PHYSICS FOR THE IB DIPLOMA: TEACHER’S RESOURCE

Starter ideas

1 Describing graphs (10 minutes)

Resources: Various distance–time and/or speed–time graphs

Description and purpose: The activity is suitable for students who have seen some graphs describing motion before. Show students the range of graphs that show the relationship between distance and time, such as the ones in the Coursebook, Figure 1.10 (without the explanations). Ask them to describe what each graph shows about the quantity on the y-axis. Students discuss the graphs in groups. They copy them into their books with explanations. They then review one another’s work. Each student can suggest what is good in the explanation from another student and how it might be improved. You can listen to the discussions to decide whether they really have understood that the slope is velocity and that it can be positive or negative.

2 Graph role-play (5 minutes or more depending on the number of graphs used)

Resources: n/a

Description and purpose: Sketch a displacement–time or velocity–time graph on the board, as in Figure 1.12 in the Coursebook. Invite a student to walk around the room showing the movement indicated by the graph. Repeat for other graphs. You can see from the student’s movement around the room (and from comments from other students) whether they have understood the graph.

Main teaching idea

1 Plotting a displacement–time graph from experimental data (30 minutes)

Resources: Ball and stopwatch or light gates, adjustable ramp, metre rulers

Practical guidance: Students can use a stopwatch or light gates to time a ball rolling down a slope, such as a tennis ball rolling from rest down the gap between two metre rules. They can measure the time taken to roll a certain distance. They can repeat the experiment for different distances. They can produce tables and graphs of the results.

Assessment ideas: Students can write an account of their method. You can assess their tables and graphs. Each student can look at the table and graph of another student. You can ask questions such as the following:

• Does each column of the table have a unit in the column header?

• Does each column have a consistent number of decimal places?

• Does the graph cover more than half the page?

• Does the graph have sensible, linear scales on both axes?

The main aim of the assessment is that each student will learn how to construct a table and draw a graph. You can see which features are being missed and can explain why these features are needed.

Differentiation: More confident students can experiment to see whether a larger or heavier ball travels further in the same time. If less confident students find planning difficult, you can help them. You should remind all students of the good features for a table and a graph.

Reflection: What are the important things to remember when drawing a graph? What are the most important things to remember when constructing a table of results?

You can also ask students how they can find the speed from a distance–time graph. They should realise that this is the gradient of the graph. You might explain the good features of obtaining a gradient from a graph. Students can suggest ways in which they can improve their graphs or remember how to draw a good graph.

Language focus: Be careful when using phrases such as ‘how long it takes’ or ‘length of time’ because some students may interpret this as two quantities: distance (long/length) and time. If this is likely to be confusing, then refer to a ‘period of time’ or ‘time taken’ or ‘number of seconds’.

Plenary ideas

1 Draw what I describe (3 minutes or more depending on number of graphs used)

Resources: n/a

Description and purpose: You can provide sets of displacement–time or distance–time graphs. Ask students to describe each motion. Alternatively, one student sketches a distance–time graph. They come to the front and describe the motion to the class but do not show the graph. All the other students sketch the graph from the description. They compare their graphs with the initial graph.

Assessment ideas: Assessment is part of the activity when they compare their graph with the original one.

Homework ideas

1 Workbook Exercise 1.3

Workbook Exercise 1.3 gives practice at sketching and interpreting graphs of motion.

2 Formula 1 telemetry

Formula 1 racing teams monitor their car’s performance on the track remotely. Part of this is a series of graphs, and one of these is a speed–time graph. Ask students to research this for themselves or given a printed version. Ask them to identify the speed–time graph and to explain why it is not appropriate in this case to have a velocity–time graph. Then ask them to describe the motion of the car from the telemetry. If time is shown, they can calculate acceleration.

1.4 Projectile motion

LEARNING PLAN

Learning objectives

• Learn how to describe the motion of a projectile

• Gain a qualitative understanding of the effects of a fluid resistance force on motion

• Gain an understanding of the concept of terminal speed

Success criteria

• Students can use the equations of kinematics separately in the vertical and horizontal components and understand that these are independent of each other

• Students can, for example, sketch the parabolic path of a projecting when air resistance is neglected, then add the path when air resistance is considered

• Students can explain why, when dropped in a fluid, the acceleration of an object decreases to zero

Common misconceptions

Misconception How to identify How to overcome

Students sometimes have difficulty understanding that the horizontal and vertical motions are independent of one another.

Drop a ball vertically from the edge of a bench. Then ask whether the time taken to reach the ground will be affected if the ball is first rolled across the bench and allowed to roll off the edge. This can be done before or after learning about vertical and horizontal components of motion.

With practice, it is possible to have two identical balls (tennis balls are ideal). One is rolled toward the edge of the bench, and the other is dropped from the same height at the instant the first one passes the edge. Students can then see that they both take the same time to fall. Alternatively, you could ask ‘In which direction does gravity act?’

Students sometimes use the wrong component in calculations.

This will be apparent when calculations are marked. Go back to the rules of trigonometry in a right-angled triangle and ensure students are secure with working out sine and cosine. Then practice resolving velocities in a variety of directions (including vertical and horizontal) into vertical and horizontal components.

Fluid resistance is a type of friction.

Students may class air resistance as a type of friction.

Friction occurs when surfaces rub together. Fluid resistance occurs because of the displacement of the fluid particles and the resulting way that they flow around the moving object.

Starter ideas

1 The monkey and hunter problem (5 minutes)

Resources: n/a

Description and purpose: The activity allows students to think about motion of a projectile. Draw a simple diagram of a monkey hanging from the branch of a tree, and a hunter with a gun pointed horizontally at the monkey, possibly from an elevated position. Ask the question: ‘If the monkey lets go of the branch and falls at the same time as the bullet leaves the gun, what will happen?’

Allow students to discuss the answer (Note: arriving at the correct answer at this stage is not essential). Then explain that, in this topic, students will find the answer to the problem.

2 Components of a vector (10 minutes)

Resources: Measuring tape, calculators

Description and purpose: Walk across a room and ask: ‘I have walked 5 m; how can we find out how far I have walked south or east?’ ‘How can we work out my velocity north and east?’ Students work out for themselves that components are of the form d cos θ and d sin θ. Students may forget these formulae. Asking them to derive them themselves may trigger the process of being able to work the formulae out if they later forget them. Students will show whether they can recall the correct use of trigonometry in a rightangled triangle readily by the speed and accuracy of their answers.