1. The Horse Before the Cart A Physics Phable By Leo Takahashi, The Pennsylvania State University, Beaver Campus

2. Giddeappe! Let’s Go ! Nope; I can’t do that. What? Why not? No point in my pulling on your cart. Says who? Isaac Newton, that’s who. No Way ! Read your physics Textbook! Newton’s 3 rd Law says: If I exert a force on the cart, the cart exerts an equal and opposite force on me! Those forces add up to zero, so I can’t move the cart. Once upon a time there was a horse. His owner hitched him to a cart. Then Things got interesting:

3. Equal-Opposite;  F = 0; a = 0; no motion 3 rd Law 2 nd Law Horse Cart Wait a minute; I did read that. There are three laws, and the fist one does say “net force of zero means a body at rest stays at rest, and the third law talks about equal and opposite forces, so that means….. OH NOOOOOO!!! HELP!!!!!!!

4. A HORSE IS NOT A CART! Look at them separately !! Physics Guru

5. Force on Cart by the Horse, F Force on Horse by the Cart, F Response to the Horse’s effort: A Forward force F G of ground on horse Backward force F g of ground on cart Equal and Opposite

6. AHA! If F G > F and F > F g The NET Force on Each is Forward!! Horse and Cart accelerate forward together!!!!! F F FGFG FgFg GIDDEAPPE!!!!! I It takes more than horse sense to grok Newton’s Laws

7. The End