The graph shows how the velocity of a small car changes with time - Edexcel - GCSE Physics - Question 3 - 2013 - Paper 1

The estimated velocity of the car at three seconds is 10 m/s.

To calculate the acceleration, we can use the formula:

$a = \frac{\Delta v}{\Delta t}$
From the graph, the change in velocity ($\Delta v$) from 0 m/s to 10 m/s over a time interval ($\Delta t$) of 2 seconds gives us:

$a = \frac{10 m/s - 0 m/s}{2 s} = 5 m/s²$.

The units of acceleration are meters per second squared (m/s²) because acceleration measures the change in velocity (in meters per second) over time (in seconds). Therefore, we have the unit of velocity divided by the unit of time, resulting in m/s².

When the car is moving at a constant velocity of 20 m/s for 3 seconds, the distance traveled can be calculated using the formula:

$ext{distance} = ext{velocity} \times ext{time}$

Thus,

$ext{distance} = 20 m/s \times 3 s = 60 m.$
When slowing down, the car decelerates to a stop from 20 m/s over 2 seconds. The average velocity while slowing down is:

$\text{average velocity} = \frac{\text{initial velocity} + \text{final velocity}}{2} = \frac{20 m/s + 0 m/s}{2} = 10 m/s.$
The distance for this period is then:

$\text{distance} = ext{average velocity} \times ext{time} = 10 m/s \times 2 s = 20 m.$

Thus, the car travels 60 m at constant velocity and only 20 m when slowing down.