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# A motor-car is uniformity retarded and brought to rest from a speed of 90km/h in ...

### Question

A motor-car is uniformity retarded and brought to rest from a speed of 90km/h in 10s. Find its acceleration.

### Options

A) 2.5 ms$$^{-1}$$

B) 2.5 ms$$^{-2}$$

C) -2.5 ms$$^{-2}$$

D) -2.5 ms$$^{-1}$$

### Explanation:

$$u = \cfrac{90\text{km}}{h} = \cfrac{90000}{3600}\text{ms}^{-1} = 25\text{ms}^{-1}$$
$$t=10\text{s}$$
$$v=u+at$$
$$0=25+a(10)$$
$$-25=10a$$
$$a=\cfrac{-25}{10}=-2.5$$m/s$$^2$$
Acceleration = -(-2.5m/s$$^2$$) = 2.5m/s$$^2$$

## Dicussion (1)

• $$u = \cfrac{90\text{km}}{h} = \cfrac{90000}{3600}\text{ms}^{-1} = 25\text{ms}^{-1}$$
$$t=10\text{s}$$
$$v=u+at$$
$$0=25+a(10)$$
$$-25=10a$$
$$a=\cfrac{-25}{10}=-2.5$$m/s$$^2$$
Acceleration = -(-2.5m/s$$^2$$) = 2.5m/s$$^2$$