Transcript Lecture 17

An object is moving down.
How does the tension T in the
string compare with the
weight "mg" of the object?
A: T=mg
B: T > mg
C: T < mg
D: Not enough information
given.
 v  v
 v
f
i
a

t
t f  ti
When an object moves in a circle, is its
velocity changing? Assume that the speed
of the bowling ball remains almost
constant and find the direction of the
acceleration of the ball assuming that the
direction of the acceleration is the same
as the direction of the velocity change
vector.
Two similar isosceles triangles

at
R
V

x at
V x V
R    V
a t a
V2
a
R
Vi

x
R
V2
a
R
a  R
2
2
mV
F  ma 
 m 2 R
R


F
R
V

a
An object moves in a circular
path with constant speed. Which
of the following statements is
true concerning the object?
A. Its velocity is constant, but
acceleration is changing
B. Its acceleration is constant,
but velocity is changing
C. Both its velocity and
acceleration are changing
D. Both its velocity and
acceleration remain constant
V
A car passes over a hill in the road
that has a circular cross-section.
How does the force exerted by the
road on the car compare with the
weight of the car?
A: N = mg
B: N > mg
C: N < mg
D: Not enough information
given.
V
A car passes over a low spot in the
road that has a circular crosssection. How does the force exerted
by the road on the car compare
with the weight of the car?
A: N = mg
B: N > mg
C: N < mg
D: Not enough information
given.
A mass is hanging from a rope and
swinging around a circular path at
constant speed. The situation is shown
in the figure. Draw a free body diagram
Support
rope
mass
circular
path
Pendulum Suspended from Spring Scale
A bob is hung by a string, attached to
a spring scale which is suspended
from a stand. First note the reading
on the spring scale when the bob is
not moving. The bob and string will
then be pulled back so that the string
makes an angle theta with the
vertical. The bob will then be released
and allowed to swing. Predict what
will happen to the reading on the
spring scale at the bottom of the
swing (more, less or same as when
the object is at rest).
L
s
A: s = L
B: s > L
C: s < L
A: S = L
B: S > L
C: S < L
A small wheel and a large wheel are
connected by a belt. The small wheel is
turned at a constant angular velocity s.
How does the magnitude of the angular
velocity of the large wheel L compare to
that of the small wheel?
There is a bug S on the rim of the small
wheel and another bug L on the rim of the
large wheel. How do their speeds
compare?
Homework
(due Monday)
Read Sections 7.4 Examples!!!
Problems # 21, 25, 27, 28, 32
Quiz
V
A car passes over a hill in the road
that has a circular cross-section with
a radius of 30m. The speed of the car
at the top of the hill is 10 m/s. What is
the force exerted by the seat of the
car on a 60kg passenger when the
car is at the top pf the hill?