At a vehicle-testing centre, a student records how an electric delivery van stops on a dry test lane. The van has a mass of 0.80 tonne. A pressure pad sends...
Question 1 Report
At a vehicle-testing centre, a student records how an electric delivery van stops on a dry test lane. The van has a mass of 0.80 tonne. A pressure pad sends a signal when the driver applies the brakes. Fig. 1 shows the measured distance from this pad to the final position of the van. Table 1 gives results from repeated runs. The braking force changes the kinetic energy of the moving van into thermal energy in the brakes and tyres.
Fig. 1
| Speed before braking / m s-1 | Braking distance / m |
|---|---|
| 8.0 | 7.0 |
| 12.0 | 15.5 |
| 16.0 | 27.0 |
Table 1
(a) State the velocity of the van when it is at the position marked “van at rest” in Fig. 1. [1]
(b) Calculate the increase in braking distance when the speed before braking changes from 8.0 m s-1 to 16.0 m s-1. [2]
(c) Explain why rainwater on the test lane would make the braking distance greater, even when the brakes apply the same force. [3]
(d) Use v2 = u2 + 2as to calculate the deceleration of the van in the 16.0 m s-1 run. [3]
Answer Details
(a) At the marked final position, the van is at rest, so its velocity is 0 m s-1. [1]
(b)
\[27.0\,\text{m}-7.0\,\text{m}=20.0\,\text{m}\]
The braking distance increases by 20.0 m. [2]
(c) Rainwater reduces friction, or grip, between the tyres and road. This reduces the available braking force. The van therefore has a smaller deceleration and travels further before stopping. [3]
(d) Use \(v^2=u^2+2as\), with \(v=0\), \(u=16.0\,\text{m s}^{-1}\), and \(s=27.0\,\text{m}\):
\[0=16.0^2+2\times a\times27.0\]
\[a=-\frac{256}{54}=-4.74\,\text{m s}^{-2}\]
The acceleration is \(-4.74\,\text{m s}^{-2}\), equivalently the deceleration is 4.74 m s-2. [3]
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