An athlete is running a 400m race around a 400m track. On the backstretch the athlete's velocity is 8m/s but he is running into
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The book falling to the floor is described by Newton's second law and Newton's third law
Explanation:
Newton's first law of motion states that:
"An object moving at constant velocity (or at rest) keeps moving at constant velocity (or will stay at rest) unless acted upon unbalanced, external forces"
For a book falling to the floor, there is an unbalanced force acting on it (the force of gravity): therefore, we cannot apply Newton's first law.
Newton's second law of motion states that:
"The net force acting on an object is equal to the product between the object's mass, m, and its acceleration, a"
Mathematically:
For the book falling to the floor, F is the force of gravity; therefore, we can apply Newton's second law, and in this case it tells us that the book has a non-zero acceleration during its fall.
In particular, the force of gravity is (where
is the acceleration due to gravity), so the acceleration of the book is
Newton's third law of motion states that:
"When an object A exerts a force (action force) on an object B, then object B exerts an equal and opposite force (reaction force) on object A".
In this case, the Earth is exerting a force (the force of gravity) on the book during its fall: therefore, the book is also exerting a equal and opposite force (reaction force) on the Earth.
Learn more about Newton laws of motion:
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