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Discussion Day: Hour: Date: Midterm 1
Physics 7C Tait (version 1)
Closed book; you may use a calculator, but no other electronic devices,
including cell phones, iPods, etc. For full credit, you must show your
reasoning and/or work in obtaining the solution. Score: Useful(?) Formulae – Midterm 1
Kinematics
A particle under constant acceleration with initial position 0 and vea
r
locity 0 at time t = 0 has position and velocity at later time t:
v
(t) = 0 + t
v
v
a
1
(t) = 0 + 0 t + t2
r
r
v
a
2
A particle moving with uniform speed v in a circle of radius R feels
centripetal acceleration of magnitude ac :
v2
ac =
R
Dynamics
A particle of mass m feeling external forces F experiences acceleration :
a
1
=
a
F
m
Forces Force of gravity: F = m . At the surface of the Earth,   = 9.8 m/s2 .
g
g
Force of kinetic friction: F = µK   where is the normal force
n
n
perpendicular to the surface responsible for the friction.
Force of static friction: F ≤ µS   where is the normal force
n
n
perpendicular to the surface responsible for the friction.
Force of ﬂuid/air resistance (1): F = −b where b depends on the
v
object and the medium through which it moves.
Force of ﬂuid/air resistance (2): F  = 1 DρAv 2 where D is the drag
2
coeﬃcient, ρ is the density of the medium, A is the crosssectional area of
the object and v is its speed. 1. A spider of mass 1.00 g hangs from two equal length strands of
spidersilk attached to the ceiling. The strands are attached to the ceiling
at points 5.0 cm apart from each other. The strand on the left makes an
angle of 30◦ with respect to the horizontal and the strand on the right
makes an angle of 60◦ with respect to the horizontal. (See ﬁgure below).
(A). Draw a freebody diagram for the spider. (B). If the spider is stable and unmoving, determine the tensions in each
of the strands of spidersilk. (C) A sudden wind blows horizontally from the left to the right (see the
ﬁgure), exerting a force which breaks one of the two strands of spidersilk
by exceeding its maximum tension. Explain why it is the strand on the
left that breaks. (D) Startled by its web breaking, the spider lets go of the still attached
strand and falls to the ﬂoor, with the broken strand acting like a parachute.
We will make the approximation that it reaches terminal velocity immediately. If the spider takes 1 s to fall 3.5 m to reach the ﬂoor below, compute
the coeﬃcient b in the expression for the force due to the air resistance,
F = −b .
v 2. A skier (of mass 70.0 kg) is at the top of a ski run, which for our
purposes is a smooth plane inclined at an angle of 20◦ with respect to
the horizontal. From where the skier stands at the top of the run, it is a
distance of 200.0 m along the run to its end at the bottom. Do not ignore
the force of friction between her skis and the snow.
(A). Draw the freebody diagram for the skier. (B). If the skier is initially stable (not moving), compute the minimum
coeﬃcient of static friction between the skis and the snow. (C). The skier pushes oﬀ and starts sliding down the hill (but with an
initial speed of close to zero). The coeﬃcient of kinetic friction between
skis and snow is µK = 0.01. How long does it take her to reach the bottom
of the run? (D) What is her speed when she reaches the bottom of the run? 3. A merrygoround is a large disk which rotates at a constant angular
speed such that any point on its surface takes a time T to make a single
revolution.
(A) Consider a child of mass m standing on the merrygoround at
radial distance R from its center. At any given instance in time, describe
the child’s velocity (both magnitude and direction) in terms of R and T
as measured by an observer standing on the ground next to the merrygoround. (B) For an observer on the merrygoround standing next to the child
(so measuring no relative velocity or acceleration between the child and
himself), what ﬁctitious force (magnitude and direction) accounts for this
observer’s observations? (C) If the period of the merrygoround is T = 3.00 s and the coeﬃcient
of static friction between the child’s shoes and the surface of the merrygoround is µS = 0.700, determine which values of the radius allow the child
to stand easily in equilibrium, and for which values the child slips oﬀ of
the merrygoround and goes ﬂying. ...
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 Winter '11
 TAIT
 Physics, Force, Friction, Mass

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