![]() I did that so you see how fast they are moving. You'll notice the two vehicles leave a trail of dots. The big red box represents the truck and the small blue box is the car. ![]() Just press play to run it and the pencil to see (and edit) the code. This content can also be viewed on the site it originates from. The initial momentum is 20 kg*m/s and the stopping force is 2 newtons.Ī plot of the x-velocity for the car and the truck looks like this: We'll say the car has a mass of 10 kg (it's a really small car) and the truck has a mass of 30 kg (three times the mass of the tiny car). ![]() Of course, that requires some actual values for the mass of the two vehicles, the starting momentums, and the stopping force. Just for fun, let's create a numerical calculation for this. The cars stop at the same time because they start with the same momentum. According to the momentum principle, they must have the same change in time.Ĭlearly, answer number 2 is correct. In the end, both vehicles will have the same force with the same change in momentum. However, it starts with a much larger velocity since the two vehicles have the same starting momentum. Yes, it's true that the car has a lower mass and a higher acceleration. This, in turn, causes the car to slow down more quickly because the truck has a large mass and a small acceleration.Īnswer number 2: They stop in the same amount of time. Since it has lower mass, the force acting on it results in larger acceleration. However, since I'm not there and you aren't here, I will just share two common answers people provide.Īnswer number 1: The light car stops first. If you like, you can check with friends to see what they think. Work is energy being transferred.OK, hopefully you have an answer by now. The cheque is a means of transferring the cash value (the work done for example when a brick is raised). Pushing this analogy to its limits helps to show that whilst you can store real cash in the bank (the energy stored, for example, in a fuel + oxygen mixture), the cheque which passes between accounts is something different. It is also impossible in this transaction to know how much is stored in each account. We have to pay the banks for doing the job for us and so although my account falls by £1 yours may only gain 95p because you have to pay bank charges. It is an instruction to my bank to pay out £1 into your account. If I transfer a £1 cheque from my account to yours then my account goes down by £1 and yours will go up by £1. An analogy to use when teaching about energy transfersĬonsider two bank accounts. Steam engines enabled the output of many Cornish mines to quadruple. Watt went even further, developing the concept of rate of working, or power, with his steam engines described in ‘horsepower’. Manufacturers such as Boulton & Watt persuaded mine owners in Cornwall to buy a steam engine in place of their pit ponies, by comparing the amount of work each could do. Early on, a major use of steam engines was pumping water out of mines. But the abstract idea of an ‘engine’ really developed with steam engines.īy the 1820s the concept of ‘work’ as mechanical effect had been introduced into discussions about what are now called power technologies. Humans first domesticated animals to do useful work and later found other ways of exploiting energy from natural sources, such as falling water and wind. You can only calculate energy that is transferred. ![]() Nor do you know how much total energy is stored gravitationally. The transfer of energy is not 100% efficient and not all the energy transferred is represented by m g h. As well as transferring energy to the raised bricks, some of the energy in your muscles warms you up. If you lift a lot of bricks, you can get too hot. However, not all the energy available does a useful job. ![]() The useful thing which you get from fuels by burning them is a transfer of energy, so that a load can be raised, or an object accelerated. It takes account of the mass, the height raised and whether the kilogram is raised on the Earth or the Moon. You can show that the equation is a good summary of what happens. This second equation is illustrated by raising kilograms onto different height shelves. When energy is transferred from energy stored chemically in muscles to energy in a raised load, or to energy stored elastically in a stretched spring, the energy transferred is a measure of how much work has been done. You can calculate the energy transferred, or work done, by multiplying the force by the distance moved in the direction of the force.Įnergy transferred = work done = force x distance moved in the direction of the force Work is done whenever a force moves something over a distance. ![]()
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