Friction: Basics and Coefficients Explained
Friction opposes motion
Friction opposes motion
Friction is dependent on the texture of the surfaces
Friction is dependent on the texture of the surfaces
Friction is dependent on normal force
Friction is dependent on normal force
Friction opposes motion
Friction opposes motion
Friction is dependent on the texture of the surfaces
Friction is dependent on the texture of the surfaces
Friction is dependent on normal force
Friction is dependent on normal force
F
F
fr
fr
=
=
F
F
n
n
μ
μ
i
i
s
s
c
c
a
a
l
l
l
l
e
e
d
d
c
c
o
o
e
e
f
f
f
f
i
i
c
c
i
i
e
e
n
n
t
t
o
o
f
f
f
f
r
r
i
i
c
c
t
t
i
i
o
o
n
n
has no units
has no units
depends on characteristics of both surfaces
depends on characteristics of both surfaces
Higher
Higher
rougher surface / more friction
Note: Friction does NOT
depend on the surface
area of contact
The coefficient of friction is different for when an object is at rest
and when it is moving.
•
μ
s
=
coefficient of static friction (object at rest)
•
μ
k
= coefficient of kinetic friction (object moving)
Static friction is greater than kinetic friction - its harder to start
an object moving than it is to keep it moving.
A 28 kg crate initially at rest on a horizontal floor
requires a 75 N horizontal force to set it in motion. Find
the coefficient of static friction between the crate and
the floor.
What do we do first?
A 28 kg crate initially at rest on a horizontal floor requires a
75 N horizontal force to set it in motion. Find the coefficient
of static friction between the crate and the floor.
Remember our strategy:
1)
Draw a free body diagram
2)
Identify all variables
3)
Identify relevant equations
4)
Solve!
5)
Check your work!
A 28 kg crate initially at rest on a horizontal floor
requires a 75 N horizontal force to set it in motion. Find
the coefficient of static friction between the crate and
the floor.
μ
s
= 0.27
A force of 40.0 N accelerates a 5.0-kg block at 6.0 m/s
2
along a horizontal surface.
a. How large is the frictional force?
b. What is the coefficient of friction?
A force of 40.0 N accelerates a 5.0-kg block at 6.0 m/s
2
along a horizontal surface.
a. How large is the frictional force?
b. What is the coefficient of friction?
F
f
= 10 N
μ
k
= 0.2
2) A 12 kg suitcase is pushed with a force of 38 N to the
left. If the coefficient of kinetic friction between the
suitcase and the floor is 0.3, how far will the suitcase
move after 5 sec?
2.8 m
FOLLOW THE STEPS! Make a plan before you plug in
numbers! Be able to explain your reasoning!
1)
A 30 kg crate requires a 53 N force to keep it moving
at 1 m/s. Find the coefficient of kinetic friction.
2)
You need to move a 105-kg sofa to a different
location in the room. It takes a 403-N force to start
the sofa moving. What is the coefficient of static
friction between the sofa and the carpet?
k
= 0.18
Question:
Question:
How does the weight of a person in an elevator
How does the weight of a person in an elevator
depend on the motion of that elevator?
depend on the motion of that elevator?
What will the scale show if the elevator is
What will the scale show if the elevator is
1.
at rest or moving with constant speed
at rest or moving with constant speed
2.
speeding up
speeding up
3.
slowing down
slowing down
Newton’s 3. law:
Newton’s 3. law:
Force with which the person acts on the scale (reading of the scale) is
Force with which the person acts on the scale (reading of the scale) is
equal to the normal force on the person.
equal to the normal force on the person.
So,
So,
if we find normal force we know the
if we find normal force we know the
reading of the scale, so called
reading of the scale, so called
APPARENT WEIGHT
APPARENT WEIGHT
Let’s assume that elevator is moving upward, and let this be positive direction.
Let’s assume that elevator is moving upward, and let this be positive direction.
1. draw free body diagram 2. apply Newton’s 2. law : F
1. draw free body diagram 2. apply Newton’s 2. law : F
net
net
= ma
= ma
F
F
n
n
– mg = ma = 0
– mg = ma = 0
→
→
F
F
n
n
= mg
= mg
apparent weight = weight
apparent weight = weight
1. elevator is at rest or moving with constant speed
1. elevator is at rest or moving with constant speed
2. elevator is speeding up: a is positive
2. elevator is speeding up: a is positive
F
F
n
n
– mg = ma
– mg = ma
→
→
F
F
n
n
= mg + ma
= mg + ma
apparent weight > weight
apparent weight > weight
the scale would show more, and you would feel heavier
the scale would show more, and you would feel heavier
3. elevator is slowing down: a is negative
3. elevator is slowing down: a is negative
F
F
n
n
– mg = - ma
– mg = - ma
→
→
F
F
n
n
= mg - ma
= mg - ma
apparent weight < weight
apparent weight < weight
the scale would show less, and you would feel lighter
the scale would show less, and you would feel lighter
You are riding in an elevator holding a spring scale with a
1kg mass suspended from it. You look at the scale and
see that it reads 9.3 N. What, if anything, can you
conclude about the elevator’s motion at that time?
1) Solve the elevator problem for an elevator
traveling downward. YOU MUST EXPLAIN YOUR
REASONING USING NEWTON’S 2
nd
LAW
2) Write down 2 things you learned friction
problems.
Friction is a force that opposes motion, influenced by surface texture and normal force. The coefficient of friction varies for objects at rest versus in motion. Learn about static and kinetic friction, key concepts, and how to solve friction problems step by step.
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Presentation Transcript
WHAT DO YOU REMEMBER ABOUT FRICTION? Friction opposes motion Friction is dependent on the texture of the surfaces Friction is dependent on normal force motion friction
WHAT DO YOU REMEMBER ABOUT FRICTION? Friction opposes motion Friction is dependent on the texture of the surfaces Friction is dependent on normal force Ffr = Fn is called coefficient of friction has no units depends on characteristics of both surfaces Higher = rougher surface / more friction motion Note: Friction does NOT depend on the surface area of contact friction
The coefficient of friction is different for when an object is at rest and when it is moving. s = coefficient of static friction (object at rest) k = coefficient of kinetic friction (object moving) Static friction is greater than kinetic friction - its harder to start an object moving than it is to keep it moving. surface-on-surface hook velcro-on-fuzzy velcro avg tire-on-dry pavement grooved tire-on-wet pavement smooth tire-on-wet pavement metal-on-metal (lubricated) steel-on-ice steel-on-Teflon s k >6.0 0.9 0.8 0.5 0.1 0.1 0.05 >5.9 0.8 0.7 0.4 0.05 0.05 0.05
FRICTION PROBLEMS WE DO A 28 kg crate initially at rest on a horizontal floor requires a 75 N horizontal force to set it in motion. Find the coefficient of static friction between the crate and the floor. What do we do first?
FRICTION PROBLEMS WE DO A 28 kg crate initially at rest on a horizontal floor requires a 75 N horizontal force to set it in motion. Find the coefficient of static friction between the crate and the floor. Remember our strategy: Draw a free body diagram Identify all variables Identify relevant equations Solve! Check your work! 1) 2) 3) 4) 5)
FRICTION PROBLEMS WE DO A 28 kg crate initially at rest on a horizontal floor requires a 75 N horizontal force to set it in motion. Find the coefficient of static friction between the crate and the floor. s = 0.27
FRICTION PROBLEMS WE DO A force of 40.0 N accelerates a 5.0-kg block at 6.0 m/s2 along a horizontal surface. a. How large is the frictional force? b. What is the coefficient of friction?
FRICTION PROBLEMS WE DO A force of 40.0 N accelerates a 5.0-kg block at 6.0 m/s2 along a horizontal surface. a. How large is the frictional force? b. What is the coefficient of friction? Ff = 10 N k = 0.2
FRICTION PROBLEMS WE DO 2) A 12 kg suitcase is pushed with a force of 38 N to the left. If the coefficient of kinetic friction between the suitcase and the floor is 0.3, how far will the suitcase move after 5 sec? 2.8 m
FRICTION PROBLEMS YOU DO FOLLOW THE STEPS! Make a plan before you plug in numbers! Be able to explain your reasoning! 1) A 30 kg crate requires a 53 N force to keep it moving at 1 m/s. Find the coefficient of kinetic friction. k = 0.18 2) You need to move a 105-kg sofa to a different location in the room. It takes a 403-N force to start the sofa moving. What is the coefficient of static friction between the sofa and the carpet?
Elevator problem Question: How does the weight of a person in an elevator depend on the motion of that elevator? What will the scale show if the elevator is 1. at rest or moving with constant speed 2. speeding up 3. slowing down Newton s 3. law: Force with which the person acts on the scale (reading of the scale) is equal to the normal force on the person. So, if we find normal force we know the reading of the scale, so called APPARENT WEIGHT
Lets assume that elevator is moving upward, and let this be positive direction. 1. draw free body diagram 2. apply Newton s 2. law : Fnet = ma Fn + 1. elevator is at rest or moving with constant speed Fn mg = ma = 0 Fn = mg apparent weight = weight mg Fn 2. elevator is speeding up: a is positive Fn mg = ma Fn = mg + ma apparent weight > weight the scale would show more, and you would feel heavier mg Fn 3. elevator is slowing down: a is negative Fn mg = - ma Fn = mg - ma apparent weight < weight the scale would show less, and you would feel lighter mg
YOU DO You are riding in an elevator holding a spring scale with a 1kg mass suspended from it. You look at the scale and see that it reads 9.3 N. What, if anything, can you conclude about the elevator s motion at that time?
EXIT TICKET 1) Solve the elevator problem for an elevator traveling downward. YOU MUST EXPLAIN YOUR REASONING USING NEWTON S 2nd LAW 2) Write down 2 things you learned friction problems.
