This is in the M1 Jan 2007 paper. When I tried part a), I used F=ma because the box is in motion. After looking at the mark scheme, they've used F=μR. I can see why you might not use F=ma now- is it because there is no acceleration as it is moving at a constant speed?
Thank you for your help, I've solved it by myself now
My original method worked- i.e. you can use F=ma. I substituted in 1 ms^2 for the acceleration when it should have been 0 due to the constant speed. No one will read this, but this thread can be closed if Moderators wish to do so.
Resolve horizontally using F=ma (Taking direction of motion as the positive) Pcos20-0.4(30g-Psin20)=30x0 Pcos20-117.6+0.4Psin20=0 Pcos20+0.4Psin20=117.6 2.3492P+Psin20=294 2.69125P=294 P=110 (N)
Sorry if it doesn't make sense. I've copied down what I wrote. Hope it helps