Question

Action and reaction forces never balance out because
A. they are equal but not opposite always
B. they are unequal in magnitude even though opposite in direction
C. they act on different bodies
D. they are unequal in magnitude

Answer 1

Hint: Action and reaction forces arise from Newton’s Third Law which states that every action has an equal and opposite reaction. These are contact forces i.e., only arise when the bodies are in contact with each other. Closer examination of Newton’s third law can give us an idea about why they never balance out.

Formula used:
${F_{12}} = - {F_{21}}$

Complete answer:
Newton’s 3rd law states that- “For every action, there is an equal and opposite reaction”. There’s a little ambiguity here in Newton’s third law. So, let us put it in another way which is more elaborate and has the same meaning.

“Forces always occur in pairs. If object 1 exerts a force F on object 2, then object 2 exerts an equal and opposite force –F on object 1, simultaneously in a straight line.” i.e.,
${F_{12}} = - {F_{21}}$
Where:
${F_{12}}$ is the force applied by object 1 on object 2
${F_{21}}$ is the force applied by the object 2 on object 1
‘−‘ indicates the opposition or the opposite direction of ${F_{21}}$.
Clearly, object 1 is exerting force on object 2 and vice-versa. They’re acting on two completely different objects. So, they’ll never cancel each other out.

Therefore the correct option is C.

Note:
If you consider rocket propulsion, the exhaust gases are released to create an action force. And as a reaction, thrust force acts on the rocket. If the forces were equal the rocket would not move forward. The same applies to the rowing of the boat. When someone paddles a boat in the backward direction, they apply force on the water, in return water pushes the boat.