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What exactly is equilibrium? I'm riding my scooter and I get this doubt. The maximum power the engine of my scooter can deliver is constant. As I pull full throttle, the scooter accelerates, the accleration decreases with time because of the resistance forces(mainly air resistance) that increase with velocity and at a particular speed (say 87 kmph) which is the top speed, the resistance forces exactly balance the thrust produced by the engine and the scooter is in equilibrium. The net acceleration at this speed is zero. This is the top speed of my scooter, right. So if I pick up two of my friends and they ride with me, now when I pull full throttle, I don't get the acceleration I got before because F=ma As m increases, a decreases at constant F. But what about my top speed? It too should decrease, right? But since the maximum force delivered by my engine is the same as before and the resistance forces reach this value only at 87kmph (the only variable affecting air resistance here is velocity, right), shouldn't I get the same top speed? If you think this regarding the acceleration produced in both cases, it makes sense to have lower top speed. Since the initial acceleration is less, the deceleration produced by air resistance as it increases with velocity matches this acceleration at a lower speed. Hence, the scooter attains equilibrium at a lower top speed. But when you think this regarding balance of forces rather than accelerations, it doesn't make sense since the maximum thrust offered by the engine never changes So I'm wondering, is equilibrium a balance of forces or a balance of the effect of forces i.e., accelerations produced by forces? Please be kind if this question is stupid.

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It is a balance of forces. The wind resistance does not impose a certain acceleration on the scooter, but a certain force. When this force balances the force produced by the engine, the speed will not increase further. With a heavier load, it will take longer to reach this state, but, ignoring any increases in wind or rolling resistance due to the bigger load, the equilibrium velocity will be the same.

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