Simple Machines

Outline of this presentation What is a force (review)? What is work (scientifically speaking)? What is power? Simple machines make our work easier by providing a mechanical advantage. We use less effort/do less work to move an object. All simple machines belong to one of two families The inclined plane family and The lever family. There are six simple machines wedge, ramp, screw, lever, wheel and axle, and pulley.

What is Force? Recall from previous study that a force is a: Push Pull Twist Forces are able to cause a change to an object and cause a change in direction, movement or form Forces are measured in Newtons (N) Force vs. Mass On Earth a 1kg mass feels 9.8N of force due to gravity

What is work? In science, the word work has a different meaning than you may be familiar with. The scientific definition of work is: using a force to move an object a distance (when both the force and the motion of the object are in the same direction.)

Work or Not? According to the scientific definition, what is work and what is not? a teacher lecturing to her class a mouse pushing a piece of cheese with its nose across the floor

What’s work? A scientist delivers a speech to an audience of his peers. A body builder lifts 150 kg above his head. A mother carries her baby from room to room. A father pushes a baby in a carriage. A woman carries a 20 kg grocery bag to her car?

What’s work? A scientist delivers a speech to an audience of his peers. No A body builder lifts 150 kg above his head. Yes A mother carries her baby from room to room. No A father pushes a baby in a carriage. Yes A woman carries a 20 kg grocery bag to her car? No

Work = Force x Distance Formula for work The unit of force is Newtons (N) The unit of distance is meters (m) The unit of work is newton-meters (N-m) One newton-meter is equal to one joule So, the unit of work is a joule (J)

W=FD Work = Force x Distance Calculate: If a man pushes a concrete block 10 meters with a force of 20 N, how much work has he done?

W=FD Work = Force x Distance Calculate: If a man pushes a concrete block 10 meters with a force of 20 N, how much work has he done? 200 joules (W = 20N x 10m)

Power Power is the rate at which work is done. Power = Work*/Time *(force x distance) The unit of power is the watt.

Check for Understanding 1.Two physics students, Ben and Bonnie, are in the weightlifting room. Bonnie lifts the 50 kg barbell over her head (approximately 0.60 m) 10 times in one minute; Ben lifts the 50 kg barbell the same distance over his head 10 times in 10 seconds. Which student does the most work? Which student delivers the most power? Explain your answers.

Answer Ben and Bonnie do the same amount of work; they apply the same force to lift the same barbell the same distance above their heads. Yet, Ben is the most powerful since he does the same work in less time. Power and time are inversely proportional.  

2. How much power will it take to lift a 20 Newton object a distance of 10 meters in 5 seconds? Work=Force x Distance Power = Work/Time

Work=Force x Distance Work = 20 x 10 Work = 200 Joules 2. How much power will it take to lift a 20 N object a distance of 10 meters in 5 seconds? Work=Force x Distance Work = 20 x 10 Work = 200 Joules Power = Work/Time Power = 200/5 Power = 40 watts

Mechanical Advantage We know that simple machines make work easier. Mechanical advantage is a measure of how much easier or faster our work has become as a result. Mathematically, this can be calculated as follows: M.A. = Load Force (output) Effort Force (input) OR M.A. = Distance over which effort is applied Distance over which load is moved

Load (R) is the output force which is also the force resisting motion. Levers Levers are one of the basic tools that were presumably first used in prehistoric times. Levers were first described about 260 BC by the ancient Greek mathematician Archimedes (287-212 BC). Effort (E) is the input force which must be supplied by the user or an engine of some kind. Load (R) is the output force which is also the force resisting motion.

Length from Fulcrum to Effort = LE Length from Fulcrum to Load (R) LR Levers Mechanical Advantage (M.A.) Length from Fulcrum to Effort = LE Length from Fulcrum to Load (R) LR M.A. = = Question: what happens to the mechanical advantage as you move the fulcrum closer to the load?

First class lever: Second class lever: Third class lever: Types of levers First class lever: The fulcrum is located in the center of the lever arm and the effort and load are at opposite ends. Example: Seesaw Second class lever: With a second-class lever the weight is located in the middle and the fulcrum and the effort or at opposite ends. Example: Wheelbarrow Third class lever: The effort is applied at the middle of the arm and the weight is held at one end while the fulcrum is at the other end. Example: Tweezers

Levers 1st Class 2nd Class 3rd Class What’s in the middle? “F-L-E is 1-2-3” This is an easy way to remember the different classes of levers.

A wheel & axle can be made from a 2nd or 3rd class lever. Wheel and Axle A wheel & axle can be made from a 2nd or 3rd class lever. R E Wheel Wheel E R Axle Axle M.A. = Radius (L) to Effort (E) = LE Radius (L) to Load (R) LR

M.A. = Radius (L) to Effort (E) = LE Radius (L) to Load (R) LR Wheel and Axle M.A. = Radius (L) to Effort (E) = LE Radius (L) to Load (R) LR Resistance = M.A. * Effort Finds Resistance if the Effort and Mechanical Advantage are known. Extension: Torque is a twisting force. The units for torque are typically ft-lbs or inch-lbs. Torque can be calculated using the formula: Torque = Force * radius

Circumference = Pi * Wheel diameter Wheel and Axle Rotary Motion is the circular motion which occurs when the wheel and axle are rotated about the centerline axis. Usually rotary motion is defined in terms of degrees of revolution. Linear Motion is the straight-line motion which occurs when a wheel rolls along a flat surface. The linear distance traveled when the wheel completes one revolution is equal to the circumference of the wheel. Circumference = Pi * Wheel diameter

A pulley is an adaptation of a wheel and axle. The Pulley A pulley is an adaptation of a wheel and axle. A single pulley simply changes the direction of a force. When two or more pulleys are connected together, they permit a heavy load to be lifted with less force. The trade-off is that the end of the rope must move a greater distance than the load.

M.A. = Total number of strands supporting the load The Pulley M.A. = Total number of strands supporting the load

M.A. = Total number of strands supporting the load The Pulley Equations and Terms: M.A. = Total number of strands supporting the load Finds the Load if the Effort and Mechanical Advantage are known Load = M.A. * Effort

The Pulley 1. Fixed Pulley is defined when a pulley is attached or fixed to a strong member, which will not move. When a fixed pulley is used the force needed to lift a weight does not change. Notice that it takes 100 N of force to lift a 100N mass (no MA). Only the direction of the force applied is altered. Also note there is no distance advantage either (i.e. 10cm moves the mass 10cm)

The Pulley 2.Movable Pulley splits the work in half. The effort needed to lift 100 N weight is 50 N. The mechanical advantage of a movable pulley is 2. Also note that the trade off is that the rope must be pulled twice as far to lift the object the same distance as in #1. (i. e. 20cm to move the mass 10cm)

The Pulley 3 &4. Block and Tackle is a system of three or more pulleys. It reverses the direction of the effort so that a downward pull can be used to lift an object. For number 3, the mechanical advantage is 3 so that 33 pounds of effort is needed to lift an object weighing 100N. (The distance the rope is pulled has tripled.)

The inclined plane is the simplest machine of all the machines. An inclined plane is a flat sloping surface along which an object can be pushed or pulled. An incline plane is used to move an object upward to a higher position.

Inclined Plane

M.A. = Length = L Height H Force = M.A. * E Effort = Force M.A. Inclined Plane Mechanical Advantage (M.A.) Mechanical Advantage for the Incline Plane M.A. = Length = L Height H Force = M.A. * E Finds the Force if the Effort (E) and Mechanical Advantage are known: Effort = Force M.A. This equation is obtained by algebraically manipulating the equation above.

The Wedge During its use, an inclined plane remains stationary, while the wedge moves. With an inclined plane the effort force is applied parallel to the slope of the incline. With a wedge the effort force is applied to the vertical edge (height) incline. M.A. = Length = L Height H

The Wedge

A screw is a combination of two simple machines: an inclined plane The Screw Can be used to change from rotary to straight line (linear) motion. A screw is a combination of two simple machines: an inclined plane a wheel and axle Inclined Plane Wheel and Axel

The Screw Equations and Definitions: Screw Pitch is the distance between two adjacent threads on a screw. The formula to calculate pitch is: Pitch = length measured Number of threads per length measured

Circumference = Pi * Diameter The Screw Equations and Definitions: The Circumference of the screw is calculated using the Geometry formula: Circumference = Pi * Diameter

M.A. = Circumference Pitch The Screw The formula for the Mechanical Advantage of a screw is: M.A. = Circumference Pitch

“Mechanical Ball Shooter” Extension Challenge Problem To create a device that will fire a ball accurately within a given range. Rules Must be able to fire a projectile (to be specified by the instructor) anywhere within 5’ to 15’ operating range (design adjustability into your device!) Must fit within a 1’x1’ footprint (in “collapsed form”) Cannot utilize high-pressure gases or combustible materials Must be constructed primarily out of materials that are provided and found.