[tom472]

For the following question I get a = -1.89ms^-2 (this is actually the answer). What I want to know is why does the acceleration have a negative value?

"A stone is projected with speed 10ms^-1 down a plane inclined at 30 degrees to the horizontal. The coefficient of friction between the slope and the stone is 0.8. Find the distance travelled by the stone before coming to rest"

[TenOfThem]

a is negative if you are slowing down

In this case the friction is slowing the stone down

[As_Dust_Dances_]

If the acceleration is negative, it means the stone is slowly decreasing in speed.

u=10
a=-1.89
v=0
s=s

Plug it into the formula: v^2=u^2+2as

[tom472]

If it was sent up the slope and friction slowed it down, would the acceleration again be negative? Thanks.

[TenOfThem]

(Original post by tom472)
If it was sent up the slope and friction slowed it down, would the acceleration again be negative? Thanks.

yes ... but even more negative as gravity would also be slowing it down