Question

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Answer 1

So work = force × displacement

or according to work energy theorem,

work= change in kinetic energy

=1/2mv^2 - 1/2mu^2

now m= 2kg. u=18km/h = 5m/'s

v=72km/h=20m/'s

so, work =1/2m (v^2-u^2)

1/2×2×(400-25)

1×375

therefore, answer is 375 joules

Answer 2

work done=final kinetic energy-inital kinetic energy

work done =(1/2)m(v)^2-(1/2)m(u)^2

=(1/2)m{(v)^2-(u)^2}

=(1/2)(2){20×20-5×5}

=400-25=375J

v=72kmph=72×(5/18)mph=20mps

u=18kmph=18×(5/18)mph=5mps

where mps =metre per second