The model airplane in figure below is flying at a speed of 21 m/s on a horizontal circle of radius 18 m. The mass of the plane is 0.82 kg. The person holding the guideline pulls it in until the radius becomes 11 m. The plane speeds up, and the tension in the guideline becomes four times greater. What is the net work done on the plane?

Question

The model airplane in figure below is flying at a speed of 21 m/s on a horizontal circle of radius 18 m. The mass of the plane is 0.82 kg. The person holding the guideline pulls it in until the radius becomes 11 m. The plane speeds up, and the tension in the guideline becomes four times greater. What is the net work done on the plane? 
The answer will be in Joules.

Elena · Answer

m•v1²/R1=T1
m•v2²/R2=T2=4•T1
v2²=4•v1²•R2/R1
W=KE2-KE1 = m•v1²/2 - m•v2²/2 =
= m•v1²/2- m• 4•v1²•R2/2•R1 =
= m•v1² { 1- (2•R2/R1)}/2=
= 0.82•21²{1-(2•18/11)}/2 = - 411 J.

Elizabeth · Answer

hmm this isnt the write answer, i wonder what went wrong?