Originally Posted by

**patrickmanning** A rectangular block of mass 30kg rests on a plane inclined at 30 degrees to the horizontal. The block is held in equilibrium by a string, which is parallel to the plane.

1 - Calculate tension in string if plane is smooth.

2 - Given that the coefficient of friction between the block and the plane is 0.6, calculate the tension in the string __if the block is just on the point of moving up the plane.__

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1 - Calculate tension in string if plane is smooth.

Since there is no friction, the tension in the string must equal the force parallel

Tension = 30 * 9.8 * sin 30˚

2 - Given that the coefficient of friction between the block and the plane is 0.6, calculate the tension in the string if the block is just on the point of moving up the plane.

Since there is friction, the tension = Force parallel – Friction force.

Tension = (30 * 9.8 * sin 30˚) – (0.6 * 30 * 9.8 * cos 30˚)