# Is mgsinθ=static frictional force on an inclined plane?

According to Newton's 3rd law, for every force there's an equal opposing force. Since a block is sitting on an inclined plane, wouldn't that mean the static friction is acting like the opposing force for mgsinθ thus, mgsinθ=static frictional force? Please respond

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Orodruin
Staff Emeritus
Homework Helper
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No, you have a very common misconception about Newton's third law. Newton's third law is a statement about forces between two different objects, not about forces on the same object. For example, it states that if the frictional force on the block from the plane is ##F_s## directed upwards along the plane, then the frictional force from the block on the plane is ##-F_s## directed upwards along the plane (or, equivalently, ##F_s## downwards along the plane). The third law does not state anything about the different forces acting on the same object.

From the second law, we can infer that the force sum on an object which is not being accelerated is zero, since ##\sum_i \vec F_i = m\vec a##.

• dkhan2
sophiecentaur
Gold Member
2020 Award
N3 describes situations where Conservation of Momentum is important. N1 is about Equilibrium situations where the sum of all forces is zero.

• dkhan2
Chandra Prayaga
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