|MadSci Network: Physics|
You've just asked a very common question concerning action-reaction. A Famous Physics Puzzle deals with this question. A very smart but lazy horse is hitched to a cart. He refuses to pull the cart telling the farmer, (apparently the horse can talk as well) "No matter how hard I pull on the cart, the cart will pull back with equal and opposite force and it will therefore be impossible for me to pull the cart." Answer: First of all when the horse pulls on the cart, the cart exerts an equal but opposite reaction on the horse, the action and reaction. If this was the only force in action the horse and cart would indeed remain stationary, however there is another force between the horse and the ground. The horse's hooves press down on the ground and the ground pushes back on the horse. If the reaction force of the ground is greater than the reaction force of the cart on the horse, then the horse will move forward. The cart will move forward when the force exerted on it by the horse is greater than the frictional force between the cart and the ground. Remember, there are two sets of forces acting on the cart as well: the action of the horse on the cart and reaction of cart on horse, and the action of the cart on the ground and the reaction of frictional force of the ground on the cart. The same thing occurs if you are pushing a box. While the box is pushing against you it is also pushing against the surface it is on and the surface is pushing back. Therefore even though pairs of action-reactions cancel out, the net or overall force resulting from different interactions allow movement. Hope this answers your question Katie Pilypas Have a look at these web sites for futher info PhysLink http://www.ph yslink.com/Education/AskExperts/ae207.cfm Newton's Laws and Forces and Gravitation http://w ww.cyberclassrooms.net/~pschweiger/s_laws_notes.htm Brain Busters http://h ydro4.sci.fau.edu/~rjordan/busters_1/push-ups_1.htm
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