Originally Posted by
John C. Ratliff
Nope, it's not the weight of the bike, but the strength of the bike that will make the difference. Some of the force will be taken up in the crushing of the front wheel, yes. But the bike would become the pivot point, and is strong enough to exert well over 175 pounds of force (deceleration) on the bicyclist if he were to keep has hands on the bar. It doesn't have to weight that amount, just exert that amount of force. Are you suggesting that in this case, linear momentum is not conserved?
No, linear momentum is not conserved. For some amount of time, there is an outside force (the pothole) acting on the bike/rider system, not to mention wind resistance as the rider stops pedalling and thus ends the external force that was keeping the system at a constant speed in spite of the wind. At some point, the rider becomes detached from the bike. A 15 lbs. protour bike cannot exert 175 lbs. of downward force on the rider. It's not anchored to the road by anything. Sure, it can exert 175 lbs. of upward force but that's only enough to get the rider going above the bars. For the rider to start swinging downwards in an arc (not just straight down), there needs to be a force pulling them in towards the pivot (a centripetal force). In the case of a cyclist going over the bars, there is no centripetal force and so there is no downward velocity gained. The rider might appear to continue in an arc because of gravity pulling them downwards and the bike being slowed down and ending up behind them.
Originally Posted by
John C. Ratliff
By the way, there is an outside force acting on Newton's Cradle--the person who lifts the ball initially to begin the process. That would be analogous to the pothole, or in this case the other bicyclist, in this system.
You are confusing force with potential energy. In Newton's Cradle, the system is given energy by the hand lifting the ball equal to
mgh (mass * gravity * height). That energy is conserved until an outside force, like a hand stopping the ball, is placed on the system. Though, in reality, the system will eventually stop on it's own from the force of wind resistance and the small amount of energy lost during the collisions between the balls.