Power, velocity and frictional resistance

AI Thread Summary
The discussion revolves around calculating the frictional resistance of a car with a mass of 1365 kg and a 60 kW engine, achieving a maximum speed of 145 km/h. The initial calculation for frictional force yielded 149,205 N. For the second part, the maximum speed of the car on a 1 in 5 gradient was calculated, but the response highlighted that friction would not remain constant as speed changes. The conclusion emphasizes the need to reconsider the assumptions regarding friction in varying conditions.
Darth Frodo
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Homework Statement


(a) A car of mass 1365kg has 60kW engine and a max speed of 145km/hr. Find the frictional resistance.

(b) Calculate max speed of car up a gradient of 1 in 5 if Friction stays the same.

Homework Equations



P = F.v

The Attempt at a Solution



60000 = F (40.2m/s)
F = 149205 N


(b) P = netF . V
60000 = (1492.5 + mgsinθ)
θ = angle of inclination
60000 = [1492.5 + (1365)(9.8)(1/\sqrt{26})]V
v = 60000/4118.6 = 14056 m/s


Is this correct?
 
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It looks good to me!

The question isn't quite right; friction will not remain the same as the speed changes.
 
Thread 'Variable mass system : water sprayed into a moving container'

Thread 'Variable mass system : water sprayed into a moving container'

Starting with the mass considerations #m(t)# is mass of water #M_{c}# mass of container and #M(t)# mass of total system $$M(t) = M_{C} + m(t)$$ $$\Rightarrow \frac{dM(t)}{dt} = \frac{dm(t)}{dt}$$ $$P_i = Mv + u \, dm$$ $$P_f = (M + dm)(v + dv)$$ $$\Delta P = M \, dv + (v - u) \, dm$$ $$F = \frac{dP}{dt} = M \frac{dv}{dt} + (v - u) \frac{dm}{dt}$$ $$F = u \frac{dm}{dt} = \rho A u^2$$ from conservation of momentum , the cannon recoils with the same force which it applies. $$\quad \frac{dm}{dt}...
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