Feeling the forces of a lever

- lever arm: length of wood about 1m long
- load: brick
- duct tape to secure brick to the end of lever
- fulcrum: paint roller or object of similar size
- optional: ruler or tape measure

Measuring and graphing the forces of a lever

- metre-long lever arm that is not too heavy e.g. wide plastic metre rule
- fulcrum: strong tube or roll of tape, stabilized with wedges of tape to stop it from rolling
- 500g weight (or 1kg works)
- duct tape or masking tape to attach weight to the lever
- 3kg spring scale (or 5kg scale if using 1kg weight)
- ruler
- worksheet (attached)

Introduce students to the parts of a lever, including standard notation for drawing a lever. The “lever arm” is simple a rigid rod, drawn as a straight line. The “fulcrum” is something that the lever can rest upon and tip back and forth over (also called the “pivot point”), drawn as a triangle under the lever. When a lever is in use, we push on one end and apply a force to the system, called the “Effort”. The lever moves and outputs a force at the other end of the lever arm, called the “Load”.

Tell students that they will be experimenting with levers, and that they will use this notation for any recording of results.

**Feeling the force on a lever**

Before class, set up the a lever with the fulcrum positioned so that students will just about be able to tip the load up.

Then ask them to experiment with changing the position of the fulcrum and see how that changes how much force is needed to lift the load.

They should find that when the fulcrum is near the load/student, they are easy to push it up in the air, but when it is far from the load, it is very hard (if not impossible).

Students can draw what they discover using standard notation:

The lever arm (plank of wood) is drawn as a straight line, and the fulcrum is a triangle under the line in the correct position. Use arrows to show where force is applied (at one end of the see saw - also called the effort), and where the resulting force is felt (under the concrete weight - also called the load).

Ask the students how the height of the ends of the see saw varies as the fulcrum is moved. They can measure the distances for more accurate recording of the results.

Less force over a greater distance (with the fulcrum near to the weight) is an easier way to lift the weight. However, in this case the weight will not move as high.

The amount of work balances: less force over a greater distance (at one end of the lever) balances more force over a smaller distance (at the other end).

**Measuring and graphing the force on a lever**

Demonstrate to students how to set up their lever, while referring to the drawing on worksheet. Tape the 500g weight (the Load) to one end of the meter rule. Tape a spring scale hanging from the other end. Lay the lever on the fulcrum then slide the system to the edge of the desk so that the spring scale hangs down just over the edge of the desk (see photo).

Show students how they will pull down on the spring scale, read off the force (Effort) required to lift the other end of the lever, while also measuring the highest point that the Load end reaches.

Show students the table on the worksheet where they will record their data. Ask them, for each position of the fulcrum, to make a drawing using standard notation showing the approximate position of the fulcrum, and to record the effort (in grams) that the scale reads for each distance (in cm) that the Load is raised by.

Finally, stress that it is important that the spring scale only just hangs over the edge of the desk, enough to take a reading but no more, so that the distance moved is recorded accurately. (If the lever hangs a long way off the desk, the lever can be pulled down past the level of the desk, so raising the other end to a random height.)

Allow students to set up their levers and record their data, while assisting as necessary to get accurate results.

Once students have all recorded data for several fulcrum positions, ask them to graph their data on the graph paper of the worksheet. See photo of a student’s graph is shown in the photo to illustrate that the data will not form a perfect line (as it is real data!), but a line can be drawn through the points with a ruler.

For a class discussion of the results, transcribe one set of data onto the board.

Ask students how the Effort and the Distance are related [as one increases, the other increases]. Ask why zero effort is not at zero height [because of the height of the fulcrum].

(If your lever bends a little with the fulcrum near the Effort and the Load out on a long arm, use the situation to discuss fair testing. If the lever arm bends a little, the distance will not be measured accurately. Hence for the testing to be completely fair, a different lever arm that is both light and strong is needed.)

Now step through the concept of trading force for distance. With a lever to demonstrate, review together at how the fulcrum position affects the distances moved at each end of the lever. Show that when the effort moves over a large distance, the load moves a small distance, and vice versa. The force is also greater on one end and smaller on the other - but opposite to the magnitude of the distance. Force and distance are traded at each end of the lever. Reiterate this important concept: at one end of the lever the force is greater and the distance is smaller, while at the other end the force is smaller and the distance is greater. This is an important concept for understanding how levers work.

Now help students to understand how the trade of force over distance can be useful to us. With a little force at one end of a lever, you can move something heavy at the other end over a small distance. Or, if you have a lot of force at one end of a lever, you can move something lighter over a greater distance at the other end. Tools that we use frequently exploit this concept. (Follow with the household levers activity.)

Turn into a second class lever by taping one end on the ground, lifting the other, and moving the load up and down the plank.

ingridscience afterschool then MacKenzie Elementary with the Scientist in Residence Program for measuring forces on the lever with a spring scale.