In your notebooks – define the terms work and joule. You do not have to do a vocabulary square on them. They can be found in chapter 12, sec. 1. February 17, 2011

What is work? When we think of work, we think of someone going to work, or some kind of manual labor. How does that compare to science’s definition of work? Work is when an object moves in the same direction as the force acting on the object. Does that match our everyday definition of work? Maybe not. Watch this demonstration and decide if our idea of work is the same as the scientific definition of work.

What is work? In the demonstration both students held a book, but only one of the books moved in the direction of the force applied to it. Does that mean that only one student did work? Yes – that is exactly what it means. In everyday language we would say that both students did work, but according to science the person holding the book did not do any work because the book did not move.

Can there be work without motion? According to the scientific definition – in order to do work on an object, that object must move some distance as a result of the force acting on that object. So NO – there can not be work without motion. Why is carrying your books to school by placing them in your backpack and then placing the pack on your back considered work? You did work putting the books in the bag, and you did work putting the backpack on your back, but after that there is no work done on the books. This is because the force you exert must be in the same direction as the motion.

Calculating Work: How do we find out how much work we have done on an object? Work is calculated by multiplying force by distance. Work = Force x Distance The SI unit for Force is the Newton The SI unit for distance is the meter Therefore; the SI unit for Work is the newton – meter AKA the Joule! The Joule is named in honor of James Prescott Joule, a Physicist from the 1800s.

Exit Slip: Is more work done when a force of 2N moves an object 3m or when a force of 3N moves an object 2m? Answer: Both scenarios result in 6J of work; therefore, they are both equal to the same amount of work.

February 18, 2011 Bellringer: Create a vocabulary square for Power – Chapter 12, section 1.

Let’s review work: Work is done when an object moves in the direction of the force applied to that object. This means that for work to be done, the object has to move in the direction of the force and without motion, no work is done on the object. But, is the amount of work always the same? Watch the demonstration and decide. Work is done on the pinwheel because the pinwheel moves in the same direction of the force. But when the dryer is on the highest setting more work is done in the same amount of time because the wheel spins faster.

What is Power? The amount of work you do on an object is not affected by the time it takes to do that work. Time is important however,when it comes to power. So what is power? Power is the rate at which work is done. So what does the rate at which work is done mean? It means the time it takes the work to be done.

What is Power? When you think of power, what comes to mind? Perhaps you think of fast cars with big engines, or a strong man lifting weights. What characteristics do all of the powerful things we have talked about share? Does science think of power the same way?

What is Power? Power equals the amount of work done on an object in a unit of time. Basically – the more quickly you are able to do the work, the more powerful you (or the objects you are using to do the work) are. For example: If it takes me 3 hours to rake leaves using a garden rake, and it only takes me 1 hour to collect the leaves using a leaf blower, which one would be more powerful? The blower has more power because it is able to do the same amount of work as the rake but in less time.

Calculate Power To calculate power we have to look at how long it takes to do a certain amount of work. Work is equal to force times distance. Power is the amount of work done in a certain amount of time. So – Power equals work divided by time Power = Work Time

Calculate Power The unit for work is the Joule. The unit for time is the second. The unit for Power is Joules per second (J/s). The shortened form of Joules per second is Watt (W). The watt is named after James Watt who made great improvements to the steam engine. The watt is a small unit of power so we normally refer to power in kilowatts (kW). 1 kilowatt = 1000 watts In the US we use horsepower, but this is not an SI unit. 1 horsepower = 746 watts

Calculate Power A tow truck exerts a force of 11,000N to pull a car out of a ditch. It moves the car a distance of 5m in 25seconds. What is the power of the truck? Power = 11,000N x 5.0m 25s Power = 55,000J 25s Power = 2,200 J/s or 2,200 W