Imbalance and Unequal Gears
project 3 of 4
The following lesson is part of a series that should be explored in a sequential manner. Students will need the full exploration each concept in order to move from one phase of Levers, Arms and Fulcrum Points to the next. It is more than just play at this level. The purpose of these exercises is to take the concept of play into the field of science. It may not be easy, but it’s certainly will be fun!
New Understanding of Mechanical Advantage
Ask students to build a fulcrum point and balance a long plank as they have previously done in the last project plan. Then ask students to add a weight to one side of the balance beam. Have students then move the plank along the fulcrum point until it is balanced. Allow time for students to play and experiment with this and to test different weights.
Challenges for Students
HINT: Students will find that the shorter one side of the beam is, the more weight it can hold. Use this discovery to explain the principle of lever arms:
A lever arm is measured from the end of the arm to the fulcrum point
Short lever arms can be used to apply more force or hold more weight
The trade-off is force for distance. The short arm can lift heavier things, but it can’t lift them as far as the long arm
Give examples of levers like scissors or wheelbarrows! Can they find the lever arms and fulcrum points? Note that on a wheelbarrow the fulcrum point is not in the middle, but on the end! Can they figure out what part of the wheelbarrow is the fulcrum point? Where are the lever arms?
Have students experiment with this and have then make a wheelbarrow.
HINT: A wheel spinning around an axle on the end of a long plank works well for this, but see what other solutions they come up with!
It may be worth spending a whole session exploring and playing with lever arms before expanding these concepts to gears of different sizes.
After ample exploration with levers, explain to students that they will now experiment with different size gears. Have students find two gears of different sizes that mesh together. Then have them draw the lever arms on the gears by drawing a straight line from the center of a gear to the middle of a gear tooth.
This allows students to visualize the lever arm as they keep track of the number of rotations the gears make.
HINT: Notice the length of the lever arm on the small gear compared to the lever arm on the big gear. As students begin to see the the two different sized gears rotate, they will see that the smaller gear is rotating at a faster speed. This is a good time to interrupt exploration to talk about what is happening.
HINT: Ask students what they notice about the rate at which the smaller gear is rotating compared to the rate of the larger gear. Students will discover that the small gear is rotating faster. This is because it is being driven by the long lever arm of the large gear. Explain that this is similar to the lever arms on the balance beam. The large gear can move a greater distance at the end of its long lever arm. This greater distance applies more speed to the small gear.
The number of rotations will be counted later in phase 3 as gear ratios are explained.
By applying this same model but then switching the perspective students will discover that if the small gear is driving the larger gear it will apply more force.
HINT: This is a result of the stronger short lever arm on the small gear pushing the teeth on the large gear. It can drive larger gears with more force, but they will turn more slowly.
Now that students have had this experience of having a larger gear drive a smaller gear and vice versa, they can compare this to how the short lever arm on the balance beam was able to hold more weight. This is mechanical advantage in action.
Real world example:
While speed is easy to visualize, the concept of force may be harder to grasp. A great practical example is a bicycle. Have any students noticed that it can be hard to pedal up a hill on a bicycle, but shifting to a lower gear makes it easier? Or that using a low gear on flat ground is slower and does not get you as far as using a high gear?
That is because bicycles have gears too! They are actually called sprockets because they are a little bit different. Notice that the sprockets on a bicycle move in the same direction as the pedal turns the back wheel. That is because they are connected with a chain in the same way pulleys are connected with belts. The sprockets work like pulleys but with the positive drive of gear teeth that do not slip.
Lower gears on a bicycle have a smaller sprocket connected to the pedal which is linked to a larger sprocket on the wheel. The opposite is true in a higher gear. There may even be a setting where the pedal moves at the same speed as the wheel! If possible bring a bicycle into the classroom to demonstrate. Set it upside down on top of a table and change gears while turning the pedal. Watch the rate of the pedal and compare to a reflector on the spokes of the back wheel. If there is no reflector, use a piece of tape or a sticker on the side of the back tire.