I got 225000N for this question but my teacher's marking (which I'm not convinced with says 25000N - he's probably right though but I must check as I can not ask him as its the christmas holidays and our exam is pretty much the first day back)
See what you make of it! Please =P
A train of mass 200000kg is moving at a constant velocity of 25m/s, it then brakes and has constant decelleration and comes to rest in 40s. In this stage there is no driving force produced by the train's engine, but a retarding force produced by the train's brakes. There is also a constant resistance force on the train of magnitude 100000N throughout the brakin. Find the retarding force produced by the breaks.
Originally Posted by topsquark
I can follow you most of the way but I do not understand where the came from! Where did it come from???
PS. We weren't taught this theorum in class but it seems like a good one to know for the exam as it seems to make the question easier to answer
Here is one way, via F = ma, which may mean if there is an acceleration, then there must be a force that is causing it.
Originally Posted by anthmoo
F = force...in N in your problem here.
m = mass, in kg here.
a = acceleration, in m/sec/sec here.
The accelaration here is actually deceleration, or acceleration to slow down, or "negative" acceleretion, or acceleration opposite in direction to the initial direction of the motion.
a = (final velocity minus initial velocity) / time
a = (0 -25)/40
a = -(5/8) m/sec/sec
The force causing this "negative" acceleration is the constant 100,000N plus the constant braking force, X, on the brakes, so,
F = ma
(100,000 +X) = (200,000)(5/8)
100,000 +X = 125,000
X = 25,000 N ..................answer.
Why is the acceleation not negative in the equation?
Because we take the F as positive. F and "a" here have the same direction. They are both opposite the direction of the motion of the train. If F is positive, so is "a".