Newton's Laws of Motion: Understanding Inertia and Forces, Lecture notes of Law

Download Newton's Laws of Motion: Understanding Inertia and Forces and more Lecture notes Law in PDF only on Docsity!

UNIT

Forces

Section

Newton’s Laws of Motion

Background

Sir Isaac Newton (1643-1727) an English scientist and mathematician famous for his discovery of the law of gravity also discovered the three laws of motion.

He published them in his book Philosophiae Naturalis Principia Mathematica (mathematic principles of natural philosophy) in 1687. Today these laws are known as Newton’s Laws of Motion and describe the motion of all objects on the scale we experience in our everyday lives.

Newton’s First Law

An object at rest wants to stay at rest and an object in motion wants to stay in motion unless acted on by an unbalanced force.

“Law of Inertia”

Objects at Rest Simply put, things tend to keep on doing what they’re already doing.

• Objects in a state of rest tend to remain at rest.
• Only a force will change that state.

Objects in Motion Now consider an object in motion.

• In the absence of forces, a moving object tends to move in a straight line indefinitely.
• Toss an object from a space station located in the vacuum of outer space, and the object will move forever due to inertia.

Inertia The tendency of an object to resist changes in its state of motion

The first Law states that all objects have inertia. The more mass an object has, the greater its inertia and the more force it takes to change its state of motion.

The amount of inertia an object has depends on its mass - which is roughly the amount of material present in the object.

Inertia Example You can tell how much matter is in a can when you kick it. Kick an empty can and it moves. Kick a can filled with sand and it doesn’t move as much.

Which has more mass, a feather pillow or a common automobile battery?

The pillow has a larger size (volume) but a smaller mass than the battery. But, clearly an automobile battery is more difficult to set into motion. This is evidence of the battery’s greater inertia and hence its greater mass.

Mass Is Not Weight

Mass is often confused with weight.

• Mass is a measure of the amount of material in an object.
• Weight, on the other hand, is a measure of the gravitational force acting on the object.

We can define mass and weight as follows:

• Mass is the quantity of matter in an object.
• Weight is the force of gravity on an object.

It is common to describe the amount of matter in an object by its gravitational pull to Earth, that is, by its weight.

In most parts of the world the measure of matter is commonly expressed in units of mass, the kilogram (kg).

At Earth’s surface, 1 kilogram has a weight of 9.81 Newton’s.

The SI unit of force is the Newton (kgm/s^2 )

The SI symbol for the Newton is N.

If you know the mass of something in kilograms and want its weight in Newton's at Earth’s surface, multiply the number of kilograms by 9.81 m/s^2.

Friction

Friction is a force that arises due to the relative motion of two surfaces.

1. Two solid surfaces: Sliding friction, rolling friction
2. A solid and a fluid: Air resistance
3. Two fluids

The direction of the friction force always acts in the opposite direction of motion.

The amount of sliding friction depends on:

1. The surface material (smoothness/roughness)
2. The normal force between the surfaces

The amount of sliding friction does not depend on:

1. Area of contact
2. Relative speed

Friction