Question #28771

1 Answer
Oct 27, 2017

Answer:

They never cancel each other out.

Explanation:

Allan exerts an action force on a small wagon. In response the wagon exerts an equal and opposite reaction force on Allan. The action and reaction pair of forces do not cancel each other out. Why? Because they were not exerted on the same object. The wagon starts moving because the horizontal forces on it are:

  • Allan's mighty push
  • a puny bit of friction in the wheels

Allan's push overwhelms the friction and the wagon starts moving.

Later that day, Allan exerts an action force on a well-built brick wall. In response the wall exerts a reaction force on Allan. Neither Allan or the wall move noticeably. The action and reaction pair of forces are equal and opposite, but do they do not cancel each other out. Why? Because they were not exerted on the same object.

Consider Allan:
He pushed on the wall but also dug his feet into the soil and pushed against it to anchor himself well for the push against the wall. The forces on him were

  • The reaction force exerted by the wall
  • The reaction force exerted by the soil

These 2 forces were equal and opposite, so Allan was in equilibrium and did not move.

Consider the wall:
The builders of the wall had started with a concrete slab below the surface of the soil. The bricks were firmly connected to that slab. (Probably a bunch of other engineering tricks to make things strong went into the situation.) When Allan pushed on the wall, it confidently applied a force to the concrete slab to remain upright. The slab held the wall solidly. The forces on the wall were

  • Allan's mighty push
  • The slab's reinforcing strength

So the wall was in equilibrium and did not move.

The message is: when this starts to confuse you, focus on one of the objects and sum up the forces acting on that object.

I hope this helps,
Steve