Inertia: Newton’s First Law of Motion
Newton’s First Law of Motion: The Law of Inertia states:
- If the forces acting on an object are balanced, then the object will not accelerate (increase in speed).
* The car and driver in the diagram to the right are stationary, at rest, thus the car and driver are not
accelerating. Because of its inertia, tendency not to change its state of motion, the car will remain at rest–unless the object has an unbalanced force acting on it.
* The car and driver in the diagram to the left are moving at a constant velocity, thus the car and driver are not accelerating. Because of its inertia, tendency not to change its state of motion, the car will continue to move at a constant velocity–unless the object has an unbalanced force acting on it. - Objects with balanced forces acting on them are said to be at equilibrium. Thus, according to the Law of Inertia, an object at equilibrium will not accelerate–unless the object has an unbalanced force acting on it. This means that the two car and driver diagrams are both examples of objects at equilibrium.
- A stationary object at equilibrium has a tendency to remain at rest–unless the object has an unbalanced force acting on it. An object moving at a constant velocity is at equilibrium, which means balanced forces are acting on the object. This object tends to continue to move at a constant velocity–unless the object has an unbalanced force acting on it.
What Inertia Is
1. Objects with mass have inertia, thus all objects composed of matter have inertia.
2. Objects at equilibrium (balanced forces acting on them) are either stationary or moving at a constant velocity, and these objects will continue in their state of motion–unless the object has an unbalanced force acting on it.
3. The state of motion of an object does not affect its inertia. The elephant in the diagram is running. The elephant has the same mass when it is running as it does when it is laying on the ground. This is because inertia depends on mass not movement.
4. Inertia is the resistance an object has to any change its state of motion. The more massive an objec
t is the more inertia is has. The more inertia, the more resistance an object has to any change in its state of motion.
5. Gravity has no effect on the inertia of an object. This is because inertia depends on the mass of an object which is a measure of the amount of matter (stuff) making up the object. Gravity is a force.
6. Astronauts in reduced or zero gravity have the same inertia as they would on Earth. This is because gravity does not affect the mass of astronauts. Only the weight of the astronat is affected by gravity. Inertia is affected when the mass of an object is changed.
Summation
The Law of Inertia states that it is the natural tendency of all objects at equilibrium (balanced forces acting on them) to maintain their state of motion–unless the object has an unbalanced force acting on it.
Stationary objects have balanced forces acting on them tend to remain stationary because of their inertia; Stationary objects do not move unless unless the object has an unbalanced force acting on it.
Objects at equilibrium that are moving at a constant velocity tend to continue to move at the same velocity —unless the object has an unbalanced force acting on it.
Objects that are not acceleration have balanced forces acting on them- this is true of stationary objects as well as objects moving at a constant velocity;
Objects at equilibrium have balanced forces acting on them; These objects do not change their state of motion–unless the object has an unbalanced force acting on it.
Changes in mass affect 
inertia-inertia and mass are directly related, which means an increase in mass will increase inertia; Inertia is not a force.
A breif summary of the Law of Inertia:
1. Stationary objects keep doing what they are doing, which is to remain at rest–unless the objects have an unbalanced force acting on them.
2. Objects moving at a constant velocity keep doing what they are doing, which is moving at a constant velocity—unless the object has an unbalanced force acting on it.
Activity: Which of the following statements is true about Newton’s first law of motion?
1. A horse that is standing at rest has the same ineria as it does when it is running.
True.
2. A penguin has less inertia in water than it does on land.
False. Any change in the inertia of an object is due to a change in the object’s mass. While the buoyancy of water affects the weight of the elephant, it does not affect the animal’s mass.
3. An unbalanced force causes a stationary object to accelerate.
False. Yes, the statement is true, but it does not apply to the Law of Inertia, Newton’s First Law of Motion. Newton’s First Law of Motion only applies to objects at equilibrium, which means objects having balanced forces acting on them. (The statement is true for Newton’s Second Law of motion.)
4. Balanced forces acting on a stationary object cause the object to remain at rest.
True: Forces can give energy to an object it acts on causing the object to change it state of motion. If forces acting on an object are balanced, they do not cause a change in motion. Balanced forces are equal in magnitude and opposite in direction.
5. Balanced forces acting on a moving object cause the object to continue to move at a constant velocity.
True.
6. The mass of a stationary object affects the amount of force required to move the object.
False. This a true statement, but it does not apply to the Law of Inetia, Newton’s First Law of Motion. It applies to Newton’s Second Law of Motion.
7. A swing is set in motion. In time the swing comes to rest. The law of inertia explains this natural tendency of all moving objects to come to rest.
False: The Law of Inertia explains that all things have a natural tendency not to change their state of motion; Stationary objects remain stationary; Moving object remain in motion.
8. A rocket moving in Earth’s atmosphere has more inertia than when the rocket is moving in a gravity-free environment.
False. Gravity has no affect on mass, therefore, changes in gravity has no effect on the inertia of any object.
9. A cat’s diet was changed so that it would loose weight. Weight loss affected the cat’s inertia.
False: Inertia is affected by changes in mass. While it is true that a change in weight is achieved by a change in mass, it is not correct to say that weight loss affects inertia.
Think About It! Weight is a measure of the force of gravity acting on an object. Weight can change if the force of gravity changes, such as in space,
on the Moon, etc… Inertia is not a force; Inertia is not affected by gravity because inertia is directly related to mass, which changes if the amount of matter making up the object changes. Technically, the cat’s diet was changed so the cat could loose part of its mass.
10. Inertia causes moving object to come to rest and stationary object to move at a constant velocity. False: Inertia does not change the motion of any object. Inertia and mass are directly related. Think About It! The mass of a vehicle does not cause the vehicle to start moving nor does it cause the vehicle to stop moving. It take a net force (unbalanced force to change the motion of an object.), which is described in Newton’s Second Law of Motion.
————————————————-Physics is the study of energy, forces and motion. You are study physics when you learn about Newton’s Laws of Motion. You are studing physics when you learn about rockets, which you will discover more about when you study Newton’s Second Law of Motion. Physics is about magnets, sound, electricity, and lot of other fun things. Understanding Newton’s Laws of Motion gives you the tools to design, build and launch homemade rockets. Physics is one of my favorite sciences and I hope that you find it as exciting as I do.
Physics for Every Kid contains fun, easy-to do physics investigations.
