Rubber band, elastic, spring
D.) In order to calculate both of them, we must know the "FORCE" on the system.
Well first of all, I think the students may have been correct.
If they didn't use distilled water, and if it wasn't exactly at
standard temperature, then the mass of 25.0 mL could
very well be 25.4 grams. We don't know that there was
any 'error' in their measurement at all.
But the question says there was, so we'll do the math:
The 'error' was (25.4 - 25.0) = +0.4 gram
As a fraction of the 'real' value, the error was
+0.4 / 25.0 = +0.016 .
To change a decimal to a percent, move the
decimal point two places that way ===> .
+ 0.016 = +1.6 % .
Their measurement was 1.6% too high.
Let's not call it an 'error'. Let's just call it a 'discrepancy'
between the measured value and the 'accepted' value. OK ?
We have the equation of motion 
, where s is the displacement, a is the acceleration, u is the initial velocity and t is the time taken.
Here s = 300 m, u = 0 m/s, a = 9.81 
Substituting
Now we have v = u+at, where v is the final velocity
Here u = 0 m/s, a= 9.81 and t = 7.82 seconds
Substituting
v = 0+9.8*7.82 = 76.68 m/s
The speed with which the penny strikes the ground = 76.68 m/s.
