Compound Machines - Student Activity

University of Virginia
Physics Department

Compound Machines

A Physical Science Activity

Materials

Inclined plane, 50-60 cm long, at least 20° incline angle
Spring scale (10 N max)
Small car with wheels, 400-800 g (Suggestion: Wheeled Cart from Frey Scientific Catalog 1-888-222-1332, item number SO4480)
2 pulleys
String, 70 cm long with loops on both ends
Metric ruler or meter stick

Procedure

Have one student hold the spring scale from the top, and another hang the cart from the bottom. Record the force reading from the spring scale (approx. 5 N) on the data chart.
Using the ruler, measure the length and height of the inclined plane. Record this.
Lay the cart on the bottom of the inclined plane, with the spring scale attached to the top. Pull the cart up the plane at a constant speed while another student reads the force from the spring scale. Record this on a data chart.
Construct a two pulley system. Hook one end of the string to the bottom of a pulley. Loop this string through the second pulley, back around the first pulley, and hook it to the end of the spring scale. Hang the cart from the second pulley (see figures below).
While one student holds the hook at the top of the first pulley, another should lift the cart by pulling the spring scale and read the force. Record this on the data chart.
While maintaining the two pulley system, set the cart at the bottom of the inclined plane. One student should hold the top of the first pulley near the top of the inclined plane. Another student should pull the cart using the spring scale and read the force. Record this on a data chart (See figures below).

Data Table

Weight of mass (N)
Height of inclined plane (m)
Length of inclined plane (m)
Force on inclined plane (N)
Force on pulley (N)
Force on pulley-plane system (N)

What is the theoretical mechanical advantage of the pulley? This is calculated by the number of ropes used to lift the cart.
What is the theoretical mechanical advantage of the plane? Divide the length of the plane by the height of the plane.
What is the theoretical mechanical advantage of the compound machine (the plane-pulley system)? This is the product of the individual mechanical advantages.
What force was required to lift the cart originally (in step one)?
What is the experimental mechanical advantage of the plane? Divide the weight of the cart by the force on inclined plane. Compare this to your theoretical advantage calculated above.
What is the experimental mechanical advantage of the pulley? Divide the weight of the cart by the force on the pulley. Compare this number to your theoretical advantage calculated above.
What is the experimental mechanical advantage of the compound machine? Divide the weight of the cart by the force on the plane-pulley system. Compare this number to your theoretical advantage calculated above.