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Unformatted text preview: Physics 7A, Section 1 (Speliotopoulos)
First Midterm, Fall 2010
Berkeley, CA Rules: This midterm is closed book and closed notes. You are allowed two sides of one-half sheet of
8.5” x 11 ” of paper on which you may write out formulas and basic facts on both sides. You may not
write out solutions to homework problems or speciﬁc examples on your sheet. You are also allowed to
use scientific calculators in general, but not ones which can communicate with other calculators through
any means. Anyone who does use wireless-capable will automatically receive a zero for this midterm.
Cellphones must be turned off during the exam, and placed in your backpacks. In particular, cell-phone-
based calculators cannot be used. Please make sure that you do the following during the midterm: - Write your name, discussion number, ID number on all documents you hand in.
— Make sure that the grader knows what s/he should grade by circling your ﬁnal answer.
- Answer all questions that require a numerical answer to three significantﬁgures. Each problem is worth 20 points. We will give partial credit on this midterm, so if you are not
altogether sure how to do a problem, or if you do not have time to complete a problem, be sure to write
down as much information as you can on the problem. This includes any or all of the following:
Drawing a clear diagram of the problem, telling us how you would do the problem you had the time,
telling us why you believe (in terms of physics) the answer you got to a problem is incorrect, and telling
us how you would mathematically solve an equation or set of equations once the physics is given and the
equations have been derived Don ’1 get too bogged down in the mathematics; we are looking to see how
'much physiCS you know, not how well you can solve math problems. If at any point in the exam you have any questions, just raise your hand, and we will see if we are able
to answer them. Copy and fill in the following information on the front of your bluebook: Name: Disc Sec Number:
Signature: Disc Sec GS]:
Student ID Number: 1. Two cars are driving side-by-side down a straight road, and they are traveling at the same speed, 170.
Suddenly, the ﬁrst car slams on its brakes, and slows down at a constant de—acceleration until it is
stopped for an instant. The car then slams on the gas and accelerates with a constant acceleration
until it is traveling once again at no. Show that if the ﬁrst car travels a distance, d, during this whole
process of slowing down and speeding up, and the second car travels a distance, D, then D = 2d
irrespective of the rate at which the ﬁrst car accelerates and de-accelerates. 2. A tennis ball launcher is placed on a grass lawn, and a target is placed
ﬂat on the ground beside it. This target is attached to a car by a rope (see
ﬁgure), and at the instant the launcher ﬁres a ball at an angle, 6, and a
speed, 170, the target travels to the right at a constant speed, W. When the
tennis ball lands, it hits the center of the target. At the instant this ball
lands, a second ball is launched at the same angle, but at a speed, v. What
MW, must 17/170 be so that the second tennis ball also hits the center of the
a target? (You should get a pure number.) _ k 3. The ﬁgure to the left shows a man with mass, m =
80 kg, standing on a plank with mass, M = 200 kg. The
man is pulling with force, F, on a rope that is connected
by a pulley to the plank below him. If F = 200 N is the
maximum force he can pull on the rope before his feet
start slipping, ﬁnd the difference, MS — ,uk, where ‘us is the coefﬁcient of static friction between his
feet and the plank, and MR is the coefﬁcient of kinetic friction between the plank and the ground. As
usual, the rope and the pulley have negligible mass, and you can assume that the plank is sliding with this F.
4. Figure A to the left shows a toy car with constant speed, v,
racing around a bowl with radius, R, and at an angle, 61 = 300,
FA R , a from the vertical. Figure B shows the same car moving at the same speed and in the same bowl, but now the bowl and its
contents are being accelerated upward with acceleration, a; the
car is now at an angle, 62 = 280, from the vertical. If the car
has the same speed in the two cases, what is a / g? All surfaces
are frictionless. Fig.A Fig.3 5. The ﬁgure below shows two masses resting on a larger block in the shape of an equilateral triangle.
The two blocks have mass m1 and m2 , with m2 > m1. They are connected to one another through a
string running over a pulley. The whole system is being accelerated to the right at an acceleration, a,
chosen such that the two small blocks do not move relative to the triangle. If at this acceleration mass
m1 is just starting to lose contact with the triangle, what is m2 /m1? Your ﬁnal answer should depend only on 6. All surfaces are frictionless. ...
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- Fall '08