Motion and Stability: Forces Flashcards

A student pushes a heavy box across a rough floor. The box moves at a constant velocity. Which of the following statements BEST describes the forces acting on the box?

Answer: The force of friction is equal to the force of the push.

According to Newton's First Law, an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force. Since the box is moving at a constant velocity, its motion is not changing, which means the forces are balanced. Therefore, the force of the student's push must be equal in magnitude and opposite in direction to the force of friction.

A rocket accelerates upward from a launchpad. According to Newton's Third Law of Motion, what is the 'reaction' force to the hot gases pushing downward from the rocket's engine?

Answer: The hot gases pushing up on the rocket.

Newton's Third Law states that for every action, there is an equal and opposite reaction. The 'action' force is the rocket's engine pushing the hot gases downward. The 'reaction' force is the hot gases pushing the rocket upward with an equal and opposite force, which causes the rocket to accelerate.

Two identical carts, Cart A and Cart B, are at rest on a frictionless surface. A student applies a force of 10 Newtons to Cart A and a force of 20 Newtons to Cart B for the same amount of time. Which of the following will be true?

Answer: Cart B will have a greater change in motion than Cart A.

The change in an object's motion (acceleration) depends on the sum of the forces acting on it and the object's mass. Since the carts have identical masses, the cart subjected to the larger force (Cart B) will experience a greater change in motion, or a larger acceleration.

A book is sitting motionless on a horizontal table. Which of the following correctly identifies an action-reaction pair of forces?

Answer: The force of the book pushing down on the table and the force of the table pushing up on the book.

An action-reaction pair of forces, as described by Newton's Third Law, acts on two different interacting objects. The force of the book pushing down on the table (action) and the force of the table pushing up on the book (reaction) is a correct pair. The force of gravity on the book and the normal force from the table are balanced forces acting on a single object (the book), not an action-reaction pair.

Which scenario provides evidence that an unbalanced force is acting on an object?

Answer: A satellite orbiting Earth at a constant speed.

An unbalanced force causes a change in an object's motion, which includes a change in speed or a change in direction. A satellite orbiting Earth is constantly changing its direction of motion, even if its speed is constant. This change in direction (and therefore velocity) is caused by the unbalanced force of gravity. The other scenarios describe situations with zero net force (balanced forces).

A student is designing an experiment to investigate how the mass of an object affects its change in motion when a constant force is applied. What are the independent and dependent variables in this investigation?

Answer: Independent: mass of the object; Dependent: change in motion (acceleration).

In a scientific investigation, the independent variable is the one the investigator intentionally changes or manipulates. The dependent variable is the one that is observed and measured to see how it is affected by the change in the independent variable. In this case, the student is changing the mass (independent variable) to see how it affects the change in motion (dependent variable) while keeping the force constant.