Energy II
Levers, Pulleys and the golden rule
A. Jørgensen Energy 9. Juli 2015 1 / 8
Forces
Remember that you need forces to change the velocity of things.
The unit of force is Newton (N)
1 N = 1 kg·m/s2. (1)
Levers
Archimedes once said: "Give me a lever long enough and a fulcrum on which to place it, and I shall move the world".
There are two kinds of levers depending on the fulcrum: einseitige und zweiseitige Hebel.
Exercise:Discuss the force diagrams on page 18.
A. Jørgensen Energy 9. Juli 2015 3 / 8
Levers
As your book says: "Hebel sind Kraftwandler". They strengthens or weakens the applied force depending on length of the lever. For two sided levers (see book page 19)
F2=F1
l1
l2. (2)
Exercise:You want to lift an elephant using your own weight and a lever. The weight of the elephant is 5 metric tons. Calculate l1/l2.
Pulleys
There are two types of pulleys: stationary and loose pulleys. For stationary pulleys
FLift=FPull. (3)
However, for loose pulleys
FLift=1
2FPull. (4)
Exercise:Take a look at page 22 and explain why this is so.
A. Jørgensen Energy 9. Juli 2015 5 / 8
Pulleys
If you havensupporting ropes (each loose pulley has 2)
Flift= 1
nFpull (5)
and the amount of rope that you have to use is
spull=n·slift (6)
Exercise:You want to lift a person of 50 kilos using one stationary pulley and 3 loose pulleys as in the picture on page 23. What force will you have to pull with? How much rope would you need to lift the person 10 m?
Inclined planes
Too lift an object you can reduce the force need using an inclined plane:
Fpull=FG
h
l. (7)
A. Jørgensen Energy 9. Juli 2015 7 / 8
The golden rule
"Bei Kraftwandlern gilt: Was man an Kraft einspart, muss man an Weg zusetzen."
Exercise:See that this applies for levers, pulleys and inclined planes.
