2
What force would 65.8 kg exert there?
F = ma = 65.8 kg * 1.6 m/s = 110 N
I have “lost” 84% of my weight!!
The FEELING of weight or weightlessness
(like that tickle in your stomach when you go over a hill)
comes from feeling the forces at work you.
Forces on your outsidelike your butt in a chair or feet on the floor
Forces on your insideyour guts being pushed by your pelvic
bone
Your apparent “weight” also depends on if you’re movement
How do you feel when?
The elevator starts upward
Heavier
after you get going
Same
Slowing down to a stop
Lighter
the elevator cord is cut
Weightless
stationary
FN,you,floor
F net= 0
you feel
“normal”
Fg,you,earth
FN,you,floor
accelerating
up
F net=
Fg,you,earth
You feel “heavier” not because you weigh more but because the floor
is pushing harder on you. (Your pelvic bone has to push harder on
your insides as well)
stationary
FN,you,floor
F net= 0
you feel
“normal”
Fg,you,earth
FN,you,floor
moving up
but
slowing
F net=
Fg,you,earth
You feel “lighter” not because you weigh more but because the floor
is barely pushing on you. You get a tickle in your “stomach”