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Finding mass

by 2much
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2much
#1
Mar5-11, 07:01 PM
P: 14
1. The problem statement, all variables and given/known data
You apply a force of .35N up to lift a fork, the resulting acceleration is .15m/s2. What is the mass in grams.


Please help I don't know where to start with this simple question.
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PhanthomJay
#2
Mar5-11, 07:13 PM
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Start with identifying the net force acting on it, and use Newton's 2nd law. Please show an attempt.
2much
#3
Mar5-11, 07:18 PM
P: 14
The net force would not be zero and I am only given the applied force. Without a mass I don't know how I can get the force of gravity.

So as far as I can tell I have .35 Fg = m .15m/s2

SammyS
#4
Mar5-11, 07:27 PM
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Finding mass

Let mass = m & use symbols where needed.

What does Newton's 2nd Law say?
2much
#5
Mar5-11, 07:46 PM
P: 14
0.35 Fg = m .15m/s2
0.35 - (m 9.8 m/s2) = m 0.15ms2

The second law says that if the net force is not zero there is an acceleration in the direction of the force.
SammyS
#6
Mar5-11, 07:51 PM
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You apply a force of 0.35 N upward.

If the mass of the for is m, what force is gravity exerting downward on the fork?

What is the net force being exerted upon the fork?
2much
#7
Mar5-11, 07:56 PM
P: 14
Wouldn't I need the mass of the fork (m) to calculate the force exerted by gravity?
SammyS
#8
Mar5-11, 07:58 PM
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You will have "m" in two places in your equation. Use algebra to solve for m.

I repeat:
What is the net force being exerted upon the fork?
2much
#9
Mar5-11, 08:15 PM
P: 14
I can get up untill
Fapp - m a(gravity) / a(applied) = m
Now I am a bit confused on my next move. Is this right so far? Since m on the left is being multiplied by the acceleration of gravity I think I should divide to get rid of it. But once I do the right side would cancel out to zero.
SammyS
#10
Mar5-11, 08:23 PM
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Fapp - m g = m a

0.35 - m g = m (0.15)

Not sure what you use for g: 10 m/s2 or 9.8 m/s2 or 9.81 m/s2

Put in the appropriate number for g & solve for m.
2much
#11
Mar5-11, 08:34 PM
P: 14
Yes, but as I said earlier I can get up until

0.35 - m g / 0.15 = m

I can't successfully eliminate the LH m. I tried dividing and adding it to the RH
PhanthomJay
#12
Mar5-11, 09:39 PM
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Before you divide by 'a', your correct equation, per post 5 2nd line, was

0.35 - (m 9.8 m/s2) = m 0.15ms2

leaving off units, then

0.35 - (9.8m) = 0.15m

Now this is algebra, add 9.8m to both sides

0.35 = 9.95m

Now solve for m and convert to grams.
2much
#13
Mar5-11, 09:54 PM
P: 14
Thanks, that makes allot of sense.


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