# mehanics 1Watch

#1
a block of mass 4kg rests on a table. It is connected by a light inextensible string passing over a smooth pulley at the edge of the table to 5kg particle hanging freely. There is a friction force of 20N acting on the block. Find the acceleration of the system and tension in the sting.

Ok so I started with eq:

T1-Mg =ma
T1-49=5a

R-Mg=0

Fr-T2=ma
20-T2=4a

so sub 1 into 3

20-4a-49=5a

a=3.2ms-1

I just don't understand how you know which way the acceleration is positive or negative from questions??? and in which equation this affects?? and how do you know it's that equation

Please can someone explain this to me?
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4 years ago
#2
(Original post by svirdi)
a block of mass 4kg rests on a table. It is connected by a light inextensible string passing over a smooth pulley at the edge of the table to 5kg particle hanging freely. There is a friction force of 20N acting on the block. Find the acceleration of the system and tension in the sting.

Ok so I started with eq:

T1-Mg =ma
T1-49=5a

R-Mg=0

Fr-T2=ma
20-T2=4a

so sub 1 into 3

20-4a-49=5a

a=3.2ms-1

I just don't understand how you know which way the acceleration is positive or negative from questions??? and in which equation this affects?? and how do you know it's that equation

Please can someone explain this to me?
Surely it is obvious that the acceleration must be in the direction in which the string is pulling so your third equation should be T-20=4a.
Note that you do not need two different tensions, the tension will be constant throughout the length of the string.
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