Thread: Problem with pulley

Hey, i have the mark scheme for this question but i simply do not understand it.the question is here : maths problem , part E . the answer is here:answer .

I don't understand, what i did was use equations of motion , from the point when Q reverses and falls 3.15 m, therefore giving me the value of V from v^=u^2+2as

v^2=0+2x(9.8x3.15)
v=7.857 m/2

so i used V=U+at
giving t=0.8017

What is wrong with my method? Would appreciate help, been bugging me all day

Originally Posted by chartsy

Hey, i have the mark scheme for this question but i simply do not understand it.the question is here : maths problem , part E . the answer is here:answer .

I don't understand, what i did was use equations of motion , from the point when Q reverses and falls 3.15 m, therefore giving me the value of V from v^=u^2+2as

v^2=0+2x(9.8x3.15)
this is wrong. the above equation is true for a particle in free fall. P is not in a free fall
v=7.857 m/2

so i used V=U+at
giving t=0.8017

What is wrong with my method? Would appreciate help, been bugging me all day

now try it.

when P hits the ground, Q is still rising at speed

time for it to rise and fall to its original position is

so...you kind of combine the upmove and the downmove into the one equation v=u+at? i didn't know you could do that , i thought you could only use equations of motion one direction at a time

what i also don't get is, why should the speed stay at 4.2 m/s? i mean, if Q falls back to where it was when P hit the ground, surely ,given that -9.8/ms^2 is greater than -4.2ms^2, the speed V would be bigger than 4.2?

Originally Posted by chartsy

so...you kind of combine the upmove and the downmove into the one equation v=u+at? i didn't know you could do that , i thought you could only use equations of motion one direction at a time

You aren't combining them. You are using them one at a time. You need to find the speed that Q is moving upward initially (the speed it gets to after P hits the ground). That gives you the v0 for the problem after P is on the ground.

-Dan

Originally Posted by chartsy

what i also don't get is, why should the speed stay at 4.2 m/s? i mean, if Q falls back to where it was when P hit the ground, surely ,given that -9.8/ms^2 is greater than -4.2ms^2, the speed V would be bigger than 4.2?

I get it! it's symmetrical because it decelerates as fast as it accelrates!