Page 194 - The City and Guilds Textbook: Plumbing Book 1 for the Level 3 Apprenticeship (9189), Level 2 Technical Certificate (8202) and Level 2 Diploma (6035)

The City & Guilds Textbook: Plumbing Book 1

Action and reaction: Newton’s third law

of motion

A push or a pull (action) on an object can often result in movement (reaction)
when the pull or push is greater than the weight of the object. If both action
and reaction are equal, then no movement takes place because the object is
pushing or pulling against the action with equal force. This is known as
contact force and is a result of contact interactions (normal, frictional,
tensional, and applied forces are all examples of contact forces). Other forces
are a result of ‘actions-at-a-glance’ interactions (gravitational pull, electrical
and magnetic). These two types of force have one thing in common: for every
force applied there is an equal opposing force and as such is subject to action
and reaction.
There are many ways in which this can be seen. For example, when a person sits
on a chair (action), the downward force of the person provokes an upward force in
the chair (reaction). The person and the chair have equal force and so equilibrium
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exists. If the person were too heavy for the chair, then the chair
would collapse (reaction).
This is Newton’s third law of motion, which states:
Every action has an equal but opposite reaction.
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This means that, for every force that an object is subjected to:
1 there is an opposing force from the object
2 both action and reaction forces are equal
p Figure 3.33 Action and reaction 3 forces always come in pairs (points 1 and 2).

Equilibrium 50N

When all the forces acting on a stationary object are balanced, the object is
said to be in a state of equilibrium. The forces are balanced when all forces (left,
right, front, back, up and down) are the same. In Figure 3.34 (left), all forces are
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p Figure 3.34 Balanced forces in equilibrium (left) and unbalanced forces in equilibrium (right)

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