Need help with some mechanics questions

for c) the answer for p is 392N but how do I go about solving it. Is friction involved because it's a rough plane but I don't get u though for 2bi) why it is -15m, I get that it is going in the negative side of the graph so does it mean whenever you go to the negative side of the graph, the number is negative as well? I thought -15 on the graph could just be 15 in calculation. For 2bii) I said because it is going in the same direction. Not sure what's a good way to explain . Thank you

How about you upload your graphs you've sketched so we can work off that?

I can't find my work at the moment because my desk is mess so could you explain how to do c first because I don't know if I should include friction in there the answer I got without friction for P is 699N. Thanks

Not a friction question. I'm heading off to bed, so hopefully someone else will see this and help you!

No one has at the moment. thanks

Displacemet is a vector quantit. Find the area between the x-axis and your lines by breaking it into triangles and rectangles.

Area above the axis is positive and are below is negative.

So displacement = area above axis - |area below axis|.

But.if I do 3*-5 I get the right answer and that's below the graph. Also I wouldn't be able to work put the area above the graph because I don't know the time.Could I halve 20 in this case because it is a constant retardation

Have to say that these are very poorly worded questions that you've been given.

As you've correctly noticed, the retardation is constant, and since we're going from 5 to -5, then the velocity will be zero half way through the interval, i.e. at the 10 second mark.

Yes, I have to agree. This is assignment questions I get every week and there are lots of wrong answers too. Can u explain c) please? Thank you

for c) by "resistance force" we must assume they are refering to the frictional force.

Consider forces parallel to the plane.

Since you know the acceleration, you can work out the resultant force parallel to the plane, F=ma.

Then this force is the sum of all the components of the forces parallel to the plane. This gives you an equation you can solve for P.

You may find a diagram useful with the forces marked.

Post some working if it's not coming out, and also say what value you're using for g. I used 9.8, and it came to 392N (3sig.fig.)