Solving the Kinetic Friction Problem - Bob's Dilemma

[brayrbob]

I need help with this problem
Bob wants to push a crate horizontally with a force of 100N. If the coefficient of kinetic friction between the crate and the floor is 0.5 and the crate has a weight of 800 N, can he move the crate.

Okay that's the problem and I don't believe that 100N is enough force to push a crate of 800N. But even if my answer is wrong I'm not sure which equation to use to prove it.
When I take the kinetic friction of 0.5 and times it by the crate's 800N, I get 400N. Is this the way to show that 100N is not enough force to push this crate?

 

[Nylex]

Yep, sounds ok to me.

 

[mrjeffy321]

Even assuming the crate is already moving, it wouldn't be enough.
The force of kinetic friction as you showed is eqaul to the objects normal force times the coefficient of kinetic friction, whcih equals 400 N.
If the pusher only supplies 100 N, then the box will slow to a stop if it is already moving.

Generally (from everything I have seen), the static friction is always higher than the kinetic friction, so if the crate starts from rest, the pusher will have even less of a chance to move the box.

 

[brayrbob]

Okay, That answer seemed logical to me, but I wanted to be sure I was right.
Thank you all very much for your help.

 

[quasi426]

If the problem only gives you the kinetic friction constant then, like someone mentioned above, you can say that the static friction coefficient is always larger then the kinetic friction coefficient. Therefore the 100N would not be larger then the static frictional force which is larger then 400N.