Homework Help: Finding the resultant force

1. May 7, 2007

monkeyhead

1. The problem statement, all variables and given/known data
Hi,
The question im a bit confused about is this:

A block is dragged across the ground by a light cable,attached to a winch. Th block is initally at rest.

Information given:
Volume: 6m³
Density 2875 kg/m³
Coefficent of friction between block and the ground: 0.4
Diameter: 1.4m

Find the force that must be applied to the block to overcome friction and cause it to accelerate at a rate of 0.4m/s²

3. The attempt at a solution

So I've first got to find the mass right? m=D.V= 2875*6= 17,250 KG

Then I can find its weight. w=m.g = 17,250*9.81= 169,222.5 N

Now I need to find the force acting down on the block right? Fa= m.a
=17,250*0.4= 6900N

And the frictional force acting Frictionalforce= μ*w = 0.4*169,222.5= 67,689N

Then the total force should equal: Tforce=√Frictional force²+Fa²= √67689²+6900²= 68,039.77308 right?

Have I gone wrong somewhere?

Many thanks
Matt

2. May 7, 2007

mbrmbrg

You're doing OK until you add forces.
Newton's seond law states:
$$\Sigma F = ma$$
Sigma means add up the forces algebraicly. Squaring stuff is when you have forces acting along x- and y- axes.

3. May 7, 2007

monkeyhead

So F - friction force = ma
where F=? Frictional force=67689 ma= (17250*0.4)=6900

So F - 67689 = 6900?

Many thanks
Matt

4. May 7, 2007

mbrmbrg

Yes.

Don't really understand what you have written here. But if you solve your last line for F, you and I will get very similar answers.

5. May 7, 2007

monkeyhead

Would that mean F= 6900+67689=74589? Right?
Many thanks
Matt

6. May 7, 2007

mbrmbrg

The number looks good to me--just slap on units and a direction, and check significant figures, and that's it.

7. May 7, 2007

monkeyhead

Thanks!
Out of interest, how would I go about finding out the angle?
Many thanks
Matt

8. May 7, 2007

mbrmbrg

Everything done solved for the x- component of the force. No information was given about the y-component.

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