1. The model car shown below has a mass of 1.733 kg and is to be accelerated fro

Question

1.      The model car shown below has a mass of 1.733 kg and is to be accelerated from rest by a 1-cm-diameter water jet moving at 75 m/s. Neglecting air drag and wheel friction, estimate the acceleration of the car at time, t = 0 sec. At a certain point in time the model car will stop accelerating and will maintain a constant velocity. Find the force required to keep the model car moving at a constant velocity of 60 m/s. Assume that the jet can follow the car without dipping.

Ans. arel= 508.8 m/s2

is this given answer wrong?

I think I got it right, but I cant find the Force to keep it at 60 m/s. Wouldn't it just be constant at 75 m/s? What is stopping it at 60 m/s?

Explanation / Answer

Thrust force = (dm/dt)*Vrel = pAVrel^2 where p is density of water & A is Area.

So, thrust force(F) = 1000*pi*(0.01/2)^2*75^2 = 441.786 N

So, accelaration = F/m = 254.925832 m/s^2.

When car maintains const. speed

Vrel = 75-60 =15m/s.

Force = 1000*pi*(0.01/2)^2*15^2 = 17.671 N.

.

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