Common misunderstandings and misconceptions ◆
◆
Students often think that, when a parachute opens, the falling person is jerked upwards. This is not true; the illusion comes from seeing films in which the cameraman continues to fall rapidly while the parachutist suddenly falls more slowly. Students are likely to predict that, when the force on an object moving in a circle is removed, it will fly directly outwards; it does not. It follows the tangent to the circle.
Homework ideas ◆ ◆
Workbook exercise 3.6 Falling If students have access to a digital camera (such as in a mobile phone), they could film objects falling under gravity and then watch the film in slow motion to check whether the object is moving at steady speed or accelerating.
Topic 4
Force, mass and acceleration
Coursebook section 3.4 Teaching ideas ◆
◆
◆ ◆
◆ ◆
You could start by demonstrating the experiment (Activity 3.2) which shows the connection between force, mass and acceleration. However, many students find the experiment difficult to understand, so it is better to present the relationship first and then demonstrate it. It is helpful to think of mass as a measure of how difficult it is to start an object moving (i.e. to accelerate it). You could show a number of balls and ask students to put them in order, from easiest to push to hardest. Include some heavy balls such as bowling balls and medicine balls. They should deduce that the force needed to give a particular acceleration is proportional to mass. Now consider an individual ball. Most students will appreciate that, the greater the force applied, the greater the resulting acceleration. Combining these gives F proportional to ma. Explain that, by choosing our units carefully, we have F = ma. Use this equation to show that 1 N = 1 kg m/s2. It is important that students understand that SI units form a coherent set whose relationships are based on underlying physical relationships. Continue with Activity 3.2 F, m and a to show that the relationship (which you can call Newton’s second law) is true. Students can practise using the relationship in Coursebook questions 3.6 to 3.9. (You may wish to go through Worked examples 3.1 and 3.2 with them.)
Common misunderstandings and misconceptions ◆
Students may think that it is weight that makes an object hard to accelerate. However, even where objects are weightless, an object with a large mass is difficult to accelerate. You may be able to show a film taken in the International Space Station which illustrates this.
Homework ideas ◆ ◆
Workbook exercise 3.4 Force, mass and acceleration You could ask students to devise their own problems involving F = ma to test their partners. Original material © Cambridge University Press 2014
© Cambridge University Press 2014 IGCSE Physics
Chapter 3: Teaching ideas
4