Mechanical proof involving inclined plane

[James10492]

Hi, me again, another mechanics problem (yay):

'A particle P is projected up a rough plane which is inclined at an angle x to the horizontal, where tan x = 3/4. The coefficient of friction between the particle and the plane is 1/3. The particle is projected from the point A with speed 20 m/s and comes to instantaneous rest at the point B.

Show that while P is moving up the plane its deceleration is 13g/15.'

(my working):

tan x = 3/4, so sin x = 3/5 and cos x = 4/5

Let the force of projection up the plane be Q. Using Newton's second law,

Q - mg sin x - (1/3)(cos x mg) = ma

Q - 3mg/5 - 4mg/15 = ma

Q - 9mg/15 - 4 mg/15 = ma

Q - 13mg/15 = ma

looks promising. Divide through by m

Q/m - 13g/15 = a

we are looking for deceleration so it makes sense that the -13g/15 term is negative, but what am I to make of the Q/m term? (I must have done something wrong but I can't see what).

 

[topsquark]

It's a problem with the English, not your Physics. When we say that a particle is "projected" up an incline we do not mean that there is a force on it, merely that it starts with a velocity up the incline. So Q = 0 N.

-Dan

 

[James10492]

It's a problem with the English, not your Physics. When we say that a particle is "projected" up an incline we do not mean that there is a force on it, merely that it starts with a velocity up the incline. So Q = 0 N.

-Dan

Ohhhhhhhhhhhhhhhhhhhhhhhhhhhhhh

(I get it now thank you).