force mass and acceleration

kamalpreet122

1. The problem statement, all variables and given/known data

A 2.00X10¹ kg mass is pulled upwards with constant acceleration by a cable attached to a motor. The cable can withstand a maximum tension of 5.00X10² N. What is the maximum acceleration possible?

The answer sheet says the answer is 15.2m/s²

2. Relevant equations

F=ma

3. The attempt at a solution

F=ma
F/m = a
500/20 = a
25 m/s²= a

some1 please tell me what i did wrong :-(

Apphysicist

Don't forget gravity!

doublemint

You have to accout for the acceleration due to gravity!

Edit: Your too fast for me Apphysicist !

Apphysicist

Indeed! F=ma is your net force = m* net acceleration. If you apply your maximum tension upward, gravity still acts downward (sum the forces for your net force, don't forget that gravity gets a minus-sign because it acts in the opposite direction of your upward tension). Then use F=ma.

kamalpreet122

so the formula would be changed to F= (mg)a ?

Apphysicist

No, it would be changed to (T-mg)=ma.

Do you see why?

kamalpreet122

yeaaa kinda ..soo the total weight subtracted from Tension gives me the net force then i apply the forumlaa .. OH that makes so much senseee =) THNXXX :D

kamalpreet122

man yur great =P i have a really tough question i was not able to figure out wanna help me with that :$

Apphysicist

Right. It's much easier to think about F=ma really being:

[tex]\sum_{\text{all}}\bold{F}\,=\,m \bold{a}
[/tex]

So it's a sum of all your vector forces (the bold meaning it's a vector...so for your problem, the +/- indicates the direction is all, since there are no components, it's just in one direction).

kamalpreet122

ohhh Thnxxx :D