Competency #1
Answer and Explanation
First, we analyze the forces acting in the vertical and horizontal directions: :
- The book has mass, so there is a force of gravity between the book and the earth (unless indicated otherwise, it is usually safe to assume a given scenario is taking place on earth). The book is touching the table, so the table must be providing a support force upward. Since the book is not accelerating upward or downward, the support force must be equal in magnitude to the force of gravity.
- The hand is exerting an applied force to the left. Since the book is moving at a constant speed, we know that there is no horizontal acceleration. However, for the acceleration to be zero, the net force must be zero (by Newton's 2nd Law). Therefore, there must be a force acting on the book to the right. This force is friction.
With the above analysis of the forces acting, we can now draw the force diagram:
We already know the horizontal net force and horizontal acceleration are both zero. Therefore, we now only need to determine the vertical net force and vertical acceleration.
- Vertical Net Force - There are 2 forces acting and they are equal in size and opposite in direction (Note: these do not constitute a 3rd law pair, since they are both acting on the same object). However, by being equal and opposite, these 2 forces do lead to a net vertical force of zero in this case.
- Vertical Acceleration - The vertical net force is zero. Below is Newton's 2nd Law:
This tells us the vertical acceleration is zero for this case.
Although the book is moving to the left, its acceleration is zero, because it is moving at a constant velocity. Remember, an object can move with a constant velocity only when the net force on the object is zero.
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