A block of unknown mass is placed on a surface in which it has a coefficient of static friction of 0.3
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A block of unknown mass is placed on a surface in which it has a coefficient of static friction of 0.3

[From: ] [author: ] [Date: 12-01-09] [Hit: ]
find the mass of the block.-Are you sure the question is posed correctly?I will explain some problems.Uk = the coefficient of kinetic friction, because when the block starts to move, it ceases to have static friction working on it.......
The surface is then titled up to a 30 degree angle. if the block takes 3 seconds to travel down the first meter of the ramp, find the mass of the block.

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Are you sure the question is posed correctly?

I will explain some problems.

How to find the force of static friction =
Fs = ma = (m*g*cos30) - (m*g*sin30*Uk)
A = (g*cos30) - (g*sin30*Uk)

Uk = the coefficient of kinetic friction, because when the block starts to move, it ceases to have static friction working on it.

And because the mass cancels out, it means that ANY mass put into the equation will render the same acceleration, despite what it is.

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a = g*(sinΘ - µcosΘ) where µ is the KINETIC coefficient of friction.

The amount of mass is irrelevant. Any mass will follow the same speed profile.

If the mass takes 3 sec for the 1st meter, a = 2*s/t² = 2*1/9 = .222 m/s²

Solving the first eq for µk → µk = tanΘ - a/gcosΘ = .551

With this value for µk, it is highly unlikely that µs is 0.3 .........
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