Transcript File

Do Now
Joshua and Brittany are working on an experiment. Joshua
pulls a 5 kg block at a constant speed across one surface with
a force of 10 N. Brittany pulls a 2 kilogram block at a constant
speed across a second surface with a force of 6 N.
a)
b)
Draw and label free-body diagrams for Joshua and Brittany’s blocks
and calculate coefficient of friction. Show your calculations and
circle your final answer.
Which conclusion do you agree with?
A.
B.
C.
D.
Joshua’s force of friction was greater and his surface had a greater
coefficient of friction.
Joshua’s force of friction was greater but his surface had a smaller
coefficient of friction.
Brittany’s force of friction was greater and her surface had a greater
coefficient of friction.
Brittany’s force of friction was greater but her surface had a smaller
coefficient of friction.
Quiz 2-6 Tracking
1: 68% (3)
2: 77% (1)
4: 72% (1)
6: 77% (6)
Homework :D
Physics to Go 2-7
Wednesday
2.
Quiz 2-7 & WDYTN 2-7
Thursday
3.
WDYTN 2-6 Rewrites
Friday
Turn
It In!
1.
January 6th
January 7th
January 8th
Objectives 2-7
We are learning…
Agenda
 Friction is a force. The coefficient of friction
BIG Questions
is a property of the two surfaces in contact and
is related to the weight of the object. Ff = µFN,
where FN is the normal force.
•
Do Now 10
•
Physics Talk 15
We will be successful when we can…
•
TWPS 5
 Apply the definition of the coefficient of
•
Team Challenge 5
•
Checking Up 10
1.What is friction?
2. How
is it µmeasured?
sliding
friction,
 Measure the coefficient of sliding friction
between
thedoes
soles ofitathletic
shoeson?
and a
3.What
depend
variety of surfaces
 Calculate the effects of frictional forces on
the motion of objects
Friction
Newton’s Second Law of Motion  ________________
Motion with no acceleration happens when there is no
net force acting on an object
 Velocity must be ___________
 All forces must add up to ___________
 Force due to friction is equal to & opposite of applied force
Friction: force that resists motion between two objects
in contact
Analyzing the Forces
 Shoe did not accelerate in
vertical direction
 Net force must be ____
 All forces must add up to ____
 Normal force is ____________
& _______________________ of
gravitational force
Normal force: force acting
perpendicular to or at right
angle to a surface
Coefficient of Friction
Coefficient of sliding friction: quantity symbolized
by the Greek letter µ; it’s value depends on the
properties of the two surfaces in contact and is used to
calculate the force of friction
 No units
 Expressed as a decimal
 Changes to the surface (material, texture, moisture,
lubrication) cause µ to change
Think & Write|Pair & Share
Athletes often wax the bottoms of skies right
before a race because…
Materials
µ
Wood on wood
Wood on snow
Waxed wood on snow
Rubber on ice
Wood on ice
Ice on ice
0.2
0.1
0.04
0.15
0.08
0.03
Team Challenge
A vehicle having a mass of 1000 kg had an accident on a wet, but level, concrete
road under foggy conditions. The tires were measured to have μ = 0.55 on wet
concrete. The driver locked the brakes, skidded for 6 seconds, and then hit the
guardrail casugin a very small dent because the vehicle stopped just as it
touched the guardrail. The driver claimed to be driving 29 m/s. You have been
hired as an investigator to determine if the driver is telling the truth.
 What ist he weight of the vehicle?
 The force force produces the negative acceleration that reduced the
velocity of the vehicle from its initial unknown speed to zero. Find the
value of the frictional force.
 Use the frictional force to calculate the acceleration (remember that it is a
negative number).
 Using the acceleration and the time over which acceleration occurred,
calculate the change in speed that the acceleration would produced.
 Use the change in speed to find the original speed of the vehicle when the
brakes were applied. Write a statement of your findings, including your
opinion of the driver’s claim.
Checking Up