Transcript F fr

A child of mass m rides on a sled down a
slick, ice-covered hill inclined at an angle
θ with respect to the horizontal.
a)What is the acceleration of the child
b)What is the normal force exerted on
the child by the sled
Quiz
1. Draw a sketch
2. Draw a free body diagram
3. Set Cartesian coordinates (make a
smart choice)
4. Find all x and y components of all
forces and the acceleration
5. Write down Newton’s Second Law for
the components
Hint: Remember, that normal force is
always perpendicular to a surface
A child of mass m rides on a sled down a
slick, ice-covered hill inclined at an angle
θ with respect to the horizontal.
a)What is the acceleration of the child
b)What is the normal force exerted on
the child by the sled
Quiz
N
a
θ
W
ΣF=Fnet
x
y
N
0
N
W
-mgsin θ
-mgcosθ
ma
-ma
0
mg sin θ = ma
a = g sin θ
N – mg cos θ = 0
N = mg cos θ
Place a block on a table. Attach a large
spring scale to the block. Pull the scale
harder and harder. The scale reading
increases more and more, then the scale
pulls hard enough, and the block starts
moving.
Place a block on a table. Attach a large
spring scale to the block. Pull the scale
harder and harder. The scale reading
increases more and more, then the scale
pulls hard enough, and the block starts
moving.
The scale
pulls on the
block, which
does not start
moving
The scale
pulls harder,
but the block
still does not
start moving
N
Ffr
W
N
N
Pull
Ffr
Pull
W
The block is on
the table, at the
moment just
before it starts
moving
Ffr
W
The scale
pulls the
block with
constant
velocity
N
N
Pull
Ffr
W
The scale
pulls harder,
so the block
moves with an
acceleration
Pull
Ffr
W
Pull
Friction
Friction
Fk   k N
Fsmax
Fk  N
Fk
Fs max  s N
0
Static
Kinetic
Pull
A child of mass m rides on a sled down a
rough hill inclined at an angle θ with respect
to the horizontal. The coefficient of the
kinetic friction is μk and static friction is μs
a)What is the acceleration of the child
b)What is the normal force exerted on the
child by the sled
x
y
N
0
N
W
-mgsin θ
-mgcosθ
Ffr
Ffr=μkN
0
ma
-ma
0
N
Ffr
a
θ
W
μkN - mg sin θ = -ma
a
N – mg cos θ = 0
a = μkN/m - g sin θ
N = mg cos θ
a = μkg cos θ - g sin θ
N = mg cos θ
A block of mass m is placed on a board that could be inclined
with respect to the horizontal. The coefficient of the kinetic friction
is μk and static friction is μs.
a=0
At what angle θ the board starts sliding?
x
y
N
0
N
W
-mgsin θ
-mgcosθ
Ffr
Ffr=μsN
0
ma
-ma
0
N
Ffr
a
θ
W
a = μsg cos θ - g sin θ
0 = μsg cos θ - g sin θ
N = mg cos θ
tan θ = μs
Homework
(due Monday)
Section 4.5, 4.6
(Examples!!!!)
Problems (p.111)
#20, 32, 35, 36, 40, 45
Friction
0
Quiz
Fk  N
Fsmax
Fk
Static
Kinetic
Fk   k N
Fs max  s N
Pull
A block of mass m is projected
with an initial speed v0 along the
horizontal plane with coefficient
of kinetic friction μk and of static
friction μs. How far will the block
go until it stops?