When you push a wall neither its state nor its direction change. Does it mean that you have not applied any force? Explain.
Answers
Work done (W) = force ∗ displacement.
Work done (W) = force ∗ displacement.Hence, work is said to be done when there is displacement. In this case the wall does not displace from its position even though the force is applied and since displacement is zero work done is said to be zero.
Answer:
Newton’s Third Law of Motion
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
F→ on body B, then B simultaneously exerts a force
−
→
F
−F→ on A, or in vector equation form,
→
F
AB
=
−
→
F
BA
.
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.
We can readily see Newton’s third law at work by taking a look at how people move about. Consider a swimmer pushing off the side of a pool ((Figure)). She pushes against the wall of the pool with her feet and accelerates in the direction opposite that of her push. The wall has exerted an equal and opposite force on the swimmer. You might think that two equal and opposite forces would cancel, but they do not because they act on different systems. In this case, there are two systems that we could investigate: the swimmer and the wall. If we select the swimmer to be the system of interest, as in the figure, then
F
wall on feet
Fwall on feet is an external force on this system and affects its motion. The swimmer moves in the direction of this force. In contrast, the force
F
feet on wall
Ffeet on wall acts on the wall, not on our system of interest. Thus,
F
feet on wall
Ffeet on wall does not directly affect the motion of the system and does not cancel
F
wall on feet
.
Fwall on feet. The swimmer pushes in the direction opposite that in which she wishes to move. The reaction to her push is thus in the desired direction. In a free-body diagram, such as the one shown in (Figure), we never include both forces of an action-reaction pair; in this case, we only use
F
wall on feet
Fwall on feet, not
F
feet on wall
Ffeet on wall.
Figure shows a swimmer pushing against a wall with her feet. Direction of acceleration is towards the left. Force F subscript feet on wall points right and force F subscript wall on feet points left. The swimmer is circled and this circle is labeled system of interest. This does not include the wall, nor the force F subscript feet on wall. A free body diagram shows vector w pointing downwards, vector BF pointing upwards and vector F subscript wall on feet pointing left.