Ch. 14
Newton’s 2 nd Law ??? F = m a Kinetic Energy = energy in motion Potential Energy = energy at rest
Transfer of energy that moves an object a distance in the direction of the applied force If there is no movement, there is no work done Work = Force x distance W = F d W (units) = Newton meter(N m) W = joule (J) SI unit = joule (J)
The rate of doing work If you increase power = you can do more work in the same amt of time OR do the same amt of work in less time Ex: Engine size indicates that it has more power Power = work / time Power = J / s Power = watt (W) SI unit = watt (W) Is there a relationship b/t Work & Power?
Your family is moving to a new apartment. While lifting a box 1.5 m straight up to put it on a truck, you exert a force of 200 N for 1.0 s. How much power is required to do this? P = work/time P = (force distance) / time Force = 200 N Distance = 1.5 m Time = 1.0 s P = (200 N 1.5 m) / 1.0 s P = 300 J / 1.0 s P = 300 W
You lift a book from the floor to a bookshelf 1.0 m above the ground. How much power is used if the upward force is 15.0 N and you do the work in 2.0 s ? P = work / time W = F d Force = ? Distance = ? Time = ? 7.5 W
You apply a horizontal force of 10.0 N to pull a wheeled suitcase at a constant speed of 0.5 m/s across flat ground. How much power is used? Force = ? Distance = ? Time = ? 10.0 N 0.5 m = 5 J 5 J / 1.0 s = ? 5 W
Another unit for measuring work = Horsepower 1 hp = 746 watts Developed comparison to show power of steam engines Horsepower was based on the power output of a very strong horse (James Watt didn’t want to exaggerate the measure of the steam engine’s power)
Machines make work easier Name some examples of machines… Tire jack, wrench, door handle, elevator Machines can: Increase Force Increase Distance Change Direction
The force you exert on a machine = input force Distance the force acts through = input distance Work done by input force acting through input distance = work input i.f. i.d. = w.i. Ex: Rowing a boat You exert force on the oar handles, the oars exert force on the water, so the boat moves
The force that is exerted by a machine = output force Distance the force is exerted through = output distance Output force output distance = work output o.f. o.d. = w.o. Moving that boat with oars… The oars are the machine
Number of times a machine can increase the input force Ex: nutcracker MA of 1 st location (nearest to hinge) MA = approx. 7 2 nd location MA = approx. 3
IMA = input distance IMA= d E output distance d R IMA can only exist if there is no friction B/c friction is always present, the AMA is always less than IMA
AMA = output forceAMA= F R input force F E Ratio of output force to input force Would the surface affect the MA? Which is better? Smooth or Rough? More force is needed to overcome the friction of a rough surface
Efficiency = Work output 100 = _______% Work Input Always less than 100% Why = Friction!!!!
6 types Lever Wheel & axle Inclined plane Wedge Screw Pulley
a rigid object that is used with an appropriate fulcrum or pivot point to multiply the mechanical force that can be applied to another objectfulcrumpivot
consists of a wheel that turns an axle, or an axle that turns a wheel. It is also a lever that turns in circles around a point or fulcrumwheel axle
a ramp used to reduce the effort needed to raise or lower an object over a vertical height It is easier to climb stairs to get to a second floor than to climb straight up a rope. It is easier to walk up a long, gentle hill than to climb up a short, steep hill Loading ramp Stairs or wheelchair ramp Mountain road Threaded screw
portable inclined plane, used to separate two objects, or portions of objects, lift an object, or hold an object in place, by the application of force to the other endinclined planeforce
an inclined plane wrapped around a cylinderinclined plane
wheel with a groove between two flanges around its circumference. A rope, cable or belt usually runs inside the groove. Pulleys are used to change the direction of an applied force wheelgrooveflangescircumferenceropecablebeltforce