The Physics of a Horse Pulling a Carriage
Date: 07/13/2005 at 01:33:12 From: Lina Subject: Physics - Newton forces Hi Dr. Math I have a question about the forces that enable an object to be moved. The question is: "How can a horse pull a carriage?" I know that Newton's Law states that an object will not move or stop in its track unless there is an unbalance force acting on it. And that F(a on b) = F(b on a). So if it is like that then where does the horse get its starting force from? And when it does start to pull the carriage, how can it stop if the force that it exerts on the carriage is the same amount that the carriage exerts on it? Also, I know that there is some friction acting too, but I'm confused about where the friction comes from and how it fits into the whole thing. Also, does the force only act between the carriage wheel and the ground, and the horse's hooves and the ground or what? Thanks for your time.
Date: 07/13/2005 at 14:23:25 From: Doctor Edwin Subject: Re: Physics - Newton forces Hi, Lina. Great question! It's always exciting to really think about something that you usually just take for granted. In school, we usually aren't taught about action and reaction very well. I remember the example I was taught was, "If you're standing in a boat and you throw a rock, the boat moves backwards." But that's not a good way to describe it. The rock-and-boat example, like the horse-and-cart example, is actually TWO action-and-reaction pairs. When I'm on the boat, I push against the rock. That's the action f(me on rock). The reaction is that the rock pushes on me with the same force f(rock on me). The result of my pushing on the rock is that the force on it is unbalanced and it accelerates away from me. The result of its pushing on me is that I accelerate away from it. In doing so, I push against the boat. It pushes back. The results are that the boat moves along with me, and I am accelerating more slowly than I would if I weren't pushing on the boat. Now, let's look at the horse example. The first action is that the horse pushes backward against the ground with force f(horse on ground). So what's the reaction to that? What's the result of the action? What's the result of the reaction? (Hint: the result of the reaction is a lot easier to observe than the result of the action). Can you carry it through from here? Now, what's the importance of friction? Would it be possible for the horse to push sideways on the ground if there were no friction? The other situation to consider is when all the forces are balanced. The cart and horse are in motion, and not accelerating. In that case, there are two forces that must be in balance. The first is the force that the horse exerts by pushing backward against the ground. Can you tell what the second is? Write back and let me know how much of this makes sense. - Doctor Edwin, The Math Forum http://mathforum.org/dr.math/
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