If forces are always equal and opposite, how can anything move?

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Here is a famous problem: A horse is pulling on a cart, and the cart pulls back with the same amount of force. If all forces are equal, how can the horse and cart move?

Answer: The horse moves because the force he exerts with his hooves is greater than the force of the wagon pulling him back.

What pushes the horse forward? It's the ground! The horse pushes backward on the ground, so the ground pushes forward with an equal force. If the horse can push back against the ground with a force greater than the cart's resisting force, then the horse will accelerate.

Acceleration will occur if one force pair (push of ground/push of horse)

is greater than another force pair (friction/pull of cart).

 

What forces act on the cart? The horse pulls it forward, and there is a backward force from the ground: friction. If the horses' pull exceeds the friction of the cart, it will accelerate.

 

 

Example 2: If the person's friction forces against the floor are greater than the refrigerator's friction forces, the fridge will accelerate.

 Acceleration will occur if one force pair (push of floor/push of legs)

is greater than another force pair (friction/refrigerator).

 

Law 1: Inertia

Law 2: F=ma

 

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