# A car moving at 70 km/hr has a mass of 1700 kg. What force is necessary to decelerate it at a rate of 40 cm/s/s?

Apr 19, 2018

#### Answer:

$F = - 680 N$

#### Explanation:

$\text{Force" = "mass"*"acceleration}$
Ref: Newton's 2nd Law commonly written $F = m \cdot a$

The acceleration is -40 cm/s/s. (I added the negative sign to the given deceleration rate because the acceleration is in the direction opposite the velocity.) That data needs to be converted to have units of $\frac{m}{s} ^ 2$.

-40 "cm/s/s" = -40 cancel("cm")/s^2 * (1 m)/(100 cancel(cm)) = -0.4 m/s^2

Plugging our data into $F = m \cdot a$,

$F = 1700 k g \cdot \left(- 0.4 \frac{m}{s} ^ 2\right) = - 680 k g \cdot \frac{m}{s} ^ 2 = - 680 N$

Minus sign indicate that force is in opposite direction.

I hope this helps.