Newton’s 3rd Law of Motion states that for every action, there is an equal and opposite reaction. In practice, this fundamental principle of physics means that whenever one object imparts a force onto another object, the second object imparts a force of equal magnitude and in the opposite direction onto the first object. The first force is called the action, and the second force is called the reaction or reaction force.
In this case, the bear applies a force of 900 N to the tree, and the tree does not move. By Newton’s 3rd Law, the tree must apply a reaction force of 900 N to the bear.
In order to exert a force on the tree, the bear uses his hind paws to push against the ground and generate the force on the tree by pushing with its front paws. Since the bear does not move, its velocity does not change, and so its acceleration is zero. From Newton's Second Law, F = ma, we conclude that the net force acting on the bear must therefore be zero, and so the force of the ground on the bear's hind paws must be equal and opposite to the force of the tree acting on the bear's front paws. All of the forces cancel out, so that the tree must apply a reaction force of 900 N to the bear.
It is helpful in this case to draw two free body diagrams, one for the bear, the other for the tree, to visualize the forces and to verify that the reaction force satisfies Newton’s 3rd Law.
