. Suppose you are pushing a 60 N uniform crate at rest with a horizont

[cmkc109]

Homework Statement

. Suppose you are pushing a 60 N uniform crate at rest with a horizontal force of 40 N
exerted at the top as shown. The crate is a cube with length of L. If the coefficient of
static friction between the crate and floor is 0.75, which statement below is correct?

(Hint: If the crate is about to pivot at point O, the
normal force will act at point O, and not through the
center of mass.)

a. The crate will slide along the surface with constant speed.
b. The crate will slide along the surface speeding with constant acceleration.
c. The crane will slide along the surface speeding to a maximum speed and
then continue to with that speed.
d. The crate does not budge to your force, it neither slides nor pivots.
e. The crate will pivot at point O.

pic : http://i49.tinypic.com/16p2x1.png

Homework Equations

The Attempt at a Solution

I did Fs = us * Fn = 0.6 x 60 = 45
So since applied force is greater than frictional force, the crate should pivot?? then how is this using torque to solve the quesiton?

 

[CWatters]

The friction means it won't slide (so that rules out a to c) but you also need to check the torque about point O to work out if it's d or e.

EDIT:

Where did you get the 0.6 from in ..

Fs = us * Fn = 0.6 x 60 = 45

 

[cmkc109]

The friction means it won't slide (so that rules out a to c) but you also need to check the torque about point O to work out if it's d or e.

EDIT:

Where did you get the 0.6 from in ..

sorry its a typo.
0.75 x 40 = 30N

but I don't understand how is it torque..
cuz in the textbook, it says something about cancel out the torque produced by friction because it acts on the pivot or something..?


or is this question just simply because the applied force is greater than the friction so the crate will pivot?

 

[cmkc109]

Fapp * L and mg L/2
but they are both tilting towards clockwise position..

imagining a force push the crate, then the crate will be moved the same direction as the direction of the force...

 

[CWatters]

Lets look at the Torque about point O and define clockwise as +ve.

Torque = force * distance

You have the force pushing on the crate at the top which produces a torque = 40 * L

You have the weight acting through the center of gravity producing an anticlockwise torque = - 60 * L/2

You have friction acting through point O so that produces a torque = 30 * 0 = 0. It's zero because the displacement from point O is zero.

Total Torque is

40L + (-30L) + 0 = 10L

That's a positive torque so the crate rotates clockwise.

 

[cmkc109]

Lets look at the Torque about point O and define clockwise as +ve.

Torque = force * distance

You have the force pushing on the crate at the top which produces a torque = 40 * L

You have the weight acting through the center of gravity producing an anticlockwise torque = - 60 * L/2

You have friction acting through point O so that produces a torque = 30 * 0 = 0. It's zero because the displacement from point O is zero.

Total Torque is

40L + (-30L) + 0 = 10L

That's a positive torque so the crate rotates clockwise.

why is the weight anticlockwise? I just couldn't picture these rotational torque... How do you picture it?

 

[CWatters]

Will post picture in a moment.

 

[CWatters]

Like this..

 

[cmkc109]

oh right, i get why is it CW and CCW now thks

 

[CWatters]

Have drawn it slightly better. This shows the crate at the point of tipping. The normal force has moved to point O.