An object has a mass of #4 kg#. The object's kinetic energy uniformly changes from #18 KJ# to # 42KJ# over #t in [0, 9 s]#. What is the average speed of the object?

1 Answer
Aug 13, 2017

Answer:

The average speed is #=121.6ms^-1#

Explanation:

The kinetic energy is

#KE=1/2mv^2#

The mass is #=4kg#

The initial velocity is #=u_1ms^-1#

The final velocity is #=u_2 ms^-1#

The initial kinetic energy is #1/2m u_1^2=18000J#

The final kinetic energy is #1/2m u_2^2=42000J#

Therefore,

#u_1^2=2/4*18000=9000m^2s^-2#

and,

#u_2^2=2/4*42000=21000m^2s^-2#

The graph of #v^2=f(t)# is a straight line

The points are #(0,9000)# and #(9,21000)#

The equation of the line is

#v^2-9000=(21000-9000)/9t#

#v^2=1333.3t+9000#

So,

#v=sqrt((1333.3t+9000)#

We need to calculate the average value of #v# over #t in [0,9]#

#(9-0)bar v=int_0^9(sqrt(1333.3t+9000))dt#

#9 barv=[((1333.3t+9000)^(3/2)/(3/2*1333.3))]_0^9#

#=((1333.3*9+9000)^(3/2)/(2000))-((1333.3*0+9000)^(3/2)/(2000))#

#=21000^(3/2)/2000-9000^(3/2)/2000#

#=1094.7#

So,

#barv=1094.7/9=121.6ms^-1#

The average speed is #=121.6ms^-1#