sorry mr bob purlsey again to annoy you.

Question

so i did all my calculations and read about hooke's law yesterday and presented to my teacher. however i was not allowed to used hooke's law because we haven't learned it yet and i did not know how to connect potential energy to force of friction.

i'll give u full values and details this time.

question :
calculate the average force of friction acting on a wodden block as it its proplled along the floor by an elastic band. remember the elastic band force is applied horizontally, it is not angled.

so i performed three trials:
distance streched was 50 cm every time.
average distance travelled by the wooden block was 0.0700m or 75.2 cm
average time for the block to travel the average distance 0.45 seconds
Fa average at 50 cm was 6 N. (only thing that worries me is that the elastic destrecthes and the force changes)

my theory:
can i find out the acceleration of the object using v1 which is 0 v2 which is 0 ( because the object comes to a rest at the end ) but we can cancel out v1 and v2 and since we have displacement and t we(with the distance,v1,v2,1/2,a equation) but now we only have d, t, 1/2 to solve for a can solve for acceleration of the object.

then i solve for fnet
fnet=ma

then i solve for the force of friction by:
we have fnet which are all the forces invloved in the object. we have force applied of 6N ( is this right) i used a newtown scale to strecth the elastic 50cm everytime to find the fa. fnormal support and fgravity are not a factor because they cancel each other out.
so i would use the equation
ff=fnet- fa

sorry mr Bob pursley. you have been very helpul to me. i would like to apologize and say thanks again.

your theory makes no sense to me. The six N force at the beginning of acceleration rapidly goes to zero, so you cant use it (Hooke's law). You have to know the velocity of the block at some point if you are going to find acceleration. Unless you measure that, you cant find acceleration (change in velocity).

Why don't you just pull the block at constant velocity with a spring scale? That reading is a measure of friction alone, since there is no acceleration. Otherwise, without assuming Hooke's law on the string, or using energy concepts, you are out of luck.
Never in the history of science have I heard someone say that they couldn't use something known to them because they have not learned it yet. Galielo, Bacon, Newton, nor Brahe would have put up with such nonsense.