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The POSITION-TIME GRAPH (not drawn to scale) for a cyclist initially moving west on a straight horizontal road is shown below: 1 Why does the position decrease after 60 seconds? 2 Calculate the velocity of the cyclist for the first 30 seconds.

Physics

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3 parts
3.2

2 m⋅s−1 west\text{2 m}\cdot\text{s}^{-1} \text{ west}

3.3

0 m⋅s−2\text{0 m}\cdot\text{s}^{-2}

3.5

85 m\text{85 m}

Step-by-step solution

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Here are the solutions to Question 3.

Part 3.1

Why does the position decrease after 60 seconds?

After 60 seconds, the cyclist is moving back towards the starting point (or in the opposite direction to their initial movement).

Part 3.2

2 m⋅s−1 west\text{2 m}\cdot\text{s}^{-1} \text{ west}

Calculate the velocity of the cyclist for the first 30 seconds.

  1. Identify the initial and final positions and times for the first 30 seconds.

    From the graph:
    At t1=0t_1 = 0 s, position x1=0x_1 = 0 m.
    At t2=30t_2 = 30 s, position x2=60x_2 = 60 m.

  2. Use the formula for velocity (gradient of a position-time graph).

    v=ΔxΔt=x2−x1t2−t1v = \frac{\Delta x}{\Delta t} = \frac{x_2 - x_1}{t_2 - t_1}
  3. Substitute the values and calculate the velocity.

    v=60 m−0 m30 s−0 sv = \frac{60 \text{ m} - 0 \text{ m}}{30 \text{ s} - 0 \text{ s}} v=60 m30 sv = \frac{60 \text{ m}}{30 \text{ s}} v=2 m⋅s−1v = 2 \text{ m}\cdot\text{s}^{-1}

    The cyclist is initially moving west, so the velocity is 2 m⋅s−12 \text{ m}\cdot\text{s}^{-1} west.
    The velocity of the cyclist for the first 30 seconds is 2 m⋅s−1 west\boxed{\text{2 m}\cdot\text{s}^{-1} \text{ west}}.

Part 3.3

0 m⋅s−2\text{0 m}\cdot\text{s}^{-2}

Write down the acceleration of the cyclist for the first 30 seconds.

The acceleration of the cyclist for the first 30 seconds is 0 m⋅s−2\boxed{\text{0 m}\cdot\text{s}^{-2}}.

Part 3.4

Explain the answer to QUESTION 3.3 referring to the shape of the graph.

For the first 30 seconds, the position-time graph is a straight line with a constant positive slope. A constant slope on a position-time graph indicates that the velocity is constant, and therefore the acceleration is zero.

Part 3.5

85 m\text{85 m}

Determine the total distance covered by the cyclist for the entire 70 second period.

  1. Calculate the distance covered in the first segment (0 to 30 seconds).

    The position changes from 0 m to 60 m.
    Distance1=∣60 m−0 m∣=60 m_1 = |60 \text{ m} - 0 \text{ m}| = 60 \text{ m}.

  2. Calculate the distance covered in the second segment (30 to 60 seconds).

    The position remains constant at 60 m.
    Distance2=∣60 m−60 m∣=0 m_2 = |60 \text{ m} - 60 \text{ m}| = 0 \text{ m}.

  3. Calculate the distance covered in the third segment (60 to 70 seconds).

    Given that the magnitude of the velocity is 2.5 m⋅s−12.5 \text{ m}\cdot\text{s}^{-1} and the time interval is 70 s−60 s=10 s70 \text{ s} - 60 \text{ s} = 10 \text{ s}.
    Distance3=magnitude of velocity×Δt_3 = \text{magnitude of velocity} \times \Delta t
    Distance3=2.5 m⋅s−1×10 s=25 m_3 = 2.5 \text{ m}\cdot\text{s}^{-1} \times 10 \text{ s} = 25 \text{ m}.

  4. Calculate the total distance covered.

    Total distance = Distance1_1 + Distance2_2 + Distance3_3
    Total distance = 60 m+0 m+25 m60 \text{ m} + 0 \text{ m} + 25 \text{ m}
    Total distance = 85 m85 \text{ m}.
    The total distance covered by the cyclist for the entire 70 second period is 85 m\boxed{\text{85 m}}.

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