# Basic Newtons law's help

1. Nov 8, 2007

### Hawk_RSA

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 plz give a solution with the steps

2. Nov 8, 2007

3. Nov 8, 2007

### Hawk_RSA

I dont know how to put it.

I can work out the force of the string and the accel but i cant 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 cant seem to work out the mass
I dont know where to start,

I dont know how to get the acceleration

4. Nov 8, 2007

### catkin

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

5. Nov 8, 2007

### 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

6. Nov 8, 2007

### 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

7. Nov 8, 2007

### 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.

8. Nov 8, 2007

### 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