Transcript 04_3-4_4 answers

Chapter 4.3-4.4
Review
1. If an 8 kg object is
accelerating at a rate
2
of 20 m/s , what is
the net force on the
object?
F = ma
F = 8 x 20
F = 160 N
2. If you applied the
same force as the
answer to question 1
to a 4 kg object, what
acceleration would
result?
F = ma
160 = 4 x a
2
a = 40 m/s
3. What are the action reaction
pairs in these examples?
A) a bat hits a baseball
B) wind causes a kite to fly
C) the moon orbits the earth
D) you turn a steering wheel
in your car
E) you use a remote to
change channels on your TV
A) a bat hits a baseball
The bat pushes on
the ball, the ball
pushes on the bat.
B) wind causes a kite to fly
The air pushes on
the kite, the kite
pushes on the air.
C) the moon orbits the earth
The earth pulls on
the moon, the
moon pulls on the
earth.
D) you turn a steering wheel in your car
Your hand pushes
on the wheel, the
wheel pushes on
your hand.
E) you use a remote to change
channels on your TV
Your finger pushes
on the button, the
button pushes on
your finger.
4. Newton’s third law says
that for every force applied,
an equal and opposite
force results. If that is true,
why don’t the forces cancel
each other out? How can
any force produce
acceleration?
The forces are not on the
same object, they are on
two different objects. Each
of these forces COULD be
unbalanced and COULD
cause acceleration.
5. The forces on a motorcycle
are 470 N north and 250 N
west. If the motorcycle and
rider have a combined mass
of 210 kg, what is the
magnitude and direction of
the acceleration?
2
470
2
250
+
=
c = 532 N
tanq = 470/250
2
c
62° north of west
F = ma
532 = 210 x a
2
a = 2.5 m/s
62° north of west
6. A bug flying east at 5 m/s hits
the windshield of a car moving
west at 20 m/s. (He won’t have
the guts to do that again!) Which
experiences the greater impact
force, the bug or the windshield?
Which experiences the greater
acceleration? Why?
The impact forces are
equal (Newton’s Third
Law). The bug has a
greater acceleration
because it has a
smaller mass (F = ma).
7. Draw a free body diagram of
each of these cases:
A) a thrown baseball (without
air resistance)
B) a thrown baseball (with air
resistance)
C) a grocery cart being pushed
D) a bottle rocket
E) a skydiver falling at terminal
velocity
A. Only
weight.
This is an
example of
projectile
motion.
B. Weight and air
resistance, only.
Assuming
the cart is
not
accelerating,
it must be in
equilibrium.
FN = mg and
Ff = FP.
I assume the
rocket is still
firing and
accelerating.
Thrust is
greater than air
resistance; the
only other
force is
gravity.
Since the
skydiver has
reached terminal
velocity, the air
resistance is
equal to the
gravity.
The skydiver is
in equilibrium
and is not
accelerating.
8. A box has a mass of
4.7 kg. What is its
weight on earth? What
is its weight on the
moon, where g is 1/6
that on earth?
Wt = mg
Wt = 4.7 x 10
Wt = 47 N
Wt = mg
Wt = 4.7 x 10 x 1/6
Wt = 7.8 N
9. A wooden block is on
an incline of 33º.
Friction is holding it in
place. What are the
magnitudes of the
frictional force and the
normal force?
The components of the weight are the
force parallel to the plane and the force
perpendicular to the plane.
sin33° = Fparallel /mg
Fparallel = 0.545mg Nparallel
cos33° = Fperpendicular /mg
Fperpendicular =
0.839mg Nperpendicular
Since the object is in equilibrium, the
frictional force is equal to the parallel force.
Fparallel = Ffrictional =
0.545mg N
Since the object is in equilibrium, the
frictional force is equal to the parallel force.
Fperpendicular = Fnormal =
0.839mg N
10. A hockey puck on the ice is
at rest. It has a mass of 1.5 kg.
A force of 11 N is required to
start the puck moving. After that
a force of 9 N is required to
keep the puck moving at a
constant velocity. What are the
coefficients of static and kinetic
friction?
Since the ice is flat, the
normal force is equal to the
weight, mg.
m = Ffriction/Fnormal
m = 11 N/(1.5 x 10) N
m = 0.733
Since the ice is flat, the
normal force is equal to the
weight, mg.
m = Ffriction/Fnormal
m = 9 N/(1.5 x 10) N
m = 0.600
11. Can the force of
air resistance on a
falling body exceed
the force of gravity?
No, the object would
accelerate until air
resistance was equal to
weight. At that point the
object would be in
equilibrium, and it would
no longer accelerate.
(terminal velocity)
The next slide is
extra! Don’t read it
unless you
understand the
previous answer!
(Couldn’t stand it could you?)
Actually, since the density of air
increases as you approach the
surface of the earth, the air resistance
WOULD increase above the pull of
gravity and slow the speed of terminal
velocity as the object approaches the
earth’s surface. But, that would only
be appreciable if the object fell for a
LONG distance.