Episode 214: Work done by a force
Lesson for 16-19
- Activity time 65 minutes
- Level Advanced
In this episode, students have to learn to think of force as a mechanism by which energy is transferred from one body to another. This only occurs when the force moves in the direction of the force.
- Discussion: Introductory discussion (10 minutes)
- Student experiment: Efficiency of a ramp (25 minutes)
- Student questions: Calculations of work done (30 minutes)
Discussion: Introductory discussion
For some students this will be a revision session. Others may be less familiar with the concepts. Use questions and answers to establish the knowledge of the group.
We say that work is done by a force when the object concerned moves in the direction of the force, and the force thereby transfers energy from one object to another. You can be a typical physics teacher and use a board rubber to illustrate your point about energy gain. Alternatively, use some other more interesting object such as a model Thunderbird One. The following questions should allow you to draw out the ideas used in the experiment which follows:
When you raise a mass (board rubber etc), how is the energy transferred stored? (Gravitationally)
What is the force acting against the movement? (mg) We say that the lifting force is doing work against gravity. Some students may feel that it takes a larger force than mg to raise the object; however, if the object is raised at a steady speed, it is in equilibrium and the lifting force will just balance weight. This of course ignores any air resistance etc
What is g? (You are looking for gravitational field strength here, not acceleration due to gravity; you may need to draw this point out.)
What two factors will determine how much energy you lose (and hence the energy transferred to the object)? (The size of the force and the distance h moved in the direction of the force.)
Can you state the equation which gives the energy gained? (mgh)
Hence how much energy is required to lift object height h? (mgh)
This suggests that energy transferred = force × distance moved in direction of force. This is known as the work done by the force. In fact, we have used the fact that students are familiar with the equation Eg = mgh to lead them to the more general equation for work done. You should now point out that mgh is simply a particular case of work done.
You can now push the board rubber or thunderbird across the lab bench to show work being done against a frictional force. In this case, the energy transferred ends up in stored thermally as the bench surface has warmed up. A relatively cheap FLIR camera will show the warmer surface.
Student experiment: Efficiency of a ramp
You may want to show the apparatus set up before the students attempt this experiment and show them how to get the block moving smoothly. The values of the masses to overcome friction depend on the apparatus you are using and therefore the experiment needs to be trialled before the lesson. Using a pulley system should allow the students to get reasonably consistent results.
This simple experiment is made more interesting by revising the resolution of forces and considering the total work done to be the sum of work against gravity and work against friction. This concept is reiterated in the questions (below).
Student questions: Calculations of work done
You may choose to use some of these in the class as discussion questions, or as worked examples.