Transcript Dynamics

Mary pulls on a lawnmower parallel to the ground. The mower
moves at a constant speed up the hill.
Which free body diagram best
represents the forces on 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
represents the forces on the mower?
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). The rope exerts a 150N force at an angle of 200 above horizontal. 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
Both (1) and (3) need
corrections.
2 (150 N) should be
(150 N) cos 20o.
5
Both (2) and (3) need
corrections.
3 Include a +(150 N) sin 20o term in
the y equation.
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 forces exerted on the car
by 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
mathematical 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
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
shown in the free-body diagram?
1

vinitial

vfinal
3

vinitial
vfinal
5
2

vfinal

vinitial

4
vinitial vfinal  0
Either 2 or 4 are possible
Which FBD below best
represents the forces
exerted on the ball as it
moves UP (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
represents the forces
exerted on the ball as it
moves 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
represents the forces
exerted on the ball at
the TOP of 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
Which motion diagram best represents the situation as
shown by the free-body diagram?
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 versus time chart best
represents this situation?
1
2
v
t
3
v
t
v
t