Download Lecture 5

Welcome to Physics 1010: The
Physics of Everyday Life
www.colorado.edu/pjysics/phys1010
HW 3 is available on CULearn
Next week: class will be covered by
Rachel Pepper
You are responsible for bringing a working
clicker every day.
Today
• Skydiving (Net force)
• Crashing Cars (Ouch)
• Driving in snow (Friction)
We have a bunch of unregistered clickers. Register clicker
today!
Confusing language
Regular people: acceleration: speeding up.
Physicists: acceleration: the rate of speeding up or
slowing down or changing direction of motion.
Regular people: kg: a weight= 2.2 pounds,
Physicists: kg: a mass. On earth this mass has a weight
of 1kg x 9.8 m/s2 = 9.8 N
Regular people: Velocity: how fast you are going = speed.
Physicists: Velocity: the speed and direction of motion.
Terminal velocity
A 100 kg man jumps from a plane. Immediately after he jumps,
a. The force of gravity on the man is 100 kg downwards, the net force on
him is 100kg downwards, and his acceleration is 9.8 m/s2 downwards.
b. The force of gravity on the man is 100 kg downwards, his net force is
0, and his acceleration is 0 m/s2.
c. The force of gravity on the man is 980 Newtons downwards, the net
force on him is 980 Newtons downwards, and his acceleration is 9.8
m/s2 downwards.
d. The force of gravity on the man is 980 Newtons, the net force on the
man is 0 Newtons, and his velocity increases until it is 9.8 m/s
downwards and then stops increasing.
e. The force of gravity on the man is 980 Newtons, the net force on the
man is 0 Newtons, and his acceleration is 9.8 m/s2 downwards.
A 100 kg man jumps from a plane.
Immediately after he jumps,
c. The force of gravity on the man is
980 Newtons downwards, the net
force on him is 980 Newtons
downwards, and his acceleration is
9.8 m/s2 downwards.
Fgravity on man
Forces always measured in Newtons! 100 kg is a mass.
Force of gravity on an object = (mass of object) x (9.8 m/s2)
and is straight downwards.
= 100 kg x 9.8 m/s2 = 980 Newtons
Sum of all forces = Fnet  In this case, Fnet = Fgravity on man
Fnet = mass x acceleration
Since Fnet is not zero, tells us man will accelerate!
Acceleration is always in the same direction as the net force!
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As man falls for some time, he finds
that he is falling at a constant
velocity. At this time…
As man falls for some time, he finds that he is falling at a
constant velocity. At this time…
a. The force of gravity on the man is 100 kg downwards, the
net force on him is 100kg downwards, and his acceleration is
9.8 m/s2 downwards.
Fair on man
e. The force of gravity on the man is
980 Newtons, the net force on the
man is 0 Newtons, and his
acceleration is 0 m/s2.
b. The force of gravity on the man is 100 kg downwards, his
net force is 0, and his acceleration is 0 m/s2.
Force of gravity
on an object (on Earth)
c. The force of gravity on the man is 980 Newtons downwards,
the net force on him is 980 Newtons downwards, and his
acceleration is 9.8 m/s2 downwards.
Fgravity on man
= (mass of object) x (9.8 m/s2)
= 100 kg x 9.8 m/s2 = 980 Newtons
and is straight downwards.
Sum of all forces = Fnet
 In this case, Fnet = Fgravity on man + Fair on man = 0 !!!!
d. The force of gravity on the man is 0 Newtons, the net force
on the man is 0 Newtons, and his acceleration is 0 m/s2.
Fnet = mass x acceleration
e. The force of gravity on the man is 980 Newtons, the net
force on the man is 0 Newtons, and his acceleration is 0
m/s2.
Since Fnet is zero, tells us man will not accelerate!
Acceleration will be zero, Velocity will be constant !
Crashing little cars:
Fair on man
Fnet
Fgravity on man
Velocity
Before reaching terminal
velocity, forces don’t quite
cancel. Still small
acceleration.
Case A: hit hard end of ramp
time
Case B: hit sponge at end of ramp
Sketch your predictions of
the velocity vs. time,
acceleration vs time, net
force, and measured force
(by probe) for this motion.
Starting when we let go and
ending after crash.
FOR CASE A AND B on
same graph.
Since Fnet is not zero, tells us man will accelerate!
Acceleration will be smaller than 9.8 m/s2 because net force
is smaller than force of gravity on man. Velocity will be
changing ! Speeding up in downwards direction
SPONGE
Crashing little cars
time
time
time
time
time
time
Velocity
Velocity
Net
force
time
Meas
Acceleration Force
Velocity
Net
Force
Meas.
Acceleration Force
time
C
Net
force
B
Meas
Acceleration Force
Case A: hit hard end of ramp
A
Case B: hit sponge at end of
ramp
time
time
time
D. Same as a, but with all curves on negative side of 0 instead of positive.
E. None of these choices.
time
Crashing little cars:
Case 1: hit hard end of ramp
SPONGE
Case 2: hit sponge at end of ramp
Just before crash:
Car in case 1 is at same velocity as Car in case 2
After crash:
Both car in 1 and 2 are at rest. Velocity = 0 m/s
Same in both
Case 1 and 2
Acceleration =
change in velocity
during crash
time elapsed during crash
Shorter in Case 1
Than Case 2
time
time
So, during crash
acceleration of 1 greater
than acceleration of 2. Force
in 1 greater … ouch!
A
Velocity
Fnet = mass x acceleration
Net
Force
 In this case, Fnet = Fgravity on man + Fair on man
time
Meas.
Acceleration Force
Sum of all forces = Fnet
Measure Net
Force
Acceleration Force
SPONGE
time
time
time
time
Wall exerts force on car.
2
Force sensor plus metal weight has mass of 1.2 kg,
It weighs approx. how many N? (How much force needed to lift it?)
a. 1.2
b. 12 c. 1.2/2.2
d. (1.2/2.2) x 9.8
Force sensor + metal weight
PULL 
Positive direction
Answer: b
Force sensor + metal weight
B
time
C
Applied
Force
Applied
Force
?N
time
Starts moving
here
Applied
Force
Friction between table and force sensor.
Predict graph of force which we must apply by pulling on string in order to
move sensor along table at a constant speed … prediction should include
force from before starting to pull until sensor is moving at constant
speed across the table. (Make guess as to specific value as well as shape.)
Friction between table and
force sensor.
Predict graph of force which
we must apply by pulling on
string in order to move
sensor along table at a
constant speed … prediction
should include force from
before starting to pull until
sensor is moving at constant
speed across the table.
Applied
Force
A
PULL 
time
-? N
Force sensor + metal weight
time
Force sensor + metal weight
PULL 
A
PULL 
time
Forces
Forces
Starts moving
here
Applied Force (by string)
A
Applied Force
Starts moving
here
Friction Force
Static friction
Net force = 0
(velocity =0)
time
Friction Force
Sliding friction
Net force = 0 (const. velocity)
Balancing sliding friction:
How much force is required to keep it moving along table at constant
speed if weight of (force sensor + metal ) = mg = 12 N ?
a. Weight x 0.3, b. Weight x 0.7, c. Weight x 1.5
d. Weight x 5
Increase the mass to 30 kg, what is force needed to keep
moving at a constant speed?
a. same as before, b. less than before, c. more than before.
Static friction
Sliding friction
Answer is A. Sliding friction force =~ 1/3 of the
weight. SO, applied force must be +Weight X 0.3.
Makes sense that takes less force to slide box
than to lift box!
Wheels and friction. (and using physics to drive better)
1. Why do you stop faster in car on snow if don’t skid?
2. How to keep from getting stuck in snow.
Answer is c. Larger mass, larger sliding friction force ….
The heavier the box, the more force it takes to push it
across the floor.
Friction allows cars to speed up or slow down.
Car tires exert force on road … road exerts force on tires.
Drive … engine rotates tire.
Tire pushes backwards on road.
Sliding friction force does not change much as speed
increases. Takes about as much force to go at slow
constant velocity as high constant velocity. (unless fast
enough that air resistance important. )
Force of road on tire (car)
Force of tire on road
Friction force- Always opposes change in motion.
No motion, no friction. No change in speed, little friction.
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Wheels and friction. (and using physics to drive better)
1. Why do you stop faster in car on snow if don’t skid?
2. How to keep from getting stuck in snow.
Friction - sliding and static. (Books say static bigger, our
experiments show not usually true. Not real law of
physics like F= ma or conservation of energy where it
always works. Is true on snow, probably more heating
and melting if slide.)
atoms of same color all hooked together by forces like tiny springs
friction
atoms (never perfectly smooth)
block
Drive … engine rotates tire.
Tire pushes backwards on road.
table
Force of road on tire (car)
Force of tire on road
Friction force- Always opposes change in motion.
No motion, no friction. No change in speed, little friction.
many intro physics books give
simple formulas describing this,
but they are wrong.
atoms
block
More force pushing surfaces together.
Squash atoms together more,
more friction. Also type of atom, amount
of surface area, …
block
move
friction force
opposing motion
table
table
As surface dragged across- start atoms vibrating= heat!
Heat energy = work= Ffriction x distance moved.
When not sliding, can get embedded and stuck more than
when sliding. So static friction force bigger than sliding
frict. force.
desk leg
floor
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Wheels- no sliding surfaces across each other! So no work,
no heating, no wasted energy.
atoms
tire
Add lubricant
slimy stuff that
does not stick to
either surface and
flows out
of the way of the
atoms, but keeps
surfaces apart.
road
Why are you supposed to pump your brakes when stopping
on wet or icy road?
a. to avoid brakes overheating and wearing out faster
b. to keep tires rolling so do not skid and wear out tires
c.to make brake lights flash on and off to get attention
of drivers behind you.
d. to keep tires rolling so will slow down more quickly
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