Newton's Third law of motion problem.

[abrowaqas]

Homework Statement

An object is free to move on a table, ex-
cept that there is a constant friction force f
that opposes the motion of the object when
it moves. If a force of 10 N pulls the object
across the table, the acceleration is 2 m/s2. If
a force of 20 N pulls the object across the ta-
ble, the acceleration is 6 m/s2. What is the
force of friction f?

a) 3.33 N
b) 1 N
c) 5 N
d) 10 N
e) none of these

Homework Equations

F= ma
and
F= -F'

and
F= μS where μ= coefficient of static friction

The Attempt at a Solution

i first find the mass in both case

F= 10 N and a= 2m/s/s
so
m= F/a = 10/2
m= 5kg

and again

F= 20N and a= 6m/s/s

so
m= F/a = 20/6
m= 3.33kg

what i do next..
or
either i must go to

F= μ S

please help me to solve it.. how i go further?

 

[Doc Al]

F= 10 N and a= 2m/s/s
so
m= F/a = 10/2
m= 5kg

10 N is the applied force, not the net force. To use Newton's 2nd law you need the net force.

 

[abrowaqas]

then Doc Al

how will i find the net force..
will it be

Fnet = 20+10 = 30N ...
what i do next?

 

[Doc Al]

how will i find the net force..
will it be

Fnet = 20+10 = 30N ...

No. In the first case, what two forces act? One is the applied force of 10 N. What's the other force?

You'll end up with an equation with 2 unknowns.

Then you'll get a second equation, using the second case (with the applied force of 20 N).

You'll solve those two equations for the two unknowns.