Your weight is 500 N and you are standing still on the ground. Choose the correct statement/s.
I. You apply a force of 500 N on the ground in downward direction
II. Ground applies a force of 500 N on you in upward direction
Answers
Whenever one body exerts a force on a second body, the first body experiences a force that is equal in magnitude and opposite in direction to the force that it exerts. Mathematically, if a body A exerts a force
→
F
on body B, then B simultaneously exerts a force
−
→
F
on A, or in vector equation form,
→
F
AB
=
−
→
F
BA
.
Newton’s third law represents a certain symmetry in nature: Forces always occur in pairs, and one body cannot exert a force on another without experiencing a force itself. We sometimes refer to this law loosely as “action-reaction,” where the force exerted is the action and the force experienced as a consequence is the reaction. Newton’s third law has practical uses in analyzing the origin of forces and understanding which forces are external to a system.
When the swimmer exerts a force on the wall, she accelerates in the opposite direction; in other words, the net external force on her is in the direction opposite of
F
feet on wall
.
This opposition occurs because, in accordance with Newton’s third law, the wall exerts a force
F
wall on feet
on the swimmer that is equal in magnitude but in the direction opposite to the one she exerts on it. The line around the swimmer indicates the system of interest. Thus, the free-body diagram shows only
F
wall on feet
,
w (the gravitational force), and BF, which is the buoyant force of the water supporting the swimmer’s weight. The vertical forces w and BF cancel because there is no vertical acceleration.
