# A car is backing up at 3 m/s sees a dog and slams on his brakes to stop over a period of 2 seconds. Is this negative or positive acceleration?

May 31, 2018

Nice question, mean teacher!

#### Explanation:

The car would normally be said to have negative acceleration, but the answer really depends on what has been chosen as a positive direction for the displacement.

This is normally the direction in which it is moving (hence displacement grows more positive over time, even if you change direction and go into reverse.) Now here we are not told which direction has been nominated as positive, so we are left to assume it is the direction in which the car is moving.

I often ask students about a similar, but simpler case: that of a bus slowing towards a set of lights - in which direction would it be accelerating? After some thought they come to the conclusion the bus is accelerating backwards whilst still moving forwards.

Hope this helps (there isnt a fixed answer, you would need to justify it by stating your assumptions in either case!)