Newton's Laws of Motion Flashcards

A passenger in a car feels pressed back into their seat as the car accelerates forward. This sensation is best explained by which of Newton's laws?

Answer: Newton's First Law (Inertia)

Newton's First Law, the law of inertia, states that an object at rest will stay at rest unless acted upon by an external force. [20] The passenger's body has inertia and tends to remain in its state of rest. As the car accelerates forward, the car seat applies a forward force to the passenger, causing the sensation of being pushed back into the seat. [17]

According to Newton's Second Law of Motion, if a constant net force is applied to two objects, one with a mass of 10 kg and the other with a mass of 20 kg, which statement is true?

Answer: The 10 kg object will accelerate twice as fast as the 20 kg object.

Newton's Second Law (F=ma) shows that acceleration is inversely proportional to mass (a = F/m). [7, 23] This means for a constant force, the object with the smaller mass will experience greater acceleration. Since the 10 kg object has half the mass of the 20 kg object, it will accelerate twice as much. [9]

A rocket propels itself forward in the vacuum of space by expelling hot gases from its engine. This is a classic example of which principle?

Answer: For every action, there is an equal and opposite reaction.

This is a direct application of Newton's Third Law. The rocket pushing the hot gases backward is the 'action'. The 'reaction' is the hot gases pushing the rocket forward with an equal and opposite force, which propels it through space. [6, 16]

Which of the following objects possesses the greatest inertia?

Answer: A large, stationary cargo truck

Inertia is an object's resistance to a change in its state of motion and is directly proportional to its mass. [1, 3, 11] An object's speed or state of rest does not determine its inertia, only its mass does. Of the options provided, the large cargo truck has the most mass and therefore the greatest inertia. [19]

If the net force acting on an object with constant mass is doubled, what is the effect on the object's acceleration?

Answer: It is doubled.

Newton's Second Law of Motion is expressed as F = ma. [12, 21] This formula shows that acceleration (a) is directly proportional to the net force (F) when mass (m) is constant. Therefore, if the net force is doubled, the acceleration will also double. [23]

Which of the following scenarios is the best example of a Newton's Third Law action-reaction pair?

Answer: A swimmer pushing water backward with their hands, and the water pushing the swimmer forward.

A Newton's Third Law pair involves two objects exerting equal and opposite forces on each other. The swimmer pushes the water (action), and the water pushes the swimmer (reaction). [14, 16, 18] In the other examples, the forces either act on the same object (gravity and normal force on the book) or involve three different objects (Earth, Moon, Sun).