# Question about Newton's Third Law of Motion

jle1092
A brick hits a glass window. The brick breaks the glass, so:
is the magnitude of the force of the brick, greater, less than, or equal to the magnitude of the force of the window?

Newton's third law of motion states that for every action, there is an equal an opposite reaction.

With this law I think I should say the magnitude is equal, but then again my gut tells me that the law is talking about the net force and this specific question is dealing with this once force itself, so I want to say the magnitude of the force of the brick is greater. What do you think?

Gold Member
magnitudes are equal, direction are opposite, effects are different, effect depends on nature and properties of the material. here, brittleness of glass.

Oscur
Well what I think you should do is think about Newton's second law, which is frequently formulated as:

$$f_{net}=ma$$

That is to say, the net force exerted on an object is proportional to its acceleration. Think about the net force acting on the brick, and then break that force down into its components; the normal reaction of the window on the brick and gravity. (It's equivalent, and more straightforward, if you first consider, for example, a skylight instead of a normal window, the brick being dropped vertically downwards onto it.)

If you still need more hints, let me know.

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