Magnitude of object with constant speed

[Kamirusen]

Homework Statement

in the physics laboratory experiment, a 6 kg box is pushed across a flat table by horizontal force F. if the box is moving at a constant speed of 0.35 m/s and the coefficient of kinetic friction is 0.12. what is the magnitude of F.

Homework Equations

F=μ*F_n
where
F = friction
μ = coefficient of friction
F_n = normal force

F_n = mass of object * acceleration

The Attempt at a Solution

i solve for F_n first,
F_n = 6 kg * 0 m/s²
and got an answer of zero for normal force... i let the acceleration to be zero because it says that it has constant speed... Is it normal to have a zero normal force?

and then when i solve for the magnitude of friction...
F = 0.12*0
which is zero also... ...

i'm not sure if i got a wrong equation, but i think its correct... or if i got some wrong interpretation of the given values that results to zero friction...

My question is, is it a normal thing for an object with constant speed to have a zero friction?

 

[Doc Al]

F_n = normal force

F_n = mass of object * acceleration

That's not how you find the normal force. Consider the vertical forces acting on the box.

 

[Kamirusen]

thanks for reply...

so ill going to consider the acceleration due to gravity of an object with no horizontal acceleration?

 

[vivekrai]

First and fore most, never solve Newton Laws Problems without FBD. Draw the figure and correctly denote the directions in which the forces acts. Use F(net)=Ma along x-y axes. See what happens.

 

[Doc Al]

so ill going to consider the acceleration due to gravity of an object with no horizontal acceleration?

Sure. You'll need it to find the object's weight.

Just because you might use the acceleration due to gravity does not mean the object must be accelerating. Think of the acceleration due to gravity as a measure of the strength of the Earth's gravitational field.

 

[-Physician]

In this case
$F_N=mg$ m = mass, g=gravity, gravity pulls down with 9.81m/s^2 so find normal force
$F_N=6kg*9.81\frac{m}{s^2}=58.86N$
If you're trying to move the body in constant velocity then
$F_{net}=0$ because:
$F_{net}=ma$
$F-F_f=ma$
$F-F_f=m * 0$
$F-F_f=0$ so friction is equal to pushing force.
so simply find friction $F_f=\mu F_N$ and that's the pushing force

 

[Kamirusen]

thanks ! its clearer now... :)