Homework Helpers: Physics

2 Forces and the Laws of Motion


Lesson 2–2: Newton’s First Law of Motion

Imagine going out to a park, and rolling a basketball down a slide. How far would the ball roll after hitting the ground? Doesn’t that depend upon the surface of the ground that the ball hits? Would it go very far if it landed in sand? What if the ground at the bottom of the slide were covered in grass? What if it were cement? How about if it were ice? It was this type of “thought experiment” that lead Galileo Galilei to come up with the concept of inertia, paving the way for Sir Isaac Newton’s first law of motion.

Before Galileo, the “physics” taught in schools was based on Aristotle’s teachings on motion. Aristotle believed that when an object on Earth had no net force acting on it, it would eventually come to a stop, because “rest” was the natural state for the object. Galileo disagreed. His experiments, both mental and actual, led him to believe that it was just as natural for an object in motion to stay in motion as it was for an object at rest to stay at rest, unless acted upon by an unbalanced or net force.

If you take the example of the ball rolling down the slide, you can imagine that the ball will travel further on a smoother surface. So, if the ball rolled down the slide onto a smooth sheet of ice it would travel farther than if it landed on grass or gravel. Galileo concluded that if an object was moving on a perfectly smooth (frictionless) surface, it would remain in motion unless something (an unbalanced force) caused it to stop. Galileo recognized friction as an unbalanced force that caused moving objects to come to a stop after you stopped applying a force to them.

If friction is what causes objects to stop, what causes them to keep moving, even after we stop applying forces to them? That would be inertia. Inertia is an object’s resistance to any changes in its motion. When a hockey player slaps a puck, the force exerted by his stick sets the puck in motion, but the puck’s own inertia allows it to remain in uniform motion until it is acted upon by another force—say, one exerted by another stick.

There are many real-life examples where you have experienced inertia. Have you ever been in a vehicle, a car or a roller coaster perhaps, that made a sharp turn? You probably felt yourself slide toward the outside of the turn, causing you to be pressed against the person or door to your side. It was your body’s own inertia that was resisting the change in direction.

Travel coffee mugs are popular items, mainly because of inertia. Have you ever tried to drink a beverage in a cup with no lid in a moving car? When the driver steps on the gas, the beverage sloshes over the front of your cup and lands in your lap. When the driver steps on the brakes, the liquid sloshes over the back of your cup and spills on your knee. When the driver makes a quick right, the drinks spills over the left side of the cup and lands on your shoe. It is no fun. The inertia of the liquid causes it to resist the changes in motion.

An interesting fact is that an object still possesses inertia even when it is weightless. This means that under weightless conditions, you could still tell the difference between two weightless objects that looked identical, but had different masses. The more massive object would still take more force to accelerate than the less massive object. They have instruments called inertial balances that can be used to mass objects by their inertia.

Galileo noted that an object in motion stays in motion unless acted upon by an unbalanced force, and an object at rest remains at rest unless acted upon by an unbalanced force. Sir Isaac Newton, who was born in 1643, the year after Galileo died, took the idea of inertia as his first law of motion.

Newton’s First Law
The Law of Inertia

In the absence of an unbalanced (net) force, an object in motion will remain in uniform motion, and an object at rest will remain at rest.

Newton, who was apparently not usually known for his modesty, once wrote in a letter to fellow scientist, Robert Hooke, “If I have seen further than others, it is by standing upon the shoulders of giants.” The choice of the word giants may have been a thinly veiled insult aimed at Hooke, who apparently was short in stature, but I like to imagine that he might have been acknowledging the work of Galileo, who deserves much of the credit for Newton’s law of inertia.

Lesson 2–2 Review

1. _______________ is an object’s tendency to resist changes in motion.

2. A boy ties a string to a rock and spins it over his head in a horizontal circle at a uniform speed. If the speed of the rock doesn’t change, is there an unbalanced force on it, or does its own inertia account for this motion?

3. Who has more inertia, a 35.0 kg boy or his 25.0 kg sister?