Work and energyForces in doing work, rate of doing work, and in energy transformation

Work

defined as the product of the force magnitude F and the displacement magnitude d

Work is done Work is done

Work is done

displacement

force

displacement

No work is done

No work is done

Work is done by the man

Work is done by gravity

lifting

falling

WorkIs done when the Force (F) applied to an object causes that object to be displaced (d) in the same direction as the force applied.

Is a method of transferring energy

How can we ?

The ground is doing work on us.

http://2.bp.blogspot.com/-EsjHgHyE0Xo/VU-Mz9UejGI/AAAAAAAALq4/MOdF14vdUv8/s1600/walk1.jpg

Work

Time for some problem solving!

Suppose a woman is pushing a grocery cart with a 500 Newton force along the 7 meter aisle, how much work is done in pushing the cart from one of the aisle to the other?

http://www.google.com.ph/imgres?imgurl=http://vectortoons.com/wp-content/uploads/2015/02/shopping-collection-sarah-004.jpg&imgrefurl=http://vectortoons.com/product/a-woman-pushing-a-shopping-cart-inside-the-supermarket/&h=904&w=1024&tbnid=lU4J2cTaNdNsnM:&zoom=1&docid=i94W7CJRHFiiOM&ei=4IN1VbmxJ5H3oAT-v4OwCQ&tbm=isch&ved=0CC4QMygqMCo4ZA

Time for some problem solving!

The librarian found a book of mass 1kg is on the floor. If the book is lifted by the librarian from the floor to the top shelf which is 2 meters from the floor, how much work is done on the book?

http://bloximages.chicago2.vip.townnews.com/nwitimes.com/content/tncms/assets/v3/editorial/9/bb/9bbd9852-8a43-5056-8293-3e2c919deff5/506d0316cf7a7.preview-620.jpg

Kinetic Energy (KE)Is the energy of a moving object

Quantifies the amount of work an object can do because of its motion

Kinetic Energy (KE)

𝐾𝐸=12𝑚𝑣2

Work-energy principle

The net work done on an object is equal to the change in the objects kinetic energy.

Concept Check

What will happen to the KE of an object if its mass is doubled but the velocity remains the same?

How about if the velocity is doubled but the mass remains the same?

Time for some problem solving!

A 1000kg jeepney has a velocity of 17 m/s. What is its kinetic energy?A 2000kg jeepney has a velocity of 17 m/s. What is its kinetic energy?A 1000kg jeepney has a velocity of 34 m/s. What is its kinetic energy?

http://www.paulancheta.com/weblog/images/07-04-2-jeepney.jpg

Potential EnergyIs energy gained in raising an object from the

ground or lost when made to fall

Also known as gravitational potential energy

Potential Energy

𝑃𝐸= h𝑚𝑔

Highest PE PE decreases as KE increases

http://scienceblogs.com/startswithabang/files/2011/12/kinetic_energy_c_la_784.jpeg

Time for some problem solving!

What is the potential energy of a 1 kg book lifted to a 0.5m height above a table?

What is the potential energy of a 1 kg book lifted to a 1.5m height above a table? http://etc.usf.edu/clipart/63200/63282/63282_books_table_lg.gif

Elastic Potential Energy

depends on an elastic material’s elasticity and how much the material is stretched or compressed

http://images.wisegeek.com/spring.jpg

Hooke’s LawWithin an elastic limit, there is direct proportionality between the force needed to stretch or compress elastic materials and their change in elongation

Elastic Potential Energy

𝐸𝑃𝐸=12𝑘𝑥2

Rest position EPE stored Work done to release energy

Power

rate of doing work or the rate of using energy

Power

rate of doing work or the rate of using energy

Time for some problem solving!

A 50.0kg marathon runner runs up the stairs to the top of the approximately 350m-tall MSHS building. To lift herself to the top in 15.0minutes, what must be her average power output?

Mechanical Energy