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```Physics 8A
Summer 2020
MIDTERM #1
I certify that I did not seek help from any other person in solving the problems on this exam.
Name: ____________________________
Signature:__________________________
1. A football is thrown at an unknown speed and angle (although we know the vertical
component of the initial velocity is upward). It then lands at a location 15.0 m below the
level at which it was thrown. Someone observes the football to be traveling at a speed of
25.0 m/s and in a direction 65.0 below the horizontal just before striking the ground.
(a) What was the initial speed of the football? At what angle was it thrown above the
horizontal? (b) How much time did the ball spend in the air? (c) How far horizontally did
the football travel?
15 m
at
Vfx n UfcosOf
fy UfSia Ef
Vix
a
Vij
Ufy
Vic t
a
Viy
n
l
10.6mls
a
22.7 m s
25.0 4s sin C 65,07
1cos C G5 O
25.0
lo G Ms
2g Oy
VI t 2gOY
VI T 2gOY
1
22.7
14.9
Vi
an
219 fm sYL 15.0
ms
T Vii
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VI
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Vix
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t
54.60
18.34
n
14.9 4
10.6ms
m
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22.7
g0
ms
14.9
Ot
as
T
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3.84 s
9.8
4 Ot
Physics 8A
Summer 2020
MIDTERM 1
2. Block A has a mass of 20.0 kg. It is attached to a rope, which is slung around a massless,
frictionless pulley and attached at the other end to block B, which has a mass of 25.0 kg. The
incline makes an angle of 15.0 with the horizontal. There is no friction between block B and the
incline. The blocks are initially at rest. (a) What minimum static friction coefficient must there
be between block A and the surface to prevent the blocks from slipping? (b) Suppose the blocks
do slip, and accelerate. If the kinetic friction coefficient between block A and the surface is 0.10,
what is the tension in the rope?
y
F
ha
f
twi
Ta
Y
TB
T
f
A
is
Block B
If system is at rest
Block B
sin 0
Mpg
Ti mpgsin 0
T
T Misgsin 0
fo f
O
Block A
Fnet a T
f
f
static friction is being pushed to its limits
Block At
Fnet
Fnet
T
fr
MA
a
0
25.0 Ks 9 8m64 sin 15.00
this is also Fornax
But also fsmay lls h Us MAg
G3
f ma
Lzo.oksk9 fm s2
µs
Mag
Los
I
T
63.4 N
Since
BY
ti
W X
Block A
Fnet
a
T Mu ma g
0.324
tlwmag
mn.AT
T
Block B
Fnet
Fnet
T
Mrsgoin G
Mrs a
MBgsinGT
T M.rs
easiest to find the
Algebraically it might be
tension from that
acceleration then
get
T Uk Ma g t mpg sin 0 T
Ma at Mpa
Ma
MBg sin G MvcMag
MB Jin G llama
a
Mat MB
25 0kg sin 15
20 0 kg t
T
0,974
µ
ma g t MA U
g
lo lol 20.0kg 9.8
25.0 Kg
2
0.1016200kg1 9 fm
39 IN
t Mp3 a
s4t 20.0kg 0.974
2
4
Physics 8A
Summer 2020
MIDTERM 1
3. A block of mass 5.00 kg is pushed against a horizontal elastic spring with a spring constant
k = 75.0 N/m. The block is then released from rest, and moves up a 25.0 incline that is 15.0 m
long. It then travels along another horizontal surface before entering a loop of radius 10.0 m. All
surfaces are frictionless. Find the minimum spring compression needed in order for the block not
l e c n ac i h he l
hen i a he
f the loop.
is.com
flueefT
c
25no
work is done by nonconservative forces
mechanical energy is conserved
But what speed does the block need to have in order
not to lose contact at the top
Since
no
I
j 14th
4
Finely h t ng
ht mg
mu
at
mr
mg 0
needstobe
3 Mgr and the kinetic energy at the top
n
So
M
Fnety
n
m
a minimum
K Ef
KEI t Ui
O t Iz k X
r
tn rYCld.om
f mu't Iz MS lzc5ookssc9
245J
K Est Ut
r
k Ef t ng y
G t Zr
ya Isin
a
I 5 Om Sin250t 2 loom
26.3 in
75.0mm X
245J
t
1534 J
X
64
m
Fooks 9 sn s4C26.3m
```
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