Finding mass

1. Mar 5, 2011

2much

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.

2. Mar 5, 2011

PhanthomJay

Start with identifying the net force acting on it, and use Newton's 2nd law. Please show an attempt.

3. Mar 5, 2011

2much

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

4. Mar 5, 2011

SammyS

Staff Emeritus
Let mass = m & use symbols where needed.

What does Newton's 2nd Law say?

5. Mar 5, 2011

2much

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.

6. Mar 5, 2011

SammyS

Staff Emeritus
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?

7. Mar 5, 2011

2much

Wouldn't I need the mass of the fork (m) to calculate the force exerted by gravity?

8. Mar 5, 2011

SammyS

Staff Emeritus
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?​

9. Mar 5, 2011

2much

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.

10. Mar 5, 2011

SammyS

Staff Emeritus
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.

11. Mar 5, 2011

2much

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

12. Mar 5, 2011

PhanthomJay

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.

13. Mar 5, 2011

2much

Thanks, that makes allot of sense.