Force question

inflames829

Homework Statement

A 380kg piano slides 3.5m down at 27 degree incline and is kept from accelerating by a man who is pushing back on it parallel to the incline. The effective coefficient of friction is 0.40. Calculate the force exerted by the man

The Attempt at a Solution

Im confused with this because i think i have to use F =ma but I am not sure because i don't know if i have to use the angle of incline as well to work out the force. Can someone please give me some hints or help

Staff Emeritus
Gold Member
A good place to start is resolving the forcing acting into their components that are parallel and normal to the incline. A free body diagram always helps.

inflames829
what do you mean by resolving the forces acting into their components that are parallel and normal to the incline

nandu11
resolving the force means making all the components of various forces on that object..since there's no acc u can take the force to b zero and thus equate the forces in opp direction..gud luck!

Nick89
If the man was not pushing against the piano, it would accelerate down the slope. The direction of this acceleration (down the slope) is usually the best choice to choose your coordinate system.

In this question, the coordinate system is therefore slightly skewed in relation to the 'ground'.

The gravitational force acting on the piano however is directed straight 'into the ground' and is therefore not aligned with the coordinate system.
Whenever you see this, you can 'split' the force into two components, one along the y-axis and one along the x-axis.
The component of the gravitational force that acts along the line of the slope will be the force that actually makes the piano move.

You can find the size of this force by using some trigonometry (you know the angle of the slope).

Finally, don't forget that there is also friction and the force of the man pushing back that is stopping the piano.