Is my method correct for this kinematic problem?

[Labkid]

I was given a problem along these lines:

A block is sliding up an inclined plane at 45 degrees to the horizontal at 1.0 m/s. The coefficient of kinetic friction is equal to 0.2, the coefficient of static friction is equal to 0.4. What force needs to be applied to the block in order to maintain a speed of 1.0m/s?

The way I approached this is that since the block is moving there is no need to worry about the static friction. Next I saw there was no acceleration meaning the object moves steadily at 1.0 m/s. I assume this means that the force required to achieve this velocity is equal to the force of kinetic friction acting against it.

uk((mg)*(cos45)) = Fk = 10.51 N

So the force needed is 10.51N

I'm not sure I'm thinking about this right though because it doesn't quite make sense to me logically. I would think the force needs to be slightly greater than the kinetic friction otherwise it would come to a stand still. Am I to assume the object was already in motion from the start and now you simply need a force to negate kinetic friction to keep it moving?

Thanks for any help.

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[NextStepTutor_3]

Since they say "maintain a speed of 1.0 m/s," you can definitely assume that the block is already moving at 1 m/s and you're just trying to prevent it from slowing down. In that case you'd just need an upward force that cancels out any downward forces (since a net force of 0 will keep the object moving at a constant velocity).

In other words, you're almost completely correct, except the block is moving upward. This means you'd also need to negate the component of gravitational force that's acting down the plane. So your final answer would be 10.51 N + mg(sin45).

 

[Labkid]

Thank you a lot that helps me understand where I went wrong.

So the question would essentially change completely if it asked "What force is required to achieve a speed of 1.0 m/s?". In that case I would need to worry about static friction plus kinetic friction with the gravity as well and then I could calculate the force required to over-come all of that?

Also I sent an e-mail to your MCAT tutor thing and I hope you guys can really help me cure my physics weakness. So far I usually get 7-8 on that section while other sections are 10 or higher.

 

[NextStepTutor_3]

You're right, it would be a completely different question. You'd need to start accelerating the block in the first place, which would require overcoming static friction, then put in enough additional force to overcome kinetic friction (and, again, gravity) as the block moved. However, they wouldn't be able to ask a question like this without giving you some more info, as there's another variable that's important here: time. (Basically, are you accelerating from 0 to 1 m/s over a short amount of time, which would require a large acceleration, or are you doing so gradually?)

But anyway, thanks for the response! We'd love to help you with physics. It's a tricky topic but feels great when you master it!