Transcript x - prettygoodphysics

Mary pulls on a lawnmower parallel to the surface so that it moves
at a constant speed up the hill.
Which
free body
diagram
best
Construct
an FBD
that best
represents
represents
forces
on the mower?
the motionthe
of the
mower.

y
Nsurface on mower
1

x

Ff surface on mower
Fstudent on mower

FEarth on mower

y Nsurface on mower
3

Ff surface on mower

FEarth on mower
x

Fstudent on mower
2

Nsurface on mower
y

Ff surface on mower

FEarth on mower
4
y
 x
Fstudent on mower

Nsurface on mower

 x
Ff surface on mower
Fstudent on mower

FEarth on mower
Stacey pulls a mower at 40-degrees relative to the ground.
Which free body diagram best
representsan
the
forces
therepresents
mower?
Construct
FBD
that on
best
the above situation.
1

y Nsurface on mower

Ff surface on mower
40

FEarth on mower
3
y

Fstudent on mower
Fstudent on mower
400
x

Ff surface on mower
0
x

Nsurface
 on mower
Fstudent on mower

Ff surface on mower

FEarth on mower
400
2

Nsurface
on mower

y
x

FEarth on mower
4
y

Nsurface
 on mower
40 Fstudent on mower

x
Ff surface on mower

FEarth on mower
0
Marsha pulls a sled on ice (no friction) with a 150-N force at
an angle of 200. The mass of the sled and her brother is 45 kg.
g = 10 m/s2. Stacey represented this situation pictorially and
mathematically.
x
x: (150 N) = (45 kg) ax
y: N – (45 kg)(10 m/s2) = (45 kg) 0
Did the student mislabel or forget to include anything?
1 (150 N) should be
(150 N) sin 20o.
4
(1) and (3) are needed
corrections.
2 (150 N) should be
(150 N) cos 20o.
5
(2) and (3) are needed
corrections.
3 Include a (150 N) sin 20o term in
the y equation.
6
The above mathematical
representation is correct.
Steve pulls a sled on ice (no friction) with a 150-N force at an
angle of 200. The mass of the sled and her brother is 45 kg.
g = 10 m/s2. Stacey represented this pictorially and
mathematically.
x
o
x: (150 N) cos 20o = (45 kg) ax
y: __ N __ (45 kg)(10 m/s2) __ (150 N) sin 20o = 0
What are the missing signs?
1
+
–
+
2
+
+
–
3
–
–
+
4
–
–
–
5
+
+
+
The motion of a car is represented with the motion diagram
below.

vfinal 

vinitial
a
o
x
Which
diagram
best represents
the forcesthe
exerted
on the car
Construct
a diagram
that best represents
forces
by
the Earth
and
the
surface?
exerted
on the
car
byEarth’s
the Earth
and the Earth’s surface.
1
y
2
y
3
x
x
4
y
x
5
x
y
y
x
x: 0 – 0.80 N = (1000 kg) ax
y: N – (1000 kg)(10 m/s2) = (1000 kg) 0
0 – 16 m/s = ax t
Which word description below best matches the above
Construct a verbal description that best matches the above
mathematical
mathematicaldescription?
description.
1 The 0.80 N force of the Earth on the object causes its speed in
the x direction to increase from 0 to 16 m/s.
2 The object slows down due to a 0.80 N frictional force.
3 The object has a 10 m/s2 acceleration in the y direction.
4 The net force causing the object to slow down is exerted in the
same direction as the initial velocity.
5 The object’s velocity increases from 0 to 16 m/s.

vfinal

vinitial
Which FBD best represents the above motion diagram for a
Construct an FBD that best represents the above motion
car?
diagram for a car.

Fsurface(

 ) on car
Fsurface (ll) on car
Fair on car
 x
FEarth on car
1
3
y
2
y
4
y
Fsurface(
 ) on car
Fsurface(ll) on car
x

FEarth on car
y

Fsurface(
 ) on car


Fsurface( ) on car
Fair on car
Fsurface (ll) on car

x
F
surface (ll) on car
x


FEarth on car
FEarth on car


Fsurface( ) on car
Fair on car


Fsurface(ll) on car
FEarth on car
Which
motion
diagram
best represents
the situation
Construct
a motion
diagram
that best represents
the
shown
in the
free-body
situation
shown
in the diagram?
free-body diagram above.
1

vinitial

vfinal
3

vinitial
vfinal
5
2

vfinal

vinitial

4
vinitial vfinal  0
Either 2 or 4 are possible
Which FBD
below
best
Construct
an FBD
that
represents
thethe
forces
best
represents
exerted
on
the
ball
as
it
forces exerted on the
is moving
UP (ignore
ball
as it is moving
UP. air
resistance)?

1
Fperson on ball

FEarth on ball

3
Fperson on ball

FEarth on ball
2

FEarth on ball

4
Fperson on ball

FEarth on ball
Which FBD below best
Construct an FBD that
represents the forces
best represents the
exerted on the ball as it
forces exerted on the ball
is moving DOWN
as it is moving DOWN.
(ignore air resistance)?

1
Fperson on ball

FEarth on ball

3
Fperson on ball

FEarth on ball
2

FEarth on ball

4
Fperson on ball

FEarth on ball
Which FBD
below
best
Construct
an FBD
that
best
represents
forces
represents
thethe
forces
exerted
ball
exerted
onon
thethe
ball
at at
the
theofTOP
of its path?
TOP
its path.

1
Fperson on ball

FEarth on ball

3
Fperson on ball

FEarth on ball
2

FEarth on ball

4
Fperson on ball

FEarth on ball
5 No forces are exerted
on the ball
y

FSurface onstroller

F Person onstroller
x

FEarthon stroller
Construct
a motion
diagram
that best represents
theas
Which
motion
diagram
best represents
the situation
situation
shown
in the free-body
shown
by the
free-body
diagram? diagram above.
1

vfinal
3
vfinal

vinitial

vinitial
2
vinitial

vfinal

4
vinitial vfinal  0
5 No diagram -- the stroller must be accelerating to the
right because the net force is to the right.
y


FSurface ( )on Car FSurface ( ll)on Car
x

FEarthon Car
Which
speed
versusversus
time chart
Construct
a speed
time best
chart that best
represents
representsthis
thissituation?
situation.
1
v
3
v
t
t
v
t