Newton's 1st Law of Motion
Newton’s 1st Law of Motion, also known as the law of inertia, can be summarized as follows:
“An object at rest will remain at rest, and an object in motion will remain in motion, at constant velocity and in a straight line, unless acted upon by a net force.”
This means that unless there is a net (unbalanced) force on an object, an object will continue in its current state of motion with a constant velocity. If this velocity is zero (the object is at rest), the object will continue to remain at rest. If this velocity is not zero, the object will continue to move in a straight line at the same speed. However, if a net (unbalanced) force does act on an object, that object’s velocity will be changed (it will accelerate).
This sounds like a simple concept, but it can be quite confusing because it is difficult to observe this in everyday life. People are usually fine with understanding the first part of the law: “an object at rest will remain at rest unless acted upon by a net force.” This is easily observable. The donut sitting on your breakfast table this morning didn’t spontaneously accelerate up into the sky. Nor did the family cat, Whiskers, lounging sleepily on the couch cushion the previous evening, all of a sudden accelerate sideways off the couch for no apparent reason.
The second part of the law contributes a considerably bigger challenge to the conceptual understanding of this principle. Realizing that “an object in motion will continue in its current state of motion with constant velocity unless acted upon by a net force” isn’t easy to observe here on Earth, making this law rather tricky. Almost all objects observed in everyday life that are in motion are being acted upon by a net force - friction. Try this example: take your physics book and give it a good push along the floor. As expected, the book moves for some distance, but rather rapidly slides to a halt. An outside force, friction, has acted upon it. Therefore, from typical observations, it would be easy to think that an object must have a force continually applied upon it to remain in motion. However, this isn’t so. If you took the same book out into the far reaches of space, away from any gravitational or frictional forces, and pushed it away, it would continue moving in a straight line at a constant velocity forever and ever, as there are no external forces to change its motion. When the net force on an object is 0, the object is in static equilibrium. You’ll revisit static equilibrium when discussing Newton’s 2nd Law.
The tendency of an object to resist a change in velocity is known as the object’s inertia. For example, a train has significantly more inertia than a skateboard. It is much harder to change the train’s velocity than it is the skateboard’s. The measure of an object’s inertia is its mass. For the purposes of this course, inertia and mass mean the same thing - they are synonymous.
Question: A 0.50-kilogram cart is rolling at a speed of 0.40 meter per second. If the speed of the cart is doubled, what happens to the inertia of the cart?
Answer: The inertia (mass) of the cart remains unchanged.
Question: Which object has the greatest inertia?
- a falling leaf
- a softball in flight
- a seated high school student
- a rising helium-filled toy balloon
Answer: (3) a seated high school student has the greatest inertia (mass).
Question: Which object has the greatest inertia?
- a 5.00-kg mass moving at 10.0 m/s
- a 10.0-kg mass moving at 1.0 m/s
- a 15.0-kg mass moving at 10.0 m/s
- a 20.0-kg mass moving at 1.0 m/s
Answer: (4) a 20.0-kg mass has the greatest inertia.
If you recall from the kinematics unit, a change in velocity is known as an acceleration. Therefore, the second part of this law could be re-written to state that an object acted upon by a net force will be accelerated.
But, what exactly is a force? A force is a vector quantity describing the push or a pull on an object. Forces are measured in Newtons (N), named after Sir Isaac Newton, of course. A Newton is not a base unit, but is instead a derived unit, equivalent to 1 kg×m/s2. Interestingly, the gravitational force on a medium-sized apple is approximately 1 Newton.
You can break forces down into two basic types: contact forces and field forces. Contact forces occur when objects touch each other. Examples of contact forces include pushing a crate (applied force), pulling a wagon (tension force), a frictional force slowing down your sled, or even the force of air accelerating a spitwad through a straw. Field forces, also known as non-contact forces, occur at a distance. Examples of field forces include the gravitational force, the magnetic force, and the electrical force between two charged objects.
So, what then is a net force? A net force is just the vector sum of all the forces acting on an object. Imagine you and your sister are fighting over the last Christmas gift. You are pulling one end of the gift toward you with a force of 5N. Your sister is pulling the other end toward her (in the opposite direction) with a force of 5N. The net force on the gift, then, would be 0N, therefore there would be no net force. As it turns out, though, you have a passion for Christmas gifts, and now increase your pulling force to 6N. The net force on the gift now is 1N in your direction, therefore the gift would begin to accelerate toward you (yippee!) It can be difficult to keep track of all the forces acting on an object.