A horse is harnessed to a cart. Beth has read the third law and sees a problem.
The cart pulls back on the horse exactly as hard as the horse pulls the cart forward. The two forces are equal and opposite. How can the pair ever start moving?
The paradox dissolves the moment each force is placed on the object it acts on.
Third-law partner forces never cancel, because the two forces of an interaction pair act on different objects, so they never combine on one object and cannot cancel.
Canceling happens inside one net force, on one object. And you know the net-force rule: only the forces acting on the object itself combine into the net force that sets its acceleration.
List the forces on the cart: the horse's forward pull on the cart, friction from the ground on its wheels, the cart's weight, and the ground's normal force.
The cart's backward pull on the horse is not on the list, because it acts on the horse.
The cart still accelerates when the horse pulls it: the cart's backward pull acts on the horse.
The horse accelerates too, when the ground's forward force on its hooves beats the cart's backward pull on it.
Why can the two forces of a third-law pair never cancel each other?
A sled-dog team exerts a forward force on a sled, and the sled exerts an equal-magnitude backward force on the team. Rowan says: 'The sled's backward force does not reduce the net force on the sled, because it acts on the team, not on the sled.' Is he correct?
A student argues: 'A wagon can never start moving, because it exerts a force on the child towing it exactly as large as the force the child exerts on it.' What is wrong with the argument?
A rocket's engine exerts a huge downward force on its exhaust gas, and the gas exerts an equal-magnitude upward force on the rocket. Why does the rocket accelerate although the two forces are equal?
Place each force on the object it acts on.
The downward force acts on the gas. The upward force acts on the rocket.
Only the forces acting on the rocket combine into the rocket's net force.
The gas's upward force is on that list; the rocket's downward force on the gas is not.
The two equal forces act on different objects, so they never combine. The upward force on the rocket can outweigh the other forces on the rocket, and then the rocket accelerates.
In the horse-and-cart scene, which of the two partner forces belongs in the net force on the horse?
The net force on the horse collects only the forces acting on the horse.
The cart's backward pull acts on the horse: it belongs.
The horse's forward pull acts on the cart: it does not belong.
Only the cart's backward pull on the horse belongs in the horse's net force.
A drone's rotors exert a downward force on the air, and the air exerts an upward force of equal magnitude on the drone. Why can these two forces not cancel each other?
A truck tows a trailer. Which force belongs in the net force on the trailer?
Which statement about an accelerating speedboat is correct?