Work is defined as the transfer of energy from one body to another. Or more rigorously:

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Presentation transcript:

Work is defined as the transfer of energy from one body to another. Or more rigorously:

We can calculate the work done on an object with: the units of work are Nm or Joules Note that these are the same units as torque yet their values are used to describe very different quantities.

Example 1 - Work against Gravity How much work is required to lift a 2.0 kg textbook from the floor to a height of 1.5 m at a constant velocity? Note: W = Fd, but what force do we need to exert to lift the book at a constant velocity? Since the velocity is constant what is the net force acting on the book?

Example 2 - Work on an object How much work is done on a 4.0 kg medicine ball that is held at a height of 1.8 m for 10 s? Note: Is energy being used to hold the ball in this position? Well, yes…but… Is work actually being done ON THE BALL? No. The amount of energy the ball has remains unchanged.

Example 3 – Forces at an angle The plucky youngster pictured to the right is pulling his sled at a constant velocity of 1.2 m/s. He pulls the 15 kg sled with a force of 35 N at an angle of 40 o to the horizontal. How much work does he do in pulling the sled 20 m? Note: Draw an FBD showing the forces at work on the sled. Break F boy into its vertical and horizontal components. Does the vertical component of the force do any work? Rule: When finding the work done on an object we only consider the component of the force that acts in the direction of displacement.

Example 4 – F net vs. F app A biology student is pushing a rope 15 m along a level surface. The student pushes the rope with a force of 220 N while the force of friction is 120 N. How much work is the student doing? Note: To find the amount of work done by the student should we used F net or F app ? Rule: When finding the total work done by a body we always use F app.

Hey wait a minute, if work is the transfer of energy what the heck kind of energy is generated by the force of friction? Rule: When finding the amount of heat energy is lost due to friction we use:

Example 5 – To scalar or not to scalar? Work is the product of a scalar and a vector so (of course) it is a scalar. However, work can be positive or negative… but how? Glad you asked. Imagine that you bring a 1.0 kg basketball from the floor to the top of a 1.0 m table. How much work did you do?

Wow, you’re awesome. Which direction did you exert that force? Now suppose the ball rolls off the table and falls straight down to the floor. How much work was done on the ball? Be careful: which direction is the force working on the ball now?

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