Explain the following statements related to Newton's third law - a) A horse is tethered to a cart to pull it forward. A student claims "According to Newtons third law, when the horse pulls forward...

Explain the following statements related to Newton's third law -

a) A horse is tethered to a cart to pull it forward. A student claims "According to Newtons third law, when the horse pulls forward on the cart, the cart will also pull backwards on the horse. The two objects are attached together, so they cannot accelerate. Discuss the validity of the statement.

b) A student stands on a skateboard and pushes on a wall. Explain why the wall does not move.

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tjbrewer | Elementary School Teacher | (Level 2) Associate Educator

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Newton's 3rd law states: "For every action there is an equal and opposite reaction."  One of the most famous illustrations of this is the "kick" of a rifle.  The action of the projectile being forced down the barrel and out has an equal and opposite reaction of the rifle being driven backwards.  Ultimately the greater mass of the rifle and its wielder means they need to accelerate less than the projectile for the forces to them to be equal so the projectile flies until it strikes an object, or is slowed by air resistance until gravity pulls it into the earth. 

In statement a) It is principally true that the Cart will "pull" on the horse.  This "pull" is known as "resistance."  If the horse can stand the "resistance" then Newton's 1st and 2nd laws come into play stating that the force exerted by the horse is an outside force causing the cart to no longer remain at rest, and that its force must be equal to the mass of the cart times the acceleration generated by the horse. 

In statement b) The situation could be looked at like this:  The action was the student pushing on the wall, the reaction was the student and skateboard accelerating in the opposite direction from the wall. 

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