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ragnarthor · Sep 21, 2011, 05:52 AM

Print ViewProblem 5.64
A 4.68 kg box sits at rest at the bottom of a ramp that is 8.36 m long and that is inclined at 40.0^\circ above the horizontal. The coefficient of kinetic friction is 0.40, and the coefficient of static friction is 0.50.

Part A
What constant force F , applied parallel to the surface of the ramp, is required to push the box to the top of the ramp in a time of 4.38 s?

ebaines · Sep 21, 2011, 09:26 AM

We won't do your homework for you, but we will help you along if you can show how you've attempted to solve this problem.

ragnarthor · Sep 22, 2011, 04:55 AM

I'm doing my homework online and only can say an anwswer 5 times,did that alreadi and got it wrong, now I know the answer is 48 but I don't know how

ebaines · Sep 22, 2011, 05:53 AM

The force needed is the sum of forces needed to:

(1) Accelerate the block so that it covers the distance in the proscribed time. Calculate what the acceleration has to be, then F1 = ma.

(2) Overcome the force of gravity that you have to pull against: F2 = the component of the block's weight along the direction of the ramp.

(3) The kinetic friction, F3= the component of the block's weight that is normal to the ramp times the coefficient of kineitc friction.

You don't need to worry about the static friction - since it's coefficient is less than the kinetic friction coefficient you have no problem overcoming it and getting the block moving.

Add up the three forces and you get your answer. Try it, and post back with your calculations.

Unknown008 · Sep 22, 2011, 06:00 AM

Originally Posted by ragnarthor

I'm doing my homework online and only can say an anwswer 5 times,did that alreadi and got it wrong, now I know the answer is 48 but I dont know how

That means that you have done workings 5 times? It wouldn't do any bad to post what you did here... :rolleyes:

mtram · Oct 28, 2012, 04:10 AM

I tried everything but I still can't get the acceleration.. I used the kinematics equations but the are too many unknowns... a small hint 'll be useful..

ebaines · Oct 29, 2012, 05:50 AM

Start by calculating the acceleration needed. Then use $\Sigma F = ma$ to determine the sum of forces required to achieve this acceleration. The forces that are acting on the box are the pushing force that you need to apply, the component of the box's weight in the direction parallel to the ramp, and the kinetic friction force. You have enough info here to solve for the pushing force.