Simple machines and Mechanical Advantage

Transcription

1 Simple machines and Mechanical Advantage Machines cannot create work. They can however make it easier to do work. Most notably, machines make an exchange to be able to make it easier in one of two ways. 1. Machines can exchange the effort moving an object a greater distance at a reduced force to producing a greater force over a smaller distance. So increased distance is exchanged for increased force. Example: A jack is a machine that makes such an exchange. You move a handle a foot or more at a reduced force so that the jack can move upward an inch or so with enough force to lift a car off the ground. This machine makes it possible to move a very heavy object with a manageable force applied over a larger distance. 2. Machines can change the direction that a force is applied and thereby give an advantage to the user.

2 Example: A flathead screwdriver is often used to open a paint can. The screwdriver acts as a lever that is balanced on the side of the rim and the tip of the screwdriver pushes on the lid. When the screwdriver is pushed down, the tip goes up and is able to remove the lid. Think about how much better this is than trying to pry off paint can lids with your fingernails! Your down ward effort force is transferred by the lever into an upward force. In your previous science classes, you would have learned about the six basic simple machines: the lever, the pulley, the wheel & axle, the incline plane, the wedge and the screw. Our standards in physical science deal with how simple machines can provide mechanical advantage. Mechanical Advantage The number of times that a machine multiplies the effort force. Unlike most things in physical science, mechanical advantage does not have a unit that goes with the number value. Units in this case cancel out and leave us with a number that tells how many times the machine multiplies the incoming force. To really understand mechanical advantage, you need to understand some basic terms:

3 Effort force (N) this is the force that the user puts into a machine. Resistance force (N) this is the force that the machine provides as an output force. Effort distance (m or other distance units) this is the distance that is the put in to the machine by the user (input). Resistance distance (m or other distance units) this is the distance that the machine moves something (output). FORMULAS Mechanical Advantage = effort distance/resistance distance Use this formula when given distances (m, etc.) in machines. Mechanical Advantage = resistance force/effort force Use this formula when given forces (N) in machines.

4 Examples: MA of Incline plane (ramp) with distance Effort distance is what is put into the machine by moving an object up the ramp. Resistance distance is the height the ramp is raising the object up (against gravity). MA = MA of Incline plane using with force Resistance force used against gravity (upward). Effort force is what the user does to move up the ramp. If resistance force is 450 N and effort force is 200 N, what is the MA?

5 MA of lever with distance Effort is the arm pushed down (should be longer) Resistance is the arm lifting the load What is the MA for distance? MA of lever with force Resistance is the force the machine puts out to lift the object. Effort is the force the user puts into the machine. What is the MA for force?

6 MA of pulley system This is a special situation. The length of rope outside the pulley system is where effort is applied. The number of rope segments inside the pulley system that are used to lift the load is the resistance. The basic rule here is to count the rope segments that are directly used inside the pulley system to find the mechanical advantage. Pulley system 1 has one rope segment supporting the weight after the pulley, so we would say the mechanical advantage is. Pulley systems with a mechanical advantage of only change the direction of force. Pulley system 2 has two rope segments after the top pulley so a MA of. Pulley system 3 has three ropes segments after the top pulley so a MA of. Pulley system 4 has four rope segments after the top pulley so a MA of.