4. November 3, 2011
A car rounds a hill while maintaining a constant
speed. Is there a net force on the car as it
rounds the hill?
No – its speed is constant.
Yes.
It depends on the sharpness of the hill and the speed
of the car.
What is the average net force needed to bring a
1500kg car to rest from a speed of 100km/h
within a distance of 55m?
-11,000 N
5. Vertical Motion
What happens when a person pulls upward on a
10.0kg box with a force equal the box’s weight?
What happens when a person pulls upward on the
same box with a 100.0N force?
6. Apparent Weight Loss
A 65kg woman descend in an elevator that briefly
accelerates at 0.20m/s2 downward. She stand on
a scale that reads in kg.
During this acceleration, what is her weight and what
does the scale read?
What does the scale read when the elevator descends
at a constant speed of 2.0m/s?
7. Pulleys
A 6kg mass and 4kg are attached on a pulley
system.
Find the force of tension on the string connecting
them.
Find the acceleration of the two masses.
4kg
6kg
8. Mechanical Advantage
A mover is trying to lift a
piano (slowly) up to a second-
story apartment. He is using
a rope looped over two pulleys
as shown. What force must
he exert on the rope to slowly
lift the piano’s 2000N
weight?
9. November 7, 2011
A force of 20N right is applied on a 4kg object to
slide it across a frictionless surface. Find the
acceleration.
5m/s2 right
Oh snap! The object actually accelerates at only
3m/s2 to the right. Guess the surface is not
frictionless. What is the force of friction?
Ff = 8N left
What is the coefficient of kinetic friction between
the two surfaces?
0.204
10. Coefficient of Friction
An object that weighs 20N requires an applied
force of 6N to start it moving across a surface.
What is the coefficient of static friction?
0.30
If the weight of the object is doubled, what
force is needed to set it in motion?
12N
11. Pushing and Pulling
A 10.0kg box is pulled along a horizontal surface
by a force of 50.0N. The coefficient of kinetic
friction is 0.30. Calculate the acceleration.
Now, the same box is pulled along the same
surface but the 50.0N force is at a 30˚ angle.
Again, calculate the acceleration.
Now, the same box is pushed at a 30˚ angle with a
force of 50.0N. Calculate the acceleration.
12. November 8, 2011
Two boxes are connected by a cord running over a
pulley. The coefficient of kinetic friction
between box A and the table is 0.20. We ignore
the mass of the cord and pulley, and any friction
in the pulley. Find the acceleration of the system.
1.4m/s2
13. The Skier
The 73kg Neil in the picture has just begun
descending the 30˚ slope. Assuming the
coefficient of kinetic friction is 0.10, calculate
the acceleration and the speed he will reach after
4.0s.
4.0m/s2
16m/s
14. Elephant and the Feather
Suppose that an elephant and a
feather are dropped off a very tall
building from the same height at
the same time. We will assume the
realistic situation that both
feather and elephant encounter air
resistance. Which object - the
elephant or the feather - will hit
the ground first?
15. Not Exactly Shocking, but… True or False?
The elephant encounters a smaller force of air resistance than the feather and
therefore falls faster.
The elephant has a greater acceleration of gravity than the feather and therefore
falls faster.
Both elephant and feather have the same force of gravity, yet the acceleration of
gravity is greatest for the elephant.
Both elephant and feather have the same force of gravity, yet the feather
experiences a greater air resistance.
Each object experiences the same amount of air resistance, yet the elephant
experiences the greatest force of gravity.
Each object experiences the same amount of air resistance, yet the feather
experiences the greatest force of gravity.
The feather weighs more than the elephant, and therefore will not accelerate as
rapidly as the elephant.
Both elephant and feather weigh the same amount, yet the greater mass of the
feather leads to a smaller acceleration.
The elephant experiences less air resistance and than the feather and thus reaches a
larger terminal velocity.
The feather experiences more air resistance than the elephant and thus reaches a
smaller terminal velocity.
The elephant and the feather encounter the same amount of air resistance, yet the
elephant has a greater terminal velocity.
16.
17.
18.
19. Elevator
Consider a person standing in an elevator that is
accelerating upward. The upward normal force
exerted by the elevator floor on the person is
_________ the downward weight.
larger than
identical to
smaller than
20. Tricky Elevator
A person stands on a bathroom scale in a
motionless elevator. When the elevator begins to
move, the scale briefly reads only 0.75 of the
person’s regular weight. Calculate the
acceleration of the elevator.
2.45m/s2 down
21. Hoisting
A 12.0kg bucket is lowered (?) vertically by a rope
in which there is 163N of tension. What is the
acceleration of the bucket?
3.75m/s2 up
22. November 17, 2011
A constant force is exerted on a cart that is
initially at rest on an air track. Friction between
the cart and the track is negligible. The force
acts for a short time interval and gives the cart a
certain final speed. To reach the same final
speed with a force that is only half as big, the
force must be exerted on the cart for a time
interval __________ that stronger force.
four times as long as
twice as long as
equal to
half as long as
a quarter of
23. A constant force is exerted for a short time interval
on a cart that is initially at rest on an air track. This
force gives the cart a certain final speed. Suppose we
repeat the experiment but, instead of starting from
rest, the cart is already moving with a constant speed
in the direction of the force at the moment we begin
to apply the force. After we exert the same constant
force for the same short time interval, the increase in
the cart’s speed
is equal to two times its initial speed.
is equal to the square of its initial speed.
is equal to four times its initial speed.
is the same as when it started from rest.
cannot be determined from the information provide.
24. The Crate’s Orientation
You are pushing a wooden crate across the floor
at a constant speed. You decide to turn the crate
on end, reducing by half the surface area in
contact with the floor. In the new orientation, to
push the same crate across the same floor with
the same speed, the force that you apply must be
________ as the force required before you
changed the crate’s orientation.
four times as great
twice as great
equally as great
half as great
one-fourth as great
25. Tilting Tables
An object is held in place by friction on an inclined
surface. The angle of inclination is increased until
the object start moving. If the surface is kept at
this angle, the object
slows down.
moves at uniform speed.
speeds up.
none of the above.
