PHYSICS  formula?
posted by Anna on .
Here is one of my questions from my physics study. It tells me the answer (35 J), but how do I find that answer? I would appreciate a formula or step by step calculation. Thanks! :)
A 30 kg kid on rollerskates is moving 1 m/s. His brother pushes him with a force of 5 N. What will be his kinetic energy after his brother pushes him 4.0 m? (35 J)

What acceleration does the pushing impart?
F=ma
5=30a
a = 1/6
How long does it take to go 4m, starting at 1m/s?
s = Vo t + 1/2 at^2
4 = t + 1/2 * 1/6 t^2
t^2 + 12t  48 = 0
t = 4
What's the speed after 4 seconds?
v = Vo+at
= 1 + 1/6 * 4
= 5/3
What's the KE then?
KE = 1/2 mv^2
= 1/2 (30)(5/3)^2
= 15*25/9
= 125/3
Hmmm. better check my math.