Easy question has me confused (Inclined plane)

[jamesweston0]

It seems to be an easy question but for some reason I can't figure it out.

Homework Statement

If the 10 lb block A slides down the plane with a constant velocity when θ = 30°, determine the acceleration of the block when θ = 45°.

Homework Equations

F = ma

The Attempt at a Solution

I actually am very puzzled by this question. I know that acceleration = 0 at 30° but I don't know how to make use of it. I tried setting the parallel force at 45° to equal ma, then solve for a, but that didn't do it either. Any insight? Thanks.

 

[matt_crouch]

first resolve the forces around the block

 

[jamesweston0]

FNormal = 10cosθ
Fparallel = 10sinθ

Just sub in 30 and 45 for θ.

What else is there to use?

 

[matt_crouch]

where is gravity in your equation remeber its the force of the block acting down the slope not the mass. Just want to check do you assume that the plane is smooth?

 

[jamesweston0]

Well the 10sinθ is the force of gravity down the ramp. Also, yes I'm assuming the plane is smooth. It doesn't say anything to lead me to believe there's friction.

 

[matt_crouch]

it should be the force of the block acting down the ramp so 10gsin45=ma

but since the mass is the same
should get gsin45=a

 

[jamesweston0]

Hmm I though 10lb is referring to a force not a mass in this case.

Regardless, gsin45 = a is not working out.

32.2sin45 = 22.77 ft/s^2

The real answer is 9.62 ft/s^2.

 

[matt_crouch]

hmmm not sure then. ill have another look
we both must be missing something obvious :/

 

[jamesweston0]

Yeah lol this is really confusing for me too. It doesn't seem to be very hard but something is obviously not clicking.

 

[jamesweston0]

Any new ideas?

 

[rock.freak667]

The only reason I think they would give you the 30 degree condition is that you need to account for friction.

try using the 30 degree criteria to get the coefficient of friction.

 

[jamesweston0]

You got it rock.freak667!

Kind of a weird question since it didn't say anything about friction. It's not in the friction chapter either. It's a few chapters later. I guess they expect us to use ideas from previous chapters even without making mention of them.

Thanks!

 

[blkjedi954]

It seems to be an easy question but for some reason I can't figure it out.

Homework Statement

If the 10 lb block A slides down the plane with a constant velocity when θ = 30°, determine the acceleration of the block when θ = 45°.

Homework Equations

F = ma

The Attempt at a Solution

I actually am very puzzled by this question. I know that acceleration = 0 at 30° but I don't know how to make use of it. I tried setting the parallel force at 45° to equal ma, then solve for a, but that didn't do it either. Any insight? Thanks.

You would use the 30 degrees to solve for the kinetic friction and then use the value for kinetic friction to solve for the acceleration. I believe its supposed to be a trick question since nowhere is friction mentioned. Cannot be solved without first finding the friction which happens to be 0.5774. Find the Normal force, and with the friction you should come out with the answer. Hope this helps.