More Simple Mechanisms Flashcards
what is a mechanism
a set of moving parts that uses movement and force to do work.
what are the kinds of movements that get mechanisms to work
linear movement - movement in a straight line (opening a door)
rotary movement - turning movement (turning on a tap)
reciprocating movement - up and down movement (bicycle pump)
oscillating movement - backwards and forwards movement (a clock’s pendulum)
how do mechanisms make work easier and what do we call this
by changing the size of the force, or the size and type of movement. we call this mechanical advantage (MA)
why do levers give us mechanical advantage
the increase the size of a force to lift heavy loads or increase the distance moved.
name two other simple mechanisms (not the lever) and what movement do they use to give us MA
the crank and the pulley use rotary motion to give us MA
describe what a crank is, how it works and the MA of cranks
what it is:
it is a simple mechanism that consists of an arm or lever joined to one end of a rotating shaft. the other end of the arm usually has a handle / pedal. they are used to lift / move objects by changing rotary movement into linear movement
How does it work? it's an adaptation of a second-class lever that is attached at one end to and axle and so that it can pivot in a complete circle about the shaft. at the opposite end to the pivot is a pin at 90 degrees to the lever. this works as a handle or pedal to turn the crank.
by turning the handle you increase the turning force of the shaft, which gives MA. you can see this in a winch.
a winch consists of a crank which turns a barrel or drum that has a length of rope or cable wound around it. when you turn the crank handle, the drum turns and winds up the rope. this makes it easy to move a load attached to the end of the rope - e.g. drawing bucket from well. winch changes rotary movement to linear movement. (see diagram P 120)
MA of Cranks:
crank provides MA like other machines. longer the crank the easier to turn axle although a greater distance needed to turn it. because length of input arm is long, crank lever increases the turning force with distance, increasing MA (MA > 1)
describe a pulley and the different types of them
pulley = a simple mechanism consisting of a grooved wheel that turns on an axle, with a cord / belt running through the groove. it’s used to lift weights
when you pull on rope, wheel turns. rope runs through groove and lifts object at other end of rope. this changes the direction of force making object easier to lift.
Single pulleys:
has one pulley wheel. the pulley is attached to a support such as a beam or a frame and is called a fixed pulley. a single fixed pulley makes it much easier to lift a load, because you can use force of gravity and your bodyweight to pull the load upwards
single pulley doesn’t give any MA:
- amount of effort needed to lift object = to load
- to raise load certain height, pull rope down equal distance. E.g. to raise flag 5 m, you need to pull distance of 5 m.
So MA = 1, but there's change in direction. the pulley acts like 2nd-class lever because fulcrum is where rope moves through pulley and load lies between the fulcrum and effort. (see diagram P 123)
Compound pulleys:
made up of 2 / more pulley wheels. when use compound pulley, less effort needed to lift load. in compound, 1 pulley usually fixed and other/s usually moveable. moveable pulley moves with load. when you pull rope, both pulley + load come up.
in double pulley system, 1 fixed combined with 1 moveable pulley. reduces effort by half. means that takes half effort lift load. because rope 2x longer, have to pull 2x far. MA equal to number of pulleys used. so double pulley system has MA = 2 (P 124)
compound pulley system, for each extra pulley used, effort decreased, but distance rope pulled increases. E.g if you use 3 pulleys, you use one third of the effort, but you have to pull the rope three times as far
why do levers give us mechanical advantage
the increase the size of a force to lift heavy loads or increase the distance moved.
name two other simple mechanisms (not the lever) and what movement do they use to give us MA
the crank and the pulley use rotary motion to give us MA
