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Working on some old exam problems

  • Thread starter eq123
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  • #1
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hey everyone .. I've been working on some old exam problems .. i couldn't figure these out :(

A 40-g bullet, with a horizontal velocity of 500 m/s, comes to a stop 20 cm within a solid wall. What is the magnitude of the force from the wall stopping it? (Assume this force to be constant) (Ans: 25000 N)

An ideal spring is hung vertically from the ceiling. When a 2.0-kg mass hangs at rest from it, the spring extends 6.0 cm from its relaxed length. A downward external force is now applied to the mass to extend the spring an additional 10 cm. Find the work done by the spring force while the spring extension changes from 6.0 cm to 16 cm. (Ans: – 3.6 J)

A body of mass M = 2.00 kg, tied to a string, rotates in a vertical circle of radius R = 1.00 m. Find the work done by the force of gravity on the body during one full revolution. (Ans: zero)

A man moves the 10-kg object shown in figure 6 in a vertical plane from position X to position Y along a circular track of radius R = 20 m. What is the work done by the force of gravity during this motion? (Ans: − 3920 J)
 

Answers and Replies

  • #2
tiny-tim
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  • #3
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thank you for your fast respond :)

i couldn't figure anything out for problems 1, 3, 4 :(

as for problem 2 .. this is what i did ..


when relaxed

wnet = ws + wg = ∆k = 0
ws = -wg
Fs d = -Fg d
Fs = -Fg
-kx = -mg
k = mg/x = 2(9.8)/(0.06) = 327 N/m

when external force is applied

ws = -0.5k(x2f - x2i) = -0.5(327)(0.162 - 0.062) = -3.6 J


i am pretty sure that this solution is wrong!

i have a question related to the spring constant .. is it always positive?

you can notice that i am having some difficulties in the last two questions with work done on circular motion .. so please .. i would appreciate it if you explained that ..
 
  • #4
tiny-tim
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hi eq123! :smile:
as for problem 2 .. this is what i did ..
yes that looks ok :smile:

what is worrying you about it? :confused:
i have a question related to the spring constant .. is it always positive?
you answer that …

if k is negative, say -100, then d2x/dt2 = 100x …

what does that mean, and is it possible to have a spring like that? :wink:

for the last two questions about https://www.physicsforums.com/library.php?do=view_item&itemid=75" you should use the work-energy theorem :smile:
 
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