What is work? When Work is done by a constant force (constant in both magnitude and direction) and is defined as … THE PRODUCT OF DISPLACEMENT TIMES THE COMPONENT OF THE FORCE PARRALLEL TO THE DISPLACEMENT The force and displacement have to be in the same direction or at least parallel. Examples: Lifting a book is work. Holding a book is not.

W = F x d measured in Joules (J) What is a Joule in fundamental units? J = N * m J = kg*m/s2*m J = kg*m2/s2

Pile Driver A pile driver is a device used to drive stakes into the ground. While building a fence, Adam drops a pile driver through a vertical distance of 8m. How much Work is done if he uses a force of 5 x 106 N?

What is work? Consider the equation W = F cos Θ in regards to displacement (Fd cos Θ): F is the force d is the net displacement of the object Θ is the angle between the directions of the force and net displacement What does this say when Θ = 0, Θ = 90, Θ = 180 in regards to displacement “-” ??? Example – Pushing a broom or pulling a box

Work Example A sled, which has a mass of 45.0 kg., is sitting on a horizontal surface. A force of 120 N is applied to a rope attached to the front of the sled such that the angle between the front of the sled and the horizontal is 35º. As a result of the application of this force the sled is pulled a distance of 500 meters at a relatively constant speed. How much work was done to this sled by the applied force?

Work Example A 75 kg crate is moved across a floor, against friction, at a constant speed. It moves a distance of 41 m, and 18kJ of work is done on it. What is the coefficient of friction?

Example Big Joe pulls a 300 N sled across level snow with a force of 520 N along a rope that is q above the horizontal. If the sled moves a distance of 60 m, and Joe does 20kJ of work, what is q?

What is power? Power is the rate at which work is done. P = W/t P = Fd/t Can two people do the same amount of work on an object but have different amounts of power? yes, how? time

The Watt power is measured in watts (W) What is a watt in fundamental units? W = J/s W = kg*m2/s2/s W = kg*m2/s3

Pile Driver Power? If Adam finishes the project in 60 seconds, How much Power was generated? Can you build a fence in 60 seconds?

Energy is defined as the ability to do work. So, if something has energy it has the ability to do work. That is it can displace an object with a force. To change the amount of energy an object has you must do work on the object.

Ex When you draw back a bow you do work on the bow. The bow now has energy and can do work on the arrow. When you lift a hammer you do work on the hammer. the hammer now has energy and can do work on the nail

Types of energy potential, kinetic, solar, electrical, nuclear, chemical etc. Mechanical energy is broken down into two types Potential energy Kinetic energy

Kinetic energy what does kinetic mean? energy of motion so things that are moving have energy What would affect how much energy a moving object has? m,v KE = ½ mv2

Work Energy Theorem vf2 = vi2 + 2ad a = vf2 – vi2 2d W = Fd W = m a d W = m vf2 –vi2 d 2d W = ½m(vf2 –vi2) W = ½mvf2 – ½mvi2 or W = DKE

Potential energy stored energy energy of position drawn bow, compressed spring, rubber band when position means height we have gravitational potential energy

Gravitational PE PE = mgh How does an object get that energy? work was done on the object W=mgh Fd=mgh mad = mgh

Conservation of energy Law of conservation of energy Energy cannot be created or destroyed. Then where does it go? Transferred

Conserving energy In a closed system the initial energy is equal to the final energy. or there is no net gain or loss of energy at any point in a closed system. or if you start with 1000 J of energy at any point in the system the energy is 1000 J

What type of energy does the ball have at this point?

How does this look conceptually in a problem? The energy of the ball is constant during the vertical loop demonstration. At the top of the ramp it has 0 velocity so, no KE It has height so it has PE So the initial energy of the ball is equal to the PE at the top of the ramp

At any other point during the demonstration the energy will be equal to the PE at the beginning of the demonstration. The energy at the bottom of the loop is equal to that at the top of the loop but is all converted to KE the ball is moving and has 0 height

At the top of the loop the ball has the same energy it had at the top of the ramp. It is made up of PE and KE the ball is moving and has height

Solving the problem Ei = Ef PEi + KEi = PEf + KEf mghi + ½ mvi2 = mghf + ½ mvf2

2(mghi + ½ mvi2 – mghf) = mvf2 Now solve it for vf mghi + ½ mvi2 = mghf + ½ mvf2 mghi + ½ mvi2 - mghf = ½ mvf2 2(mghi + ½ mvi2 – mghf) = mvf2 2(ghi + ½ vi2 – ghf) = vf2 vf = 2(ghi + ½ vi2 – ghf)

Machines : device that helps do work by changing the magnitude or direction of applied force Why do we use machines? To make work EASIER Machines do not change the amount of work done!!! So what does EASIER mean?

Wi = Wo Winput = Woutput Fi di = Fo do Easier means less force has to be used. therefore that force has to be applied over a larger distance Winput = Woutput Fi di = Fo do

the inclined plane Wi = Wo di Fi do Fo

the lever di di Fi do Fo do

How much do machines help us? we measure this with ideal and actual mechanical advantage di do IMA = Fo Fi AMA =

Key Terms – Mechanical Advantage Force Out = Resistance Force In = Effort Distance In = Effort Distance Distance Out = Resistance Distance

How well do machines work? we measure this with efficiency Wo Wi AMA IMA Efficiency = = x 100 = %