Transcript Lecture12

RQ8: A car is driving at a constant
speed in a perfect circle on a flat
parking lot. The centripetal force
acting on the car is:
A. Gravity.
B. The normal force from the road.
C. Friction from the road.
Lecture Quiz 1:
When it is directly overhead, why doesn’t the liquid fall in a
bucket full of water?
A. Centripetal force pushes it up.
B. Rotating objects have their own gravity.
C. It does fall, but not fast enough to leave the bucket.
D. Because of its high speed, air is pushing it into the glass.
E. Newton’s Third Law.
Quiz 2
You cut the string on a ball that is
moving in uniform circular motion.
The ball will fly off in a direction
a.) along the circle
b.) towards the persons arm
c.) exactly opposite the persons arm
d.) on a line tangent to the circle at
the point
e.) continue to move along a
circular path but in big and big
circles to the left.
Fig. 5.4
Centripetal Acceleration:
a = v2/r
Directed inwards, towards the center.
Fig. 5.5
The tension in the string can be divided into two components:
Horizontal = centripetal force
Vertical = opposes gravity
Note: There is no vertical acceleration.
Fig. 5.8
Here the NORMAL force from the road has a horizontal
component, providing the centripetal force. This ADDS to the
frictional force that might be present on a level road.
Lecture Quiz: Question 3
On a Ferris Wheel, when is the
NORMAL force on you largest?
A. At the top
B. At the bottom
C. At the sides
D. It must be constant
E. Its magnitude must always equal gravity.
Consider again the Ferris Wheel….
Ferris Wheel AT REST
N
There is no acceleration, so no
net force on the riders. Weight is
exactly cancelled by the Normal
force from the seat.
W
N
N
N + W = 0 = ma
W
W
N
W
Ferris Wheel IN MOTION
Circular acceleration
Ntop
a = v2/r
Centripetal force = mv2/r,
always directed inward.
Weight is constant;
Normal force varies.
Nbottom
Lecture Quiz: Question 4
What happens if the speed v gets so large that the
centripetal acceleration equals g? (a=v2/r =
g)
A.
B.
C.
The normal force at the top disappears (N=0),
and you feel “weightless”.
The normal force at the bottom disappears
(N=0), and you feel extra heavy.
The normal force exactly equals your weight.
Ntop
Nbottom
Fig. 5.17
Is the Moon falling?
LECTURE QUIZ - Question 5:
The Moon is in a circular orbit about the
Earth, with a period of about 28 days.
A. The Moon is continuously accelerating towards the Earth.
B. The Moon never falls and crashes into the Earth because
no force is pushing or pulling it towards the Earth.
C. The Moon does not fall to Earth because the forces acting
on the Earth add up to zero. (They cancel out.)
Fig. 5.17
Is the Moon falling?
RQ9: The gravitational force exerted by the Earth on the
Moon is much larger than the gravitational force exerted
by the Moon on the Earth, which is why the Moon
revolves around the Earth.
A. True
B. False