MadSci Network: Physics

Re: If every action has an equal but opposite reaction, how can anything move?

Date: Sat Mar 2 05:54:40 2002
Posted By: Katie Pilypas, Undergraduate, Physics, Maths, Flinders University of South Australia
Area of science: Physics
ID: 1012878756.Ph

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 

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 

Hope this answers your question
Katie Pilypas

Have a look at these web sites for futher info


Newton's Laws and Forces and Gravitation

Brain Busters

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