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Humans’ Intelligent Inventions with Simple Physics Tricks

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David Lee


AP Physics C

26 November 2015

Humans’ intelligent inventions with simple physics tricks


In this lab, we used devices called simple machines that change the amount of force or direction of force need to do work. The simple machines that we used were pulleys, levers, and wheel and axle. Each machines use different kinds of physics as in changing the distance of work done with different directions, using equilibrium of torques, or requiring both.

The purpose of this lab was to understand the concept of torque’s application to the simple machines. Specifically, the objective was to calculate the mechanical advantage of pulleys and wheel and axle, finding the equilibrium of torques in levers, and finding the mass of the ruler by using moments.

Mechanical advantage is the measure of the advantage gained by the use of a tool, specifically in this lab, the simple machines. In order to get the value of the Mechanical Advantage, you must use the formula

MA=Weight of LoadApplied Force(A) If the value of the MA is 1, then it means that no mechanical advantage was gained.

Pulley is one of the types of the simple machines that uses the mechanical advantage in work. The formula of work is W=Fd(B) and pulleys larger the value of d to make the same value of W with a small value of F which is the force applied by us.

Lever is also one of the types of the simple machines but it uses the equilibrium of torques in clockwise and counterclockwise. Levers can obtain the multiplication of force but not energy. The multiplication of force can occur by the change in distance of the forced applied from the pivot. The conservation of energy show that the work input should equal the work output which can be shown in the lever as in the work clockwise is equal to the work counterclockwise as shown in the formula FeLe=FrLr(C).

Wheel and Axle is the last type of simple machine used in this lab which can be zexplained as a combination of lever and pulley. How the wheel and axle works is very similar to the lever because the lever arm influences the strength of the force. The longer the lever arm, the stronger the force will be. The operation of a pulley can be shown in the equilibrium of torques of the wheel and axle acting like pulleys. The formula for the wheel and axle combines the formulas (B) and (C) which is Fr2r=Fe2R.

Fr2r is the counterclockwise work done by the smaller wheel and the Fe2Ris the clockwise work done by the bigger wheel. When these two work reach equilibrium, the value of Fr and Fe can be seen with the proportion with the value of r and R. Therefore a final equation can be concluded with FrFe=Rr=MA(D). The greater value of the R, the greater value of the mechanical advantage is shown in the equation.

Moment is when the system reached balance or equilibrium. In a lever, the moments can be calculated by calculating the counterclockwise torque and the clockwise torque. When the two values are equal, the moment is found. In order to find the two values, you must multiply the force applied by the distance of the force from the pivot. The picture below shows how the mass of the ruler can be found by using the moment of the system. The counterclockwise torque is the value of the mass of the ruler times the distance of the original center of mass of the ruler from the pivot. This makes sense because the mass of the ruler is trying to even out the force applied by the weight of the mass attached. The formula for this system can be written as Wp=Rq (E) where w is the weight of the mass attached, p is the distance of the mass from the pivot, q is the distance of the center of mass from the pivot and R is the weight of the ruler.

Methods(Materials underlined)


Make the three pulley configurations as shown in the picture

Add a mass of 0.5kg to the pulleys and record the force required to lift each weight using the forcemeter.

You must have the forcemeter attached to the side without the mass attached.

Calculate the mechanical advantage by using the formula (A)

Balance Beam

Have a balance beam ready

Attach the balance beam to a clamp and stand

Have the mass ready as in the configurations

Arrange the masses until the beam is balanced (Make sure you only have 1 mass in each hole maximum)

Record the locations where the masses are.


Configuration 1: 50g and 20g

Configuration 2: 100g and 2 x 20g

Configuration 3: 100g, 50g, and 2 x 20g

Wheel and Axle

Have the wheel and axle ready

Attach the wheel and axle to a clamp and stand

Add different sizes of masses to the small hooks at different wheel sizes

have the forcemeter attached to the place as in the table and measure the force needed to lift

Rearrange the configurations to complete the table.

Calculate the Mechanical Advantage by using the formula (A)

Finding the mass of the ruler

The specific rulers are set up with masses attached at different places

Try to balance each rulers and find the center of mass

Record the weight of the mass attached to the ruler and where the weight is attached and where the center of mass is.

Plot the graph with these x and y values

y: weight of mass attached x distance from the center of mass to the mass x: distance from the pivot to the original center of mass

The gradient will give you the mass of the ruler as shown in the equation (E)


50g on 5cm

50g on 30cm, 50g on 80cm

2 x 50g on 15cm



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