Work and Energy

6.1 Work Done by a Constant Force Which of the following is NOT work? Pushing a Stalled Car Pulling a Wagon Climbing stairs Falling Down Carrying a Heavy Backpack Down the Hall

Definition of Work Work equals Force times Displacement W = Fs The Force and Displacement MUST be in the same direction They must be in the same direction since they are both vectors.

Units of Work W = Fs F  N, s  m Unit of Work is Nm = Joule (J)

Back to the Definition of Work W = Fs If s = 0, what work is done? W = 0 If you spend an hour pushing on a wall, you do zero work since s = 0

Example 1 Marcy pulls a backpack on wheels down the 100-m hall. The 60-N force is applied at an angle of 30° above the horizontal. How much work is done by Marcy? W = 5200 J W=Fs Need to get the horizontal component of force. Fx = 60N cos 30 = 51.96 N W=51.96N(100m) = 5196 J

Redo the Work Formula Since we have to find the component of the force in the direction of the displacement W = (F cos )s Where  is the angle between the F and s vectors.

Example 2 Mark is carrying books (200 N) down the 100-m hall. How much work is Mark doing? W = 0 J The force is vertical and the displacement is horizontal. W=(F cos )s  W = (200 N cos 90)100 m = 0

Example 3 You carry some books (200 N) while walking down stairs height 2 m and length 3 m. How much work do you do? W = -400 J F = 200 N (lift up) S = -2 m (down) W = Fs = (200 N)(-2 m) = -400 J

Check your understanding A suitcase is hanging straight down from your hand as you ride an escalator. Your hand exerts a force on the suitcase, and this force does work. Which one of the following is correct? The W is negative when you ride up and positive when you ride down The W is positive when you ride up and negative when you ride down The W is positive The W is negative

Practice Problems How much work do you actually do for homework? P173 CQ1-3, P1 – 5, 7 – 8 Total of 10 Problems