Newton’s First Law of Motion Inertia Chapter 4 Newton’s First Law of Motion Inertia

Where we’ve been and where we’re going…. So far we have discussed what happens as an object moves. We have been able to predict where a projectile will move and we have seen the effects of gravity on falling objects. We aren’t done discussing motion, now we will discuss why things move instead of how they move.

Say you are watching TV one day and the TV starts to slide across the room. Crazy right? It wouldn’t just happen that way, something has to make it move.

Force A force is a push or a pull.   Friction is a force that opposes an object’s motion.

What is friction? If you got a microscope and looked at any surface, even one that looked really smooth, through the microscope you’d be able to see lots of irregularities in the surface of the surface. If you put two surfaces next to each other they try to “grab” on to each other. This is the cause of friction .

The Godfather of Force Galileo Galilei studied the effects of forces on objects. He asked the following: a ball rolling up a hill will slow down, a ball rolling down a hill will speed up, so what will happen to the speed of a ball that is rolling across a flat surface if there is no friction?

The motion of the ball The ball would continue to roll at the same speed. Let’s look at it another way

Force and Motion A force is NOT needed to keep an object moving.

If I slide a book across a desk, what will eventually happen?

The book in motion on the table top does not come to a rest position because of the absence of a force; rather it is the presence of a force - that force being the force of friction - which brings the book to a rest position. In the absence of a force of friction, the book would continue in motion with the same speed and direction - forever! (Or at least to the end of the table top.) A force is not required to keep a moving book in motion. In actuality, it is a force that brings the book to rest.

Newton’s 1st Law Every object continues in a state of rest, or of motion in a straight line at a constant speed, unless it is compelled to change that state by forces exerted upon it.

Newton’s 1st Law Stated in everyday language: An object in motion remains in motion, an object at rest remains at rest until acted on by an unbalanced force. This is sometimes called the law of inertia.

Friction On Earth there are no situations where friction is absent. Some surfaces have less friction such as a bowling lane or a frozen pond. Anytime we talk about an example in class where an object is on ice, we will say that no friction exists.

Space Although we always have friction on Earth, there is no friction in space. If you throw a ball in space, what type of motion will it have?

The ball will continue moving in a straight line at a constant speed.

Check for Understanding Supposing you were in space in a weightless environment, would it require a force to set an object in motion?

Yes, it would require a force to start the motion Yes, it would require a force to start the motion. Once the object is set in motion it will continue on forever with no force necessary to keep it moving.

Who would be willing to let me throw a golf ball to them Who would be willing to let me throw a golf ball to them? Would anyone let me throw a basketball to them? How about a 14 pound bowling ball?

Why would it not be a good idea for me to throw a bowling ball at you as hard as I possibly could?   The bowling ball is heavier. Once that bowling ball is in motion it wants to remain in motion so it’s hard to stop.

All objects resist changes in their state of motion. This is inertia. All objects have this tendency - they have inertia. But do some objects have more of a tendency to resist changes than others? Absolutely yes! The tendency of an object to resist changes in its state of motion varies with mass.

An object that has a lot of mass will also have a lot of inertia. Mass is measured in units of kilograms (kg).

Check for Understanding Mac and Tosh are arguing in the cafeteria. Mac says that if he flings the Jell-O with a greater speed it will have a greater inertia. Tosh argues that inertia does not depend upon speed, but rather upon mass. Who do you agree with? Explain why.

Tosh is correct. Inertia is that quantity which depends solely upon mass. The more mass, the more inertia. (Momentum is another quantity in Physics which depends on both mass and speed. Momentum will be discussed in a later unit).

Check for Understanding Fred spends most Sunday afternoons at rest on the sofa, watching pro football games and consuming large quantities of food. What effect (if any) does this practice have upon his inertia? Explain.

Fred's inertia will increase! Fred will increase his mass if he makes a habit of this. And if his mass increases, then his inertia increases.

Check for Understanding The group of physics teachers are taking some time off for a little putt-putt golf. The 15th hole at the Hole-In-One Putt-Putt Golf Course has a large metal rim which putters must use to guide their ball towards the hole. Mrs. Anderson guides a golf ball around the metal rim When the ball leaves the rim, which path (1, 2, or 3) will the golf ball follow?

The answer is 2. Once leaving the rim, the ball will follow an "inertial path" (i.e., a straight line). At the instant shown in the diagram, the ball is moving to the right; once leaving the rim, there is no more unbalanced forces to change its state of motion. Paths 1 and 3 both show the ball continually changing its direction once leaving the rim.

Unbalanced Forces Newton’s 1st Law again: An object in motion remains in motion, an object at rest remains at rest until acted on by an unbalanced force. What is an unbalanced force?

Balanced Forces To understand unbalanced forces it is best to start with balanced forces. Right now, as you sit in your chair, the forces on you are balanced. The amount that you push down on your chair is equal to the amount that the chair pushed back up on you.

Balanced Forces The two forces: you pushing on the chair and the chair pushing on you are balanced. They are equal in amount but opposite in direction. When there are only balanced forces on an object, the object is in equilibrium.

Balanced Forces

Balanced Forces When only balanced forces act on an object, it will not have a change in its state of motion.

A question for you If an object is not changing its state of motion, does that mean that the object is at rest?

A question for you If an object is not changing its state of motion, does that mean that the object is at rest? No, it just means that the object will continue doing what it was already doing, whether it was at rest OR in motion.

Back to the book sliding on the table When a book slides across a table it will come to a stop because of friction. Friction is an unbalanced force that acts on the book

Unbalanced Forces When an unbalanced force acts on an object, the object will change its state of motion. This means that the object will speed up, slow down or change direction.

Categories of Forces Forces are said to be either contact forces or field forces. Contact forces occur when two objects are actually touching one another. An example of a contact force is your hiney and the chair you are sitting on. Field forces occur between objects that are not in contact. The most common example of a field force is the gravitational force.

Force Force is measured in units called Newtons. Newton is abbreviated N.

Mass vs Weight Mass is a measure of the matter in you. Weight is the force of gravity that acts on an object.

Mass vs Weight If you were to go to the Moon, Mars, Jupiter, etc. your mass would not change. You would still have the same amount of matter in you. In these different locations however, your weight would change because the value of gravity has changed.

Mass vs Weight Mass is measured in kilograms (kg). Because weight is a force, it is measured in Newtons. Here in America we use pounds to measure weight. A Newton and a pound measure the same thing, just like inches and centimeters both measure distance. This is not to say that 1 pound = 1 Newton.

Weight The weight of an object can be found with the following equation: W = mg In this equation, W stands for weight, m stands for mass and g stands for gravity. We will now start using 9.8 m/s² for g if we are looking for the weight of an object in Newtons We will use 2.2 ft/s² for g if we are looking for the weight of an object in pounds.

Inertia As you sit at your desk, are you really at rest? Where would you land if you were to jump straight up?

Since the Earth is moving you are not really at rest, rather you are moving with the Earth. If you jumped up, you’d land in the same place. What if you were on a plane and you tossed a penny straight up. Where would it land? In front of you, in your hand, or behind you?

The penny would land in your hand because even though the plane is moving at about 600 mph, the penny is also moving that fast and it will continue to move that fast as it is thrown up. The penny was in motion before you threw it and it wants to continue in motion. It has inertia.