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?

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.

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

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.

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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

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

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..

Start by calculating the acceleration needed. Then use 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.