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Forces
Three forces act on an object. In which direction does the
object accelerate?
Three forces act on an object. In which direction does the
object accelerate?
An elevator suspended by a cable is moving upward and
slowing to a stop. Which free-body diagram is correct?
An elevator suspended by a cable is moving upward and
slowing to a stop. Which free-body diagram is correct?
The acceleration must be down, so the net force must be
down.
A Martian lander is approaching the
surface. It is slowing its descent by
firing its rocket motor. Which is the
correct free-body diagram for the
lander?
A Martian lander is approaching the
surface. It is slowing its descent by
firing its rocket motor. Which is the
correct free-body diagram for the
lander?
To slow down while
descending, the
acceleration must be up.
To accelerate up, the net
force must be up. Choice
a is the only free body
diagram consistent with
that determination.
You’ve just kicked a rock, and it is now sliding across
the ground about 2 meters in front of you. Which of
these forces act on the ball? Check all that apply.
A. Gravity, acting downward.
B. The normal force, acting upward.
C. The force of the kick, acting in the direction of
motion.
D. Friction, acting opposite the direction of motion.
You’ve just kicked a rock, and it is now sliding across
the ground about 2 meters in front of you. Which of
these forces act on the ball?
A. Gravity, acting downward.
B. The normal force, acting upward.
C. The force of the kick, acting in the direction of
motion.
D. Friction, acting opposite the direction of
motion.
Once the foot is no longer in contact with the rock, it no
longer exerts a force on the rock
Rank order, from largest to smallest, the size of the
friction forces
to
in these 5 different situations.
The box and the floor are made of the same materials in all
situations.
A.
B.
C.
D.
E.
fc >
fb >
fa >
fa =
fb >
fd >
fc >
fc =
fb >
fc =
fe >
fd >
fd =
fc =
fd =
fb > fa.
fe > fa.
fe > fb.
fd = fe.
fe > fa.
Rank order, from largest to smallest, the size of the
friction forces
to
in these 5 different situations.
The box and the floor are made of the same materials in all
situations.
A.
B.
C.
D.
E.
fc >
fb >
fa >
fa =
fb >
fd > fe > fb > fa.
fc > fd > fe > fa.
fc = fd = fe > fb.
fb > fc = fd = fe.
fc = fd = fe > fa.
The is no friction in case a. Case b is near maximum static friction which is
larger than kinetic friction. Kinetic friction does not depend on speed.
A car is rolling over the top of a hill at speed v.
At this instant,
A. n > w.
B. n = w.
C. n < w.
D. We can’t tell about n without knowing v.
A car is rolling over the top of a hill at speed v.
At this instant,
A. n > w.
B. n = w.
C. n < w.
D. We can’t tell about n without knowing v.
The acceleration is towards the center of the
circle, in this case down. The net force must be
down by Newton’s second law.
A ball on a string is swung in a vertical circle. The string
happens to break when it is parallel to the ground and the
ball is moving up. Which trajectory does the ball follow?
A ball on a string is swung in a vertical circle. The string
happens to break when it is parallel to the ground and the
ball is moving up. Which trajectory does the ball follow?
The velocity
when the string
breaks is straight
up. The only
force acting on
the ball is gravity
which has no x
component, so
there is no
subsequent
motion in the x
direction.
Car B is stopped for a red light. Car A, which has the same
mass as car B, doesn’t see the red light and runs into the
back of B. Which of the following statements is true?
A. B exerts a force on A but A doesn’t exert a force on B.
B. B exerts a larger force on A than A exerts on B.
C. B exerts the same amount of force on A as A exerts on B.
D. A exerts a larger force on B than B exerts on A.
E. A exerts a force on B but B doesn’t exert a force on A.
Car B is stopped for a red light. Car A, which has the same
mass as car B, doesn’t see the red light and runs into the
back of B. Which of the following statements is true?
A. B exerts a force on A but A doesn’t exert a force on B.
B. B exerts a larger force on A than A exerts on B.
C. B exerts the same amount of force on A as A exerts on B.
D. A exerts a larger force on B than B exerts on A.
E. A exerts a force on B but B doesn’t exert a force on A.
This is true by Newton’s third law.
Boxes A and B are sliding to the right across a frictionless
table. The hand H is slowing them down. The mass of A is
larger than the mass of B. Rank in order, from largest to
smallest, the horizontal forces on A, B, and H.
A.
B.
C.
D.
FB on H = FH on B = FA on B = FB on A
FB on H = FH on B > FA on B = FB on A
FB on H = FH on B < FA on B = FB on A
FH on B = FH on A > FA on B
Boxes A and B are sliding to the right across a frictionless
table. The hand H is slowing them down. The mass of A is
larger than the mass of B. Rank in order, from largest to
smallest, the horizontal forces on A, B, and H.
A.
B.
C.
D.
FB on H = FH on B = FA on B = FB on A
FB on H = FH on B > FA on B = FB on A
FB on H = FH on B < FA on B = FB on A
FH on B = FH on A > FA on B
The equalities are true by Newton’s third law. Block B must be accelerating to the
left, so the net force is to the left. This makes FHB > FAB.
All three 50 kg blocks are at rest. Is the tension in rope 2
greater than, less than or equal to the tension in rope 1?
A. greater than
B. less than
C. equal to
All three 50 kg blocks are at rest. Is the tension in rope 2
greater than, less than or equal to the tension in rope 1?
A. greater than
B. less than
C. equal to
Newton’s first law for the block
on the left proves that the
tension equals the weight.
Newton’s first law for either of
the individual blocks on the right
proves that the tension equals
the weight for an individual.
The block on the far right is moving up with constant
speed. Is the tension in rope 2 greater than, less than or
equal to the tension in rope 1?
A. greater than
B. less than
C. equal to
The block on the far right is moving up with constant
speed. Is the tension in rope 2 greater than, less than or
equal to the tension in rope 1?
A. greater than
B. less than
C. equal to
This is still a Newton’s
first law situation!
In the figure to the
right is the tension in
the string greater than,
less than, or equal to
the weight of block B?
A. Greater than
B. Less than
C. Equal to
In the figure to the
right is the tension in
the string greater than,
less than, or equal to
the weight of block B?
A. Greater than
B. Less than
C. Equal to
This is a Newton’s second law situation for each of the blocks. Block A will
accelerate to the right and block B will accelerate down. The net force on B must
be down by Newton’s second law. The tension force exerted by the rope on block
B must be less than the weight force exerted by the earth on block B.
A small car is pushing a
larger truck that has a
dead battery. The mass
of the truck is larger than
the mass of the car. Which of the following statements is true?
A. The car exerts a force on the truck but the truck doesn’t
exert a force on the car.
B. The car exerts a larger force on the truck than the truck
exerts on the car.
C. The car exerts the same amount of force on the truck as
the truck exerts on the car.
D. The truck exerts a larger force on the car than the car
exerts on the truck.
E. The truck exerts a force on the car but the car doesn’t
exert a force on the truck.
A small car is pushing a
larger truck that has a
dead battery. The mass
of the truck is larger than
the mass of the car. Which of the following statements is true?
A. The car exerts a force on the truck but the truck doesn’t
exert a force on the car.
B. The car exerts a larger force on the truck than the truck
exerts on the car.
C. The car exerts the same amount of force on the
truck as the truck exerts on the car.
D. The truck exerts a larger force on the car than the car
exerts on the truck.
E. The truck exerts a force on the car but the car doesn’t
exert a force on the truck.
This is true by Newton’s third law.