EXAMPLE• A 105 g hockey puck is sliding across the ice. A player exerts a constant 4.50 newton force over a distance of 0.150 m. How much work does the player do on the puck? What is the change in energy?
LIFTING A BOOK• When is the work positive?• When is the work negative?• When is the work zero?
WORK AGAINST GRAVITY• W = Fd• The work of lifting something is equal to the weight of the object times the distance lifted• Weight =• So W =
WORK• Since work equals the change in KE, the unit is the same• Work is measured in joules• One joule happens when a force of 1 N acts for 1 m• An apple is approximately a newton, so lifting an apple 1 meter is about 1 Joule of work
WORK• What if our force is not applied in a straight line?• Will it be as effective?• How do we account for this?
WORK• W = Fdcosɵ• ɵ is between the force and the direction of displacement• If he pushes the car 10.0 m, how much work did the man do?
WHAT TO INCLUDE IN WORK• Which direction do the normal force and gravity point?• ɵ is …• What about friction?
EXAMPLE• A sailor pulls a boat a distance of 30.0 m along a dock using a rope that makes a 25.0° angle with the horizontal. How much work does the sailor do on the boat if he exerts a force of 255 N on the rope?