Physics Summary

Let's have a look at how you can use Newton's Second Law to figure out how fast your rocket will go. First we need to understand

• Velocity
• Acceleration
• Force

Then we'll put them together to find your rocket's speed.

You might notice that we make all our measurements here using the metric system. That's because the metric system is designed for all the equations to work without any conversion factors just to fix the funny units. If we measure in imperial units, feet and inches and pounds, then there are fudge factors all over the place... and it can get very confusing very fast so lets keep this as simple as we can by using metric measurements.

Velocity

This is the measure of your speed, defined as the distance covered each second, or each minute, or each hour. In equation form, then, you can find the distance you've covered by multiplying the velocity at which you're traveling by the time you're going at that speed:

Distance = velocity x time

or in short form,

s = v x t

Example:

Toy car rolling at 2 meters per second -
Time Distance
1 second 2 meters
2 seconds 4 meters
3 seconds 6 meters
4 seconds 8 meters

Acceleration


But now what if your velocity is changing, like when you're rocket's engine is burning and the rocket is rapidly speeding up? How do you measure how you're speeding up?

The term for this is acceleration, and the measure is to figure out how much faster you're going after each second (or after each minute or hour). If it takes my car 10 seconds to go from 0 to 60 miles per hour, then my acceleration is 6 miles per hour each second - we would say 6 miles per hour per second.

If I measure velocity in meters per second instead of miles per hour, then acceleration is in meters per second per second. It sounds funny but that's how you say it.

So now if I know my acceleration and how long I accelerate, I can find my velocity (speed) by multiplying the acceleration times the time, or in equation form,

Velocity = acceleration x time

Or in short form,

v = a x t

Example:

Car accelerating at 5 meters per second per second
After the car is going
1 second 5 meters per second
2 seconds 10 meters per second
3 seconds 15 meters per second
4 seconds 20 meters per second


Note: 1 meter per second (m/s) equals 2.237 miles per hour.

Force

Newton's Second Law:

Force = mass x acceleration, or

F = m x a (sometimes written F = ma)

You usually think of force in pounds - in the metric system force is measured in "Newton’s", yes, named after Sir Isaac. A Newton is not a lot of force - one pound is 4.45 Newton’s.

Example

Pushing a 0.25 kilogram rocket
Motor's thrust rocket's acceleration
1 newton 4 meters/sec/sec
2 newtons 8 meters/sec/sec
3 newtons 12 meters/sec/sec
4 newtons 16 meters/sec/sec

 

So What?

So now you can find the speed your rocket will go:

If I know the force applied to the rocket, and the mass of the rocket, I can use Newton's second law to find the acceleration.

Once I know my acceleration and how long I accelerate, I can find velocity (v = a x t).
The force, of course, is the rocket's thrust as given by the motor spec. We find the burn time by dividing the impulse by the thrust, as described on the NAR Rocket Motor Coding web page.

Example:

My rocket weighs 3.5 ounces = 0.1 kg.
Note: 1 kilogram equals 35.27 ounces

If we use a "C5" motor, it will have a thrust of 5 Newton’s for 2 seconds, according to its specifications.

That's all we need to know! Putting it all together, we have:

F = 5 Newton’s

m = 0.1 kg

t = 2 seconds

F = m x a, therefore 5 = 0.1 x a, so a = 50 meters/sec/sec

v = a x t = 50 x 2 = 100 meters/second = 223.7 mph!

...and you won't get a ticket! Some of the faster model rockets will get up to 300 mph and even more, so this speed is actually pretty typical.

Sir Isac Newton (04-01-1643 / 31-03-1727

Ok let’s start with those dates you are going to argue that Isaac was born on 25-12-1643 and died on 20-03-1726 it all depends on which calendar you use, Isaac was born during the period when the Julian calendar was used but died just after the start of use of the modern Gregorian calendar, with the modern Gregorian calendar the dates in the title are correct, if however you use the old Julian calendar then the alternative dates in the text are correct.

Sir Isaac Newton was an English Physicist and Mathematician, Newton was a very influential scientist and shaped a lot of the way we think of things, many of his findings are still key today as per his first, second and third laws opposite.

 

 

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