3 CHANGING THE MOVEMENT OF AN OBJECT REQUIRES A NET FORCE TO BE ACTING ON IT

Task 3

WORKSHEET

Materials and tools for a working group:

dynamometer, chair, board (inclined plane), bucket

As pupils have a tool to measure the force (the dynamometer they used in the previous task), it is possible to start exploring how individual simple machines work. First of all, it will be the task of the pupils to find out whether we use less or equal force to pull the load on the chair by using an inclined plane, compared to a situation where we do not use the inclined plane. For comparison, they will use a dynamometer or a teacher can lead them to design their own procedure. In the first case, the teacher can follow the task 3, in which he leads the pupils to make predictions as to whether by pulling on the inclined plane they pull a bucket using less effort comparing direct lifting the bucket on the chair. The situations are shown so pupils can imagine the way in which they are going to compare the situations using dynamometer. In this case, making predictions is easier.

Very important is the discussion with pupils on how to determine whether or not the prediction is confirmed. This is to determine whether pupils are aware of how to use a dynamometer to compare the effort required to pull a load on an inclined plane and without using it. As the teacher suggests the procedure, it is important to find out whether the proposed process of verifying predictions is understandable to pupils. The goal of the task is not only to verify whether the inclined plane reduces the effort to pull the load up, but also to make pupils realize, by way of example, how to use a force measuring tool to examine commonly observed phenomena.

After making the predictions, the teacher guides the pupils to verify them and record the observation result (verification of prediction) in the relevant part of the table. They will evaluate the predictions by comparing them with the result of the observation, emphasizing at the end what the conclusion can be made.

To make a proper conclusion it is suitable to discuss the force we use to pick the load up. In the pictures pupils can mark with a green arrow to indicate which direction they forced on load. At the same picture, they mark a red arrow to indicate in which direction it is pulled down due to the force of gravity. Despite the fact that at this age we do not expect children to understand the net forces, it is appropriate to lead them to realize that when pulling on an inclined plane, we force the load in a different direction compared to direct pulling of the bucket on the chair.

In addition to the action of gravitational force, pupils can also try to mark the friction force. In previous tasks, they have found that friction can be reduced, and thus, in this task, they can investigate whether it is possible to reduce the effort required to pull the load upwards by reducing friction when moving on an inclined plane. It is good to highlight the direction of the forces acting on moving objects in all of the following tasks, so as to gradually create the correct notion that different forces are constantly acting on the objects and the objects change their movement only if these forces are in imbalance.