4.2 Newton's First Law of Motion: Inertia

Multiples of this standard unit of force can be stated for the magnitude of all other forces.

There are many other possibilities for standard forces.
Later in the chapter, some alternative definitions of force will be given.

A stretched spring can be used as a standard unit of force.

The hook exerts force on whatever is attached to it.

There are 6 units in the force standard being employed.

Get two rubber bands to investigate standards.

Use a paper clip to attach a small household item to the rubber band, and then use it as a weight to investigate the stretch of the rubber band.
Take the amount of stretch produced in the rubber band and divide it by the number of items suspended from the rubber band.

If left alone, an object in motion tends to slow down and stop unless some effort is made to keep it moving.

A body at rest remains at rest unless acted on by a net external force.

The status quo of motion can be preserved by this law.

Net external force will be defined in the next section.

An object sliding across a table or floor because of the net slows the force of friction on it.

The idea of cause and effect is important in describing situations.

An object sliding along a rough horizontal surface is an example.
The object is grinding to a halt.
The object will move farther if we spray the surface with talcum powder.
If lubricating oil is rubbed on the surface, it will make it more smooth.
We can imagine the object sliding in a straight line indefinitely.
The slowing is caused by Friction.
The object wouldn't slow down if it were completely free of friction.
There is an air hockey table.

The puck glides long distances without slowing down when the air is turned on.

We can accurately predict how quickly the object will slow down if we know enough about the friction.
Friction is a force outside.

Newton's first law can be applied to anything from an object sliding on a table to a satellite in the sky.

Experiments have shown that any change in speed or direction must be caused by an external force.
It is a basic feature of all laws of physics that the idea of generally applicable or universal laws is important.
Identifying these laws is like finding patterns in nature.

Some objects have more inertia than others.

It is more difficult to change the motion of a boulder than it is of a basketball.

Mass is the amount of stuff in something.

The amount of matter in an object is determined by the number of atoms and Molecules in it.
Mass doesn't vary with location.
The mass of an object on Earth is the same as it is on the Moon.
It is difficult to count and identify all of the atoms and molecules in an object, so mass determination is not often done this way.

The standard kilogram is used to determine the mass of objects.

A kilogram is equal in mass to a kilogram of another substance.

Volume and density are the quantities that might differ between them.

There is a relationship between force and changes in motion.

The second law of motion is used to calculate what happens when a force is used.
We need to sharpen some of the ideas that have already been mentioned in order to write down the exact relationship of force, mass, and acceleration in a simple equation.

A change in motion is the same as a change in velocity.

Newton's first law says that a net external force causes a change in motion.

Another question arises.

The other children exert two external forces.
The motion of a system is only affected by external forces.

Before you can determine which forces are external, you have to define the boundaries of the system.

Sometimes the system is obvious and other times it is more subtle.
The correct application ofNewton's laws is fundamental to many areas of physics.
We will revisit this concept many times on our journey through physics.

Different forces on the same mass produce different results.

The arrows show all external forces.

The wagon and its rider are the system of interest.
The weight of the system and the support of the ground are assumed to be canceled.
The motion of the wagon is being acted on by the vector, which acts to the left.
The system of interest is shown in the free-body diagram.
The center of mass is represented by a dot.
From this dot, each force is extended.
There are two forces acting to the right.

It seems reasonable that the force acting on a system should be proportional to the force acting on the system.

A smaller force causes a smaller force to accelerate.
The vertical forces are assumed to cancel since there is no acceleration in the vertical direction.
The weight and the support of the ground are the two forces that make up the horizontal force.
In later sections, these will be discussed in more detail.

The techniques are the same as for the addition of other vectors, and are covered in the same states as for the net external force.

It is important to ignore the internal forces once the system of interest is chosen.

It seems reasonable that the mass of the system should be taken into account.

The bigger the mass, the smaller the force that creates the acceleration.
External force applied to a car produces a much smaller acceleration than it does when applied to a basketball.

Where is the mass of the system?

Experiments show that acceleration is linearly proportional to the net external force.

Different accelerations are produced by the same force on different systems.

As you do more problems, there will be a series of patterns for the free-body diagram.

The mass of the object and the net external force are the only factors that affect the object's speed.

The second law of motion is given by combining the two proportionalities.

The net external force on the system is proportional to the mass of the system.

The first equation gives more insight into what the second law means.

There is a cause and effect relationship between the law and three quantities.
The validity of the second law is based on experimental verification.

The pound is the most familiar unit of force in the United States, where 1 N is 0.225 lbs.

The object moves toward the center of Earth when dropped.

The second law states that a net force on an object is responsible for its speed.
Weight is a downward force because it has a direction, and down is the direction of gravity.
The magnitude of weight is shown as.
In the absence of air resistance, all objects fall with the same speed.
We can derive an equation for weight using Galileo's result andNewton's second law.

Consider an object falling down.

The downward force of gravity has a magnitude.
The magnitude of the net external force on an object is stated in the second law.

The downward force of gravity only affects the object.

The acceleration of an object due to gravity is known as.

Depending on the direction of the coordinate system, recall can take a positive or negative value.

When working with weight, be sure to consider this.

The only force on the object is gravity.

In the real world, objects fall downward toward Earth because there is always some upward force from the air acting on the object.

The weight of an object depends on location and is not an inherent property of the object, because the acceleration due to gravity varies slightly over the surface of Earth.

If one leaves Earth's surface, the weight can change a lot.
The acceleration due to gravity is only on the Moon.
A 1.0-kg mass has a weight of only 1.7 N on the Moon.

The weight of an object is the force it exerts on the nearest large body, such as Earth, the Moon, the Sun, and so on.

In physics, this is the most useful definition of weight.
It is different from the definition of weight used by NASA and popular media in relation to space travel and exploration.
"weightlessness" and "microgravity" are really referring to the phenomenon we call "freefall" in physics.
We will make distinctions between free-fall and actual weightlessness by using the above definition of weight.

It is important to know that weight and mass are not the same.

In classical physics, mass is the quantity of matter, whereas weight is the force of gravity.
It is tempting to equate the two, since most of our examples take place on Earth, where the weight of an object only varies a little with the location of the object.
Our medical records often show our weight in kilograms, but never in the correct units of newtons, as the terms mass and weight are used interchangeably in everyday language.

Mass and weight are often used in the same sentence.

These terms are different in science.
Mass is the measure of the amount of matter in an object.
The kilogram is the most common measure of mass.
Weight is a measure of the force of gravity on an object.

The weight is equal to the mass of the object.

If the mass of an object is kept intact, it will remain the same.

The weight of an object can change when it enters a region with stronger or weaker gravity.
The acceleration due to gravity on the Moon is less than the acceleration due to gravity on Earth.
If you measured your weight on Earth and then on the Moon, you would find that you weigh less, even though you are not fat.
The force of gravity is not as strong on the Moon.
When people say they are losing weight, they really mean they are losing mass, which causes them to weigh less.

The scale has springs that are similar to rubber bands.

The springs give a measure of your weight.

This is a force in pounds.
In most countries, the measurement is divided by 9.80 to give a reading.
The scale provides information about mass.
Push down on a table while standing on a bathroom scale.

The net external force on a lawn mower is about 11 lbs parallel to the ground.

The mower has a mass of 24 kilograms.

The lawn mower's net force is to the right.

The law states that the acceleration can be calculated from the second law.

The net force is parallel to the ground and the direction of the acceleration is the same.

We can say something about the relative magnitudes of the external forces that are acting on the system.
The force that the person pushing the mower exerts must be greater than the force that the other side of the mower exerts.
It is reasonable for a person to push a mower.
It wouldn't last long because the person's top speed would soon be reached.

A sled is propelled to the right by a rocket thrust.

The same thrust is created by each rocket.
The vertical forces cancel when there is only horizontal acceleration.
The ground exerts an upward force on the system that is equal in magnitude and opposite in direction to its weight.
The sled, its rockets, and rider are the only objects that are considered.
The arrow is larger than scale.

We assume the vertical forces cancel since there is no vertical acceleration.

We have only horizontal forces and a simpler problem.
The positive direction is indicated by plus or minus signs.
There is a free-body diagram in the figure.

The thrust of the engines can be found by looking for ways to find the mass of the engines.

We need to only consider the magnitudes of the quantities in the calculations since we have defined the direction of the force and acceleration as acting to the right.