A bobsled has a momentum of 1500 kg*m/s to the south. Air resistance reduces its momentum to 750 kg*m/s to the south. What impulse is applied to the bobsled by the air resistance?

The impulse is exactly the change in momentum,

and they have the same units.

The impulse imparted to the bobsled by the air

resistance is

(750 - 1500) = - 750 kg-m/s.

It's negative because it reduced the bobsled's momentum.

If the impulse had come from a rocket engine on the back of

the bobsled, then it would have been a positive impulse.