Work and Power May 18, 2015

Think about it….. You are sitting on your bed doing your homework. Are you doing work?

Think about it….. You are not doing work when you are sitting on your bed doing your homework.

Today, I can describe and calculate work and power. May 18, 2015

Work the transfer of energy to an object by using a force that causes the object to move in the direction of the force.

Difference between Force & Work Applying a force doesn’t always result in work being done. – Whenever you apply a force to an object and the object moves in the direction of the force, you have done work on the object.

Is it work or not? (p. 211) Work is done on an object if two things happen: (1)the object moves as Force is applied (2)the direction of the object’s motion is the same as the direction of the Force

How much work is done? The amount of work (W) done in moving an object force (F) applied to the object by the distance (d) through which the force is applied SI units: Newton for Force … meter for length/distance … work is the Newton-meter (N × m) … more simply called a joule (J)

Work (p. 213)

Let’s work it out…. A horse applies a force of 120 N to pull a wagon a distance of 50,000 m along a level road. How much work is done? W = F x d Step 1: List the given and unknown values F = 120 Nd= 50, 000 mW = ? Step 2: Enter the known values into the equation and solve. W = F x d W = 120 N x 50,000 m W = 6,000,000 Nm = 6,000,000 J

Now you try…. How much work is done on a 16 N sack of potatoes when you lift the sack 1.5 m?

Solution How much work is done on a 16 N sack of potatoes when you lift the sack 1.5 m? W = F x d W = 16 N x 1.5 m W = 24 J

Let’s try another problem… A rope is thrown over a beam, and one end is tied to a 300 N bundle of lumber. You pull the free end of the rope 2 m with a force of 400 N to lift the lumber off the ground. How much work have you done?

Problem Solved….. A rope is thrown over a beam, and one end is tied to a 300 N bundle of lumber. You pull the free end of the rope 2 m with a force of 400 N to lift the lumber off the ground. How much work have you done? W = F x d W = 400 N x 2 m W = 800 J

Calculate how fast work is done by calculating power. Power the rate at which energy is transferred.

Calculating Power To calculate power (P), you divide the amount of work done (W) by the time (t) it takes to do that work, as shown in the following equation: P = W t The unit used to express power is joules per second (J/s) … also called the watt. One watt (W) is equal to 1 J/s.

Calculate the Power A stage manager at a play raises the curtain by doing 5,976 J of work on the curtain in 12 s. What is the power output of the stage manager? Step 1: Write the equation for power. P = W t Step 2: Replace the variables with the known values and then solve the equation for power. P = 5,976 J 12 s P = 498 W

Now you try…. If you do 50 J of work in 5 s, what is your power?

Now you try…. If you do 50 J of work in 5 s, what is your power? Step 1: Write the Equation P = W t Step 2: Replace the variables with the known values and then solve the equation for power. P = 50 J 5 s P = 10 J/s or 10 W

Try another problem …. If you did 100 J of work in 5 s, what was your power output?

Try another problem …. If you did 100 J of work in 5 s, what was your power output? P = 100 J 5 s P = 20 W