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Speed and Velocity

Speed and Velocity

Although speed and velocity are often used interchangeably in everyday life, they represent different quantities in physics. 

What is speed? 

Speed is a measurement of how fast an object moves relative to a reference point. It does not have a direction and is considered a magnitude or scalar quantity. Speed can be figured by the formula: 

Speed = Distance/Time 
or 
s = d/t

How to Measure Speed 

In the United States we mostly think of speed in miles per hour or mph. This is the way the speed of a car is typically measured. In science and physics the standard unit of measure for speed is generally meters per second or m/s. 

The measurement of speed can reflect two different scalar quantities.

• Instantaneous Speed - The speed of an object at a given moment. The car may be travelling at 50 mph at this moment, but it may slow down or speed up during the next hour.
• Average Speed - The average speed is calculated by the distance that an object traveled over a given interval of time. If a car traveled 50 miles over the course of one hour then its average speed will be 50 mph. It may be that the car traveled at instantaneous speeds of 40 mph and 60 mph during that time, but the average speed is 50 mph.

What is velocity? 

Velocity is the rate of change in an object's position. Velocity has a magnitude (speed) and a direction. Velocity is a vector quantity. Velocity is represented by the formula: 

Velocity = the change in distance/change in time 

Velocity = Δx/Δt

How to Measure Velocity 

Velocity has the same unit of measure as speed. The standard unit of measure is meters per second or m/s. 

What is the difference between speed and velocity? 

Speed is the magnitude of velocity. Velocity is the speed of an object plus its direction. Speed is called a scalar quantity and velocity is a vector quantity. 

Speed of Light 

The fastest possible speed in the universe is the speed of light. The speed of light is 299,792,458 meters per second. In physics this number is represented by the letter "c." 

Interesting Facts about Speed and Velocity

• The first scientist to measure speed as distance over time was Galileo.
• A speedometer is a great example of instantaneous speed.
• The speed of light can also be written as 186,282 miles per second.
• The speed of sound in dry air is 343.2 meters per second.
• The escape velocity of Earth is the speed needed to escape from Earth's gravitational pull. It is 25,000 miles per hour.

Why are Rainclouds Dark?

Force

Force

What is force? 

In physics, force is a push or pull on an object. A force can cause an object to accelerate, slow down, remain in place, or change shape. 

How to Measure Force 

The unit of measure for force is the newton which is abbreviated as "N". One newton is the force needed to accelerate one gram of mass by one centimeter per second squared. Other units of force include the dyne and the pound-force. 

 
Examples of force

Force, Mass, and Acceleration 

Force can be figured out if you know the mass and acceleration of an object. This equation comes from Newton's Second Law of Motion: 

f = m * a

Where f = force, m = mass, and a = acceleration.

Forces and Vectors 

Force not only has a magnitude (which is what we get in newtons when we use the equation above), but it also has a direction. This makes force a vector. Vectors are shown by an arrow that indicates the direction of the force and a number that indicates the magnitude. See the pictures to the right to see how the arrow is used to show the direction of the force. 

Forces in Equilibrium 

Sometimes there can be many forces acting on an object, but the object remains still. In this case the forces are in equilibrium. The sum of the forces, or the net force, is zero. 

The picture below shows an object sitting on a table. The object isn't moving. This is because the force of gravity pulling the object down is equal and opposite to the force of the table pushing up. The net force is zero and the forces are in equilibrium. 

Combined Forces 

When multiple forces are acting on an object, the resultant force is the sum of the vectors of the individual forces. We won't get into complex vector math here, but take for example a tug of war. The two sides are each pulling. If one side is pulling with a force of 2 N in left direction and the other side is pulling with a force of 3 N in the right direction, then the resultant force is 1 N in the right direction. 

Types of Forces

• Friction - Friction is a force caused when one object rubs against another. It works in the opposite direction of the main force.
• Gravity - Gravity is a force caused by a large body, such as the Earth. Gravity pulls objects toward the Earth with an acceleration of "g" which equals 9.8 m/s².
• Electromagnetic - Electromagnetic force is a force associated with electric and magnetic fields.
• Nuclear - Nuclear forces are the forces that hold atoms and their particles together.
• Tension - A pulling force that is exerted by a string, cable, or chain on another object.
• Elastic - An elastic force is a force exerted by an object trying to return to its natural length. This is modeled by a spring that has been pulled by an external force, but is pulling back while trying to return to its original length.

Interesting Facts about Force

• An object that is accelerating in a circular motion experiences "centripetal" force.
• The four fundamental forces are gravity, electromagnetic force, the strong nuclear force, and the weak nuclear force.
• Torque is a type of force that measures changes in the rotational speed of an object. Torque is an important feature of automobiles, especially trucks.
• Drag is a force that decreases the velocity of an object. Thrust is a force that increases the velocity of an object.

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Where Traditional Physics Stops

We're about to move into the modern age of physics. In the early 1800's, scientists began examining the basis of matter, space, and time. Sometimes it gets very confusing, but the big idea is that Newton's physics describe about 90% of the way things work in the universe (mechanics). His ideas start to break down when you talk about ideas such as objects moving at the speed of light, the inside of atoms, extreme temperatures, and when the objects are huge (like galaxies interacting with each other). 

Into the Atom

The original idea of atoms developed by Niels Bohr showed a structure based on various shells and a center area called the nucleus. The electrons were found in those shells while the protons and neutrons were found in the nucleus. There are other ways to look at the structure of atoms (you may have heard of "spdf"), but we're going to stick with the classic view for many of our discussions. This view of the structure of an atom was one of the foundations for modern physics. 

Into the Universe

Albert Einstein also played a large part in modern physics. He developed formulas that described the way matter and energy were related. Just about everyone has heard of the formula E=mc^2. That formula explains how energy is related to mass. The idea found its way into the study of fission reactions, and it was proved that enormous amounts of energy were stored in even one atom of a substance. 

Current Studies

Even now, scientists are still testing the boundaries of physics and the laws of physics. Only a few years ago a new state of matter was created. The Bose-Einstein condensate was theorized decades ago, but scientists have only recently been able to create it in a lab. Every day astronomers are studying space and learning how black holes and galaxies interact. Stephen Hawking is one of the more famous scientists working in that field. Our point is, there is still much to discover. 

Forming the Present Simple tense in English

Forming the Present Simple tense in English