Carla demonstrates Newton’s First Law of Motion by slowly pushing a book across a table. The book moves at a constant rate of speed so long as she is pushing it.

Question

Which of the following is NOT true.
Her push balances the force of friction
The upward force exerted by the table balances the book’s weight
There is an unbalanced force on the book, acting in the direction of motion
The forces acting on the book are all balanced.

Answer 1

To determine which statement is NOT true in the context of Carla pushing the book across the table, let's analyze each option in relation to Newton's First Law of Motion.

1. Her push balances the force of friction: This statement is true. If the book moves at a constant speed, the force Carla exerts while pushing must be equal and opposite to the friction force opposing its motion.

2. The upward force exerted by the table balances the book’s weight: This statement is also true. The table exerts an upward normal force that balances the gravitational force (weight) acting downward on the book when it is on the table.

3. There is an unbalanced force on the book, acting in the direction of motion: This statement is NOT true. If the book is moving at a constant speed, there is no unbalanced force acting on it; the forces are balanced (the push and friction forces are equal and opposite). According to Newton's First Law, an object in motion tends to stay in motion at a constant speed unless acted on by an unbalanced force, which is not the case here.

4. The forces acting on the book are all balanced: This statement is true. Since the book is moving at a constant speed, the net force acting on it is zero, indicating that all forces (the push and friction) are balanced.

Thus, the statement that is NOT true is:

There is an unbalanced force on the book, acting in the direction of motion.