An object has a mass of #6 kg#. The object's kinetic energy uniformly changes from #120 KJ# to # 720 KJ# over #t in [0, 4 s]#. What is the average speed of the object?

1 Answer
Jul 21, 2017

Answer:

The average speed is #=365.3ms^-1#

Explanation:

The kinetic energy is

#KE=1/2mv^2#

The mass is #=6kg#

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=120000J#

The final kinetic energy is #1/2m u_2^2=720000J#

Therefore,

#u_1^2=2/6*120000=40000m^2s^-2#

and,

#u_2^2=2/6*720000=240000m^2s^-2#

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

The points are #(0,40000)# and #(4,240000)#

The equation of the line is

#v^2-40000=(240000-40000)/4t#

#v^2=50000t+40000#

So,

#v=sqrt((50000t+40000)#

We need to calculate the average value of #v# over #t in [0,4]#

#(4-0)bar v=int_0^4(sqrt(50000t+40000))dt#

#4 barv=[((50000t+40000)^(3/2)/(3/2*50000))]_0^4#

#=((50000*4+40000)^(3/2)/(75000))-((50000*0+40000)^(3/2)/(75000))#

#=240000^(3/2)/75000-40000^(3/2)/75000#

#=1461#

So,

#barv=1461/4=365.3ms^-1#

The average speed is #=365.3ms^-1#