Question

Which of the following is the best example of Newton's first law (the law of inertia), which states that an object at rest stays at rest and an object in motion stays in motion at the same speed in the same direction unless acted upon by an unbalanced force? (A) a train slowing to a stop as it enters the station (B) the recoil of a cannon when a cannonball is fired into the air (C) a skydiver who has already reached terminal velocity and is falling at a constant speed toward the ground (D) a rocket blasting off on its way to the moon

Short answer

The best example of Newton's first law of motion (the law of inertia) is: (C) a skydiver who has already reached terminal velocity and is falling at a constant speed toward the ground.

Step-by-step solution

Option A: A train slowing to a stop as it enters the station

A train slowing to a stop is not a good example of the law of inertia because it is experiencing a net force. The train is slowing down due to external forces such as friction between the rails and the train's wheels and the force applied by the train's brakes. These forces cause the train's motion to change, so it is not consistent with Newton's first law.

Option B: The recoil of a cannon when a cannonball is fired into the air

Though the recoil of a cannon demonstrates Newton's third law (action-reaction), it does not illustrate the law of inertia. In this case, the cannon's motion changes as a result of the explosion that propels the cannonball. The external force of the explosion causes the cannon to recoil, or move backward, which is inconsistent with Newton's first law.

Option C: A skydiver who has already reached terminal velocity and is falling at a constant speed toward the ground

A skydiver at terminal velocity is a good example of the law of inertia. Terminal velocity occurs when the force of air resistance (drag) is equal and opposite to the force of gravity, resulting in a net external force of zero. In this situation, the skydiver's velocity remains constant (hence, they fall at a constant speed) because there is no unbalanced force acting on them. This example is consistent with Newton's first law of motion.

Option D: A rocket blasting off on its way to the moon

A rocket blasting off is not an ideal example of Newton's first law, as it experiences a significant net external force. The rocket's engines produce a large force that propels the rocket off the ground and accelerates it upward. The presence of a net force causing the rocket's motion to change is not consistent with the law of inertia. From the analysis of all options, we conclude that the best example of Newton's first law of motion (the law of inertia) is: (C) a skydiver who has already reached terminal velocity and is falling at a constant speed toward the ground.

Key concepts

Law of Inertia

Newton's First Law, also known as the Law of Inertia, is a fundamental principle in physics. It describes the behavior of objects in terms of motion and rest. According to this law, an object at rest will stay at rest, and an object in motion will continue moving at the same speed and in the same direction unless acted upon by a net external force. This means if there are no changes in external influences, an object's state of motion remains constant. Hence, it inherently resists any change to its motion. Consider a book lying on a table. It will remain there until you apply a force to move it. Similarly, a ball rolling down a straight path will keep moving unless friction or another force changes its trajectory or speed. This predictability is due to inertia, the tendency of objects to resist changes to their state of motion.

Net External Force

A net external force is the sum of all the forces acting on an object from outside sources. When multiple forces act on an object, they can combine or cancel each other out.

• If the resultant (net) force is zero, the object will either remain at rest or continue moving at a constant velocity, much like the states described by Newton's First Law.
• If there is a non-zero net external force, it means that the object's velocity will change.

For instance, if you push a shopping cart in a straight line and encounter friction from the ground, the cart will slow down unless you continue pushing with sufficient force to overcome that friction. The greater the net force, the more significant the change in motion. Therefore, balancing forces effectively explain the steady motion of objects.

Terminal Velocity

Terminal velocity is the constant speed reached by an object freely falling through a fluid or gas, such as air. It occurs when the gravity pulling the object downward is balanced by the drag force (air resistance) opposing the motion. At terminal velocity, the forces are balanced, resulting in a net external force of zero. This state means that there is no further acceleration, according to Newton's First Law. For a skydiver, as they fall, their speed increases until air resistance matches the pull of gravity, resulting in terminal velocity. This steady speed is where the skydiver's velocity becomes constant, aligning perfectly with the law of inertia as no additional unbalanced force is acting on them.

Constant Speed

Constant speed refers to the uniform motion of an object where it covers equal distances in equal intervals of time, without acceleration or deceleration. When an object moves at constant speed, it implies that there is zero net external force acting on it, according to Newton's First Law of inertia.

• A vehicle set on cruise control exemplifies constant speed, as it maintains a steady pace without speeding up or slowing down, assuming no external forces like wind or inclines interfere.
• Objects moving at constant speed indicate balanced forces, where all acting influences cancel each other out, resulting in zero acceleration.

In simpler terms, if you are riding a bike downhill without pedaling or braking, if the downhill slope and air resistance create balance, the bike may continue at a constant speed thanks to the forces being in equilibrium.