Calculating Mass on a Frictionless Surface

[Hawk_RSA]

http://img75.imageshack.us/img75/9735/prob1ve7.jpg

Frictionless surface, pulled along a horizontal line, string has no mass

Calculate the mass m


The answer says 17,1 kg, i don't know if it is correct,
can sum1 please give a solution with the steps

 

[chaoseverlasting]

The answer is correct. You have to show some work for us to help you.

 

[Hawk_RSA]

I don't know how to put it.

I can work out the force of the string and the accel but i can't do it the other way around

i have taken the answer and tried to work backwards and see if i get 80N force on the sting between the block

For block m (17,1 kg's)

Fr = ma
F2 = 17,1a

For 15 kg mass

Fr = ma
150 - F1 = 15a
F1 = 150 - 15a

Newton's 3rd law F1=F2

150 - 15 = 17,1a

32,1 a = 150
a = 4,67289..

F = ma
F = 17,1 * 4,67289..
F = 79,906..
F = 80 N

so the answer is correct, but i can't seem to work out the mass
I don't know where to start,

I don't know how to get the acceleration

 

[catkin]

Have you tried drawing the free body diagram for the 15 kg mass?

 

[temaire]

You calculate the acceleration by isolating the 15 kg mass, and taking into consideration the applied force and the tension force.
Fnet = Fapp - Ft
ma = 150 - 80
15a = 70
a = 4.67

 

[temaire]

Now to find the unknown mass, you have to use the entire free body diagram. This time you have to ignore the tension force.

Fnet = Fapp
ma = 150
(15 + m)(4.67) = 150
(15 + m) = 32.1
m = 32.1 - 15
m = 17.1 kg

 

[Precursor]

You must first isolate the 15 kg mass and find its acceleration. So now, you can use this value to determine the mass of the unknown mass.

 

[Hawk_RSA]

Thanks guys
this is the other sullution i got:
F = ma
a = F/m
call the object of mass 15kg object 1
call the object of mass m kg object 2
acceleration of system (object 1 + object 2) = acceleration of object 1 = acceleration of object 2 = 150N / (15kg + m kg)
acceleration of object 2 = 80N / m kg
therefore:
150 / (15 + m) = 80 / m
multiply through etc
1200 + 80m = 150m
1200 = 70m
m = 17.14