Stuff you asked about:
Imagine for a moment friction didn't exist. That would be hilarious. Cars and people
sliding all over the place like "WHOOAAAAAA!“
It would be great if you could make the equal versus less than or equal to rules for
kinetic and static friction more clear. When would there be a need for the less than
sign? Thanks!
Why did the cat slide off the roof? It lost it's mu
What causes friction? Why is the coefficient of static friction always larger than the
coefficient of kinetic friction (why is it easier to keep a heavy object moving than it is
to get it moving in the first place)?
Please focus on the concepts that trick us up. I you probably know them better than
me.
I often jump to conclusions instead of actually writing out the newton's second law
stuff....and I often get it wrong.
I think I actually understand the concept of friction this time. I still dont necessarily
understand circular motion though.
I feel like I'm becoming increasingly overwhelmed in all of my classes. It's as if I'm not
learning anything, information is just punching me in the face and then bouncing
away.
Even though I hate it, thank you for making us do these pre-lectures and
checkpoints twice a week. Otherwise, I would never look at my physics
outside of class.
I can't wait for the explosion demo!!!!
Physics 221
Lecture 6
Today’s Concept:
Friction
Stuff you asked about:
Ramp problems are still difficult. could you literally do one of the
problems with us in lecture? (break up the component of gravity…)
90
q
90-q
90
y
mg
x
q
Friction
Direction: Always opposes the relative motion of two surfaces
Magnitude: Kinetic fk=mkN
Static: Does what needs to do up to a point: fs  msN
Checkpoint
A box sits on the horizontal bed of a moving truck.
Static friction between the box and the truck keeps the box from
sliding around as the truck drives.
mS
a
If the truck moves with constant accelerating to the left as
shown, which of the following diagrams best describes the
static frictional force acting on the box:
A
B
C
Describe more about flat bed truck problem.
Clicker Question
A box sits on the horizontal bed of a moving truck.
Static friction between the box and the truck keeps the box from
sliding around as the truck drives.
mS
a
Friction is the only horizontal force that acts on the box:
A) True
B) False
Checkpoint
mS
a
If the truck moves with constant accelerating to the left as shown,
which of the following diagrams best describes the static frictional
force acting on the box:
A
B
C
A) its acceleration is to the left, so the force is to the left.
B) The frictional force always acts opposite to the direction of
motion;
Checkpoint
mS
a
If the truck moves with constant accelerating to the left as shown,
which of the following diagrams best describes the static frictional
force acting on the box:
A
B
C
Be Careful…
The truck is accelerating to the left so the box wants to go to the right.
The frictional force is what is keeping the box from doing this so it must
be pointing in the same direction as the acceleration.
I'm still confused on how acceleration can act in one direction but
make things move in the opposite.
Clicker Question
A box of mass M sits on a horizontal table. A horizontal string having
tension T applies a force on the box, but static friction between the
box and the table keeps the box from moving.
What is the magnitude of the net force acting on the box?
T
f
A) Mg
B) mMg
C) T
D) 0
M
Since acceleration is zero.
Checkpoint
A box of mass M sits on a horizontal table. A horizontal string
having tension T applies a force on the box, but static friction
between the box and the table keeps the box from moving.
What is the magnitude of the static frictional force acting on the box?
A) Mg
B) mMg
C) T
D) 0
T
f
M
B) f=μ(N)=μMg
C) Friction must equal T because the net force must be
zero if the box is not moving.
Clicker Question
Great question:
In checkpoint 2 isn't the frictional force equal to both μMg and T?
T
f
A) YES
B) NO
M
f can be less than uMg
(i.e. suppose T = 0)
In Your Prelecture
What is the magnitude of the static frictional force acting on the box?
Does Theta(max) refer to the maximum angle at which the box will
stand still rather than sliding down the ramp, or the angle at which the
box will begin to slide?
YES
I find the concept of the car travelling in a circle to be the most difficult.
I can't wait for the explosion demo!!!!
Checkpoint
A block slides on a table pulled by a string attached to a hanging
weight. In Case 1 the block slides without friction and in Case 2
there is kinetic friction between the sliding block and the table.
m2
m2
Case 1
(No Friction)
g
Case 2
m1
(With Friction)
In which case is the tension in the string biggest?
A) Case 1 B) Case 2 C) Same
60% got this right
g
m1
Clicker Question
A block slides on a table pulled by a string attached to a hanging
weight. In Case 1 the block slides without friction and in Case 2
there is kinetic friction between the sliding block and the table.
m2
m2
Case 1
(No Friction)
T
m1
a
m1g – TCase
= m12a
(With Friction)
T = m1g - m1a
m1g
What is the tension in the string in Case 1?
A) T = 0
B) T = m1g
C) T is between 0 and m1g
g
m1
m2
m2
Case 1
(No Friction)
Case 2
m1
(With Friction)
In which case is the acceleration of the blocks biggest?
A) Case 1 B) Case 2 C) Same
m1
m2
m2
Case 1
(No Friction)
Case 2
a1 m
1
(With Friction)
In which case is the tension in the string biggest?
A) Case 1 B) Case 2 C) Same
B) in case 2 the block system will accelerate less due
to the friction, therefore the net force on the
suspended block must be less. since the gravitational
force can not change the tension must increase in the
opposite direction
a2 m
1
Lets work it out
A block (m2) slides on a table pulled by a string attached to
a mass (m1) hanging over the side. The coefficient of kinetic
friction between the sliding block and the table is mk. What
is the tension in the string?
m2
m1
Pleaaaaaaaaaase go over pulleys and tension in strings I don't
understand how it works at all.
Clicker Question
A block (m2) slides on a table pulled by a string
attached to a mass (m1) hanging over the side. The
coefficient of kinetic friction between the sliding block
and the table is mk. What is the tension in the string?
T2
T1
m2
What is the relationship between the magnitude
of the tension of the string at block 2 and the
magnitude of the tension in the string at block 1?
A) T1 > T2 B) T1 = T2 C) T1 < T2
m1
Clicker Question
A block (m2) slides on a table pulled by a string attached to a
mass (m1) hanging over the side. The coefficient of kinetic
friction between the sliding block and the table is mk. What is
the tension in the string?
a2
m2
What is the relationship between the magnitudes of
the acceleration of the two blocks?
A) a1 = a2 B) a1 < a2 C) a1 > a2
a1
m1
m2
1) FBD
m2
m1
m1
1) FBD
m2
N
f
m2
T
T
m2g
m1
m1
m1g
1) FBD
2) SF=ma
m2
N
f
T
m2
T
m1
m1
m2g
Block 2
N = m2g
m1g Block 1
T – m m2g = m2a
m1g – T = m1a
add
m1g – m m2g = m1a + m2a
m1g – m m2g
a=
m1 + m2
1) FBD
2) SF=ma
m2
N
f
m2
T
T
m1
m1
m2g
m1g
m1g – m m2g
a=
m1 + m2
T = m1g – m1a
T is smaller when a is bigger