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ConcepTest 4.1 Tension I
Two tug-of-war opponents each
1) 0 N
pull with a force of 100 N on
2) 50 N
opposite ends of a light rope.
3) 100 N
What is the tension in the light
(ignore the mass) rope?
4) 150 N
5) 200 N
ConcepTest 4.1 Tension I
Two tug-of-war opponents each
1) 0 N
pull with a force of 100 N on
2) 50 N
opposite ends of a light rope.
3) 100 N
What is the tension in the light
(ignore the mass) rope?
4) 150 N
5) 200 N
The tension is not 200 N !! Whether the other end
of the rope is pulled by a person, or pulled by a
tree, the tension in the rope is still 100 N !!
ConcepTest 4.2 Three Blocks
Three blocks of mass 3m,
3m 2m,
2m and m
1) T1 > T2 > T3
are connected by light strings and
2) T1 < T2 < T3
pulled with constant acceleration a.
What is the relationship between the
3) T1 = T2 = T3
tension in each of the strings?
4) all tensions are zero
5) tensions are random
a
3m
T3
2m
T2
m
T1
ConcepTest 4.2 Three Blocks
Three blocks of mass 3m,
3m 2m,
2m and m
1) T1 > T2 > T3
are connected by light strings and
2) T1 < T2 < T3
pulled with constant acceleration a.
What is the relationship between the
3) T1 = T2 = T3
tension in each of the strings?
4) all tensions are zero
5) tensions are random
T1 pulls the whole set
of blocks along, so it
a
must be the largest
largest.
g
T2 pulls the last two
masses, but T3 only
pulls the last mass.
3m
T3
2m
T2
m
T1
Follow--up: What is T1 in terms of m and a?
Follow
ConcepTest 4.3 On an Incline
Consider two identical blocks,
blocks
1) case A
one resting on a flat surface,
2) case B
and the other resting on an
incline. For which case is the
normal force greater?
3) both the same (N = mg)
4) both the same (0 < N < mg)
5)) both the same (N
( = 0))
ConcepTest 4.3 On an Incline
Consider two identical blocks,
blocks
1) case A
one resting on a flat surface,
2) case B
and the other resting on an
incline. For which case is the
normal force greater?
3) both the same (N = mg)
4) both the same (0 < N < mg)
5)) both the same (N
( = 0))
In Case A,
A we know that N = W.
y
In Case B,
B due to the angle of
the incline, N < W. In fact, we
N
f
can see that N = W cos(θ).
θ Wy
θ
W
x
ConcepTest 4.4 Tetherball
In the game of tetherball,
tetherball
1)) Toward the top
p of the pole
p
2) Toward the ground
the struck ball whirls
3) Along the horizontal component of the
tension force
around a pole.
pole In what
4) Along the vertical component of the
tension force
direction does the net
force on the ball p
point?
5) Tangential to the circle
T
W
ConcepTest 4.4 Tetherball
In the game of tetherball,
tetherball
the struck ball whirls
around a pole.
pole In what
direction does the net
force on the ball point?
1)) Toward the top
p of the pole
p
2) Toward the ground
3) Along the horizontal component of the
tension force
4) Along the vertical component of the
tension force
5) Tangential to the circle
Th vertical
The
ti l componentt off the
th
tension balances the weight
weight. The
horizontal component of tension
W
T
T
provides the centripetal force that
points toward the center of the
circle.
W
ConcepTest 4.5 Loop
A ping pong ball is shot into a
circular tube that is lying flat
(horizontal) on a tabletop. When
the ping pong ball leaves the
track which path will it follow?
track,
ConcepTest 4.5 Loop
A ping pong ball is shot into a
circular tube that is lying flat
(horizontal) on a tabletop. When
the ping pong ball leaves the
track which path will it follow?
track,
z
Once the ball leaves the tube, there is no longer
a force to keep it going in a circle.
circle Therefore
Therefore, it
simply continues in a straight line, as Newton’s
First Law requires!
Follow--up: What physical force provides the centripetal force?
Follow
ConcepTest 4.7 Going in Circles I
You’re on a Ferris wheel moving in a
vertical circle. When the Ferris wheel is
at rest,
rest the normal force N exerted by
1) N remains equal to mg
2) N is smaller than mg
your seat is equal to your weight mg
mg..
3) N is larger than mg
How does N change at the top of the
4) None of the above
Ferris wheel when you are in motion?