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succeed. Hey class I'm Mr. Thornton and I'm going to help you Succeed in your
GCSE and IGCSE! This lesson Kinetic Energy. This topic was requested by Maccag 2 HD.
If there's a topic you'd like me to cover then just leave a comment
below. Kinetic energy is just movement energy. Any object with mass which is
moving has some kinetic energy, and increasing the mass and/or the velocity
will increase the kinetic energy. That's why getting hit with a foam ball
would hurt a lot less than getting hit by a similar-sized rock moving at the
same velocity. The rock would have more mass so more kinetic energy, and your
body would have to absorb that energy on impact. Remember we can't create or
destroy energy; just store it and change what type we have, so if the rock stops
when it hits you then it has no velocity, so it no longer has any kinetic energy
either, and that energy must have been transferred into you. It may sound a bit
ghoulish talking about people getting hit by rocks,
but understanding this idea of kinetic energy and how it behaves is actually
central to a lot of our ideas about preventing serious injuries in
collisions, which I'll come back to shortly. The equation for kinetic energy
is kinetic energy equals 1/2 times the mass times the square of the velocity.
This is an equation you need to learn the way this is represented varies from
exam board to exam board. There's no single standardized way to represent it which
physicists all over the world have agreed on, so some boards write kinetic
energy as KE while others write it as Ek with the k as a subscript. I'm going to
stick with KE because I think it's a clearer notation at GCSE, but the
calculation is the whichever notation you prefer. The
important thing about this equation is that squared symbol over the velocity.
The reason squaring the velocity is so important is clearer if we look at some
examples. Let's pick some values which make our calculation nice and easy to
follow. Let's say that in all cases the mass is going to be 6 kilograms and
let's start out with a velocity of 2 meters per second.
KE equals 1/2 m v squared so that's 1/2, or 0.5 when you type it into a
calculator, times 6 kilograms times the square of 2 meters per second. 2 squared
equals 4 so our calculation is 1/2 times 6 times 4 which is equal to 12. It's a
type of energy so like all energy is measured in joules. The symbol for that
is a capital J, so a six kilogram mass moving at 2 meters per second has a
kinetic energy of 12 joules. Hopefully that was pretty simple to
follow. The one mistake I sometimes see people make is squaring the whole thing
instead of just the velocity. If you're typing this into a calculator I
recommend either using some brackets or squaring the velocity first and then
multiplying by 0.5 and the mass to avoid this problem. Now let's see
what happens when we double the mass. Our calculation will now be KE equals 1/2
times 12 kilograms times 2 squared which gives us 24 joules. Doubling the mass has
doubled the kinetic energy. This is why getting hit by a heavier object hurts
more. Now let's try going back to our six kilogram mass and doubling the velocity
to four meters per second. Now KE equals 1/2 times 6 times 4 meters per second
squared. 4 squared is 16 so our calculation is 1/2 times 6 times 16,
giving us 48 joules. Doubling the velocity has given us 4 times the
kinetic energy. We would say that the kinetic energy is
proportional to the mass, that if we double mass we double kinetic energy. If we
triple mass we triple kinetic energy and so on, but the kinetic energy is also
proportional to the square of the velocity, so doubling velocity gives us
four times the kinetic energy, tripling velocity gives us nine times the kinetic
energy, and so on. Small changes in velocity can have a really big effect on
the kinetic energy because KE is proportional to the square of v. This is
why bullets are dangerous although their mass is just a few grams. When they're
fired from a gun they're traveling at hundreds of meters per second so they
have a large amount of kinetic energy concentrated into a very small object.
Things get even more dangerous in space. There are thousands of pieces of space
junk, mainly things like debris from previous missions, in orbit around our
planet. I want you to imagine a one-gram steel bolt drifting through space. To
stay in a Low Earth Orbit it would have to be travelling at around 27,000
kilometres per hour, that's about 17,000 miles per hour. This
is the orbital velocity of the International Space Station, which does a
complete orbit of the planet every hour and a half. That's seven and a half
thousand meters per second. If this one gram bolt meets a spacecraft coming the
other way at the same velocity then their relative velocity will be 15,000
meters per second. Let's calculate the kinetic energy of our one gram bolt.
Remember one gram is 0.001 kilograms. KE equals 1/2 times 0.001 kilograms times
15,000 meters per second squared. The square of 15,000 is 225 million.
Multiplying through by 1/2 and by 0.001 kilograms gives us a kinetic
energy of 112,500 joules. By comparison, a large
high velocity bullet fired from a rifle would typically only have a kinetic
energy of around ten thousand joules. This tiny 1 gram piece of space junk
could hit a spacecraft with the energy of 10 high-velocity bullets. Spacecraft
need to be better than bullet-proof, and all because kinetic energy is
proportional to the square of the velocity. This same principle is
important here on the ground. Although the standard speed limit here in the UK
in a built-up area is 30 miles per hour, there are an increasing number of 20
miles per hour speed limits being introduced in town centres and outside
schools to reduce the stopping distance and the kinetic energy of cars in places
where collisions with pedestrians are likely. A car moving at 20 miles per hour
has just 44% of the kinetic energy it would have at 30 miles per hour.
Again, that tiny change in velocity has a big effect on the kinetic energy, meaning
that any collision with this slower moving car is going to be significantly
less dangerous for whoever or whatever that car collides with. If we come back
to the car stopping distance, when a driver applies the brakes the car's
kinetic energy is changed by friction mainly into heat and some sound as the
brake pads rub against the brake discs. To stop the car all of the stored
kinetic energy that that car is carrying as it moves has to be changed into other
types of energy, such as heat. This can even cause some types of brakes to
overheat under heavy braking, causing them to stop working properly and making
the driver lose control. If a car has less kinetic energy then it's easier to
remove its kinetic energy because there's less kinetic energy to remove,
and so it's easier to stop the car because the ease of stopping the car
depends upon the amount of kinetic energy
and the kinetic energy is proportional to the square of the velocity. Doubling
the velocity will make a car four times more difficult to stop because it's got
four times the kinetic energy. If we look at the highway code here in the UK it
specifies typical stopping distances at different velocities. Let's just focus
upon the braking distance and rearrange these bars into a graph. You can see that
this is actually just a y equals x squared graph. Doubling the velocity from
30 miles per hour to 60 miles per hour increases the braking distance by four
times. This is all because kinetic energy is proportional to V squared. I'll talk
more about stopping distances in a future video and I'll add a card with a
link to that video up here when it's uploaded. If you want to see how to
rearrange the equation for kinetic energy then I have a video which runs
through all possible rearrangements and there'll be a link in the description or
you can click on this card instead. LIGHTNING ROUND
Kinetic energy is movement energy and can be calculated as 1/2 times mass times
the square of the velocity. KE equals 1/2 m v squared. Because the kinetic energy is
proportional to the square of the velocity, tiny changes to the velocity
can have a big effect, making fast moving objects dangerous even if their mass is
low. To stop an object all of the stored kinetic energy it's carrying must be
changed into other types of energy such as heat in a car's brakes. Doubling the
velocity quadruples the kinetic energy so quadruples the stopping distance of a
car, and under the heavy braking the brakes may overheat and fail. Most countries
have speed limits on most roads to reduce the kinetic energy of cars, making
them easier to stop and making them have less kinetic energy, and therefore do
less damage in the event of a collision. I hope that video really helps you. If it
did it'd be great if you let me know in the comments.
Remember to Like, Subscribe, and hit the bell to get a notification the next time
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you can click here to check out this related video. Good luck in your GCSEs
and IGCSEs, and thanks very much for watching.
For more infomation >> Noticias Telemundo Mediodía, 7 de enero de 2019 | Noticias Telemundo - Duration: 21:46. 
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