Question

A 2.5m pendulum is pulled back at an angle of 15.0 degrees and released. it's speed at the bottom of the swing is,


A) 1.29 m/s
B) 1.67 m/s
C) 6.88 m/s
D) 7.00 m/s

Answer 1

Conservation of mechanical energy:

Down is all kinetic, when released is all potential:

1/2*m*v^2 = mgh, the initial height h = L(1-cos(alpha)) (nice to think why)

so, v^2 = 2gL(1-cos(alpha))

v = sqrt( 2gL(1-cos(alpha)) = sqrt( 2*10*2.5*(1-cos(15))

Beware of having the calculator in degrees

v = sqrt( 50* 0.0341 ) = 1.305 m/s

It is A) and they may have used 9.81 m/s^2 instead of 10 m/s^2