Work F The force, F, pushes the box for a short distance. This causes the box to start moving!!!!!! I just don’t have any energy  Whoa!!! Now I do!!!

Work If a force, F, is used to move an object a distance, d, then the force does work on the object. F d W = Fd We can say that the force added (gave) energy to the box!!!!!!!

Units for Work  The units for force are Newtons (N) and the units for distance are meters (m).  Since Work is equal to force times distance, We could use a Newton-meter (N·m).  But the more common unit is the Joule (J)  1 J = 1 N·m  1 J is defined as the amount of work for 1- N of force to move an object 1-m

In order for work to be done:  There must be a force applied.  The object must move a distance.  The force must be responsible for the motion.

How much work is being done by this weightlifter? NONE!!!!! Since he doesn’t move the weights any distance, the work done is 0 J.

 If work is done on an object its energy is changed  If the work done is positive, the energy of the object is increased.  If the work done is negative, the energy is decreased. Work & Energy

Potential Energy

Lifting a Ball This ball has a mass of 2-kg. What is its weight? w = mg = (2)(9.8) = 19.6-N What force must be used to lift this ball? Its weight: F = w = 19.6-N How much work will be done to lift this ball to a height of 5-m? F = 19.6-N d = 5-m W = Fd = (19.6)(5) = 98-J 19.6-N 5-m Since 98-J of work was done on the ball its energy was increased by 98-J. This was added to a form of energy called Potential Energy.

Potential Energy Potential Energy (PE) is energy stored in an object due to its position, and is stored by doing work. For our ball, its position is its height, 5-m, above the ground. This form of potential energy is technically called Gravitational Potential Energy Gravitational Potential Energy is found by multiplying the mass (m) of the object by gravity (g) and height (h). 5-m PE = mgh 98-J of work was done lifting this ball. It now has 98-J of potential energy stored in it. It now has the ability to do 98-J of work. PE = (2)(9.8)(5) = 98-J

Pile Driver A pile driver is a machine that uses the conversion of potential energy into work to drive beams and poles into the ground. The block at the top of the pile driver has a mass of 275-kg and is 9-m above the pole it is driving into the ground. What is the potential energy of the block? m = 275-kg h = 9-m PE = mgh = (275)(9.8)(9) = 24,255-J This means the pile driver (block) can do 24,255-J of work on the pole to drive it into the ground!!!!!!

4-m A 3-kg ball is at a height of 4-m when it starts to fall. At 4-m its PE = (3)(9.8)(4) = J PE = J At 3-m its PE = (3)(9.8)(3) = 88.2-J 3-m 2-m 1-m 0-m PE = 88.2-J At 2-m its PE = (3)(9.8)(2) = 58.8-J PE =58.8-J At 1-m its PE = (3)(9.8)(1) = 29.4-J PE =29.4-J At 0-m its PE = (3)(9.8)(0) = 0-J PE =0-J As the ball falls it loses potential energy because its height is decreasing. Where does this potential Energy go??? It is transferred into Kinetic Energy – Energy of Motion. I see more notes in your future!!!!!!!!!!!!!!