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Define a vector.

During a training session, an athlete runs 120 m East along a straight road (A to B). After a short rest, she runs 80 m West (B to C) and then 60 m West (C to D). The total time for this session took two minutes.

2.2.1 Draw a vector scale diagram to represent the three displacements (A to B; B to C; C to D). Label all the vectors clearly. (Use a scale of [tex]1 \, \text{cm} = 20 \, \text{m}[/tex]) (3)

2.2.2 On the vector scale diagram, draw and indicate the resultant vector for her motion. (1)

2.2.3 Calculate the total distance covered by the athlete. (1)

2.2.4 What is her change in position at D, relative to A? (1)

2.2.5 Calculate the average velocity of the athlete. (4)

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Sagot :

### 2.1 Define a vector

A vector is a mathematical object that has both magnitude and direction. Vectors are often represented as arrows in a coordinate system, where the length of the arrow represents the magnitude and the arrowhead indicates the direction. Common examples of vectors include displacement, velocity, and force.

### 2.2 Training Session Analysis

#### 2.2.1 Draw a vector scale diagram

Considering the scale of [tex]\( 1 \, \text{cm} = 20 \, \text{m} \)[/tex], we can represent the displacements as follows:

- [tex]\( A \)[/tex] to [tex]\( B \)[/tex]: 120 m East → [tex]\( \frac{120 \, \text{m}}{20 \, \text{m/cm}} = 6 \, \text{cm} \)[/tex] East
- [tex]\( B \)[/tex] to [tex]\( C \)[/tex]: 80 m West → [tex]\( \frac{80 \, \text{m}}{20 \, \text{m/cm}} = 4 \, \text{cm} \)[/tex] West
- [tex]\( C \)[/tex] to [tex]\( D \)[/tex]: 60 m West → [tex]\( \frac{60 \, \text{m}}{20 \, \text{m/cm}} = 3 \, \text{cm} \)[/tex] West

To create the diagram:

1. Draw an arrow 6 cm to the right (East) from point [tex]\( A \)[/tex] to [tex]\( B \)[/tex].
2. From point [tex]\( B \)[/tex], draw an arrow 4 cm to the left (West) to point [tex]\( C \)[/tex].
3. From point [tex]\( C \)[/tex], draw another arrow 3 cm to the left (West) to point [tex]\( D \)[/tex].

#### 2.2.2 Draw and indicate the resultant vector

The resultant vector is the net change in position from point [tex]\( A \)[/tex] to point [tex]\( D \)[/tex]:

- Start at point [tex]\( A \)[/tex].
- The resultant vector extends from [tex]\( A \)[/tex] to [tex]\( D \)[/tex], combining all intermediate movements.
- Given the previous vectors, it should be drawn directly from [tex]\( A \)[/tex] to [tex]\( D \)[/tex], indicating the final position relative to the start.

#### 2.2.3 Calculate the total distance covered by the athlete

The total distance covered is the sum of the absolute values of all the segments run, regardless of direction:

[tex]\[ \text{Total Distance} = 120 \, \text{m} + 80 \, \text{m} + 60 \, \text{m} = 260 \, \text{m} \][/tex]

#### 2.2.4 What is her change in position at [tex]\( D \)[/tex], relative to [tex]\( A \)[/tex]?

The net displacement is the sum of the directional movements:

1. [tex]\( A \)[/tex] to [tex]\( B \)[/tex]: 120 m East
2. [tex]\( B \)[/tex] to [tex]\( C \)[/tex]: 80 m West
3. [tex]\( C \)[/tex] to [tex]\( D \)[/tex]: 60 m West

The net displacement:

[tex]\[ 120 \, \text{m} - 80 \, \text{m} - 60 \, \text{m} = 120 \, \text{m} - 140 \, \text{m} = -20 \, \text{m} \][/tex]

This means the athlete ends up 20 m West of her starting position.

#### 2.2.5 Calculate the average velocity of the athlete

Average velocity is defined as the total displacement divided by the total time taken.

1. The total displacement is -20 m (20 m West).
2. The total time taken is 2 minutes, which needs to be converted to seconds for velocity calculation:

[tex]\[ 2 \, \text{minutes} \times 60 \, \text{seconds/minute} = 120 \, \text{seconds} \][/tex]

Average velocity is then calculated as:

[tex]\[ \text{Average Velocity} = \frac{\text{Total Displacement}}{\text{Total Time}} = \frac{-20 \, \text{m}}{120 \, \text{s}} = -\frac{1}{6} \, \text{m/s} \, \approx \, -0.167 \, \text{m/s} \][/tex]

The negative sign indicates the direction is towards the West.

### Summary of Answers

1. Definition of a vector (2 marks)
2. Scale diagram and resultant vector (4 marks)
3. Total distance covered: 260 m (1 mark)
4. Change in position: 20 m West (1 mark)
5. Average velocity: -0.167 m/s (4 marks)

[Total: 12 marks]