Question

A sled and rider, gliding over horizontal, frictionless ice at 4.7 m/s , have a combined mass of 80 kg . The sled then slides over a rough spot in the ice, slowing down to 3.4 m/s . What impulse was delivered to the sled by the friction force from the rough spot? Express your answer to two significant figures and include the appropriate units.

Answer 1

Impulse is the change in momentum.
All we need is the momentum before and after.

Momentum  =  (mass) x (speed)

Before:        =  (80 kg) x (4.7 m/s)  =  376 kg-m/s

After:           =  (80 kg) x (3.4 m/s)  =  272 kg-m/s

Change in momentum  =  (376 - 272) =  104 kg-m/s


Notice that when you work with impulse, you're USUALLY calculating

                           (force) x (time).

The force is in Newtons, and the time is in seconds,
so the impulse is usually in units of

                             Newton-seconds .

So how can I say that impulse and change of momentum are
the same thing ?  Am I trying to pull a fast one on you ?

Remember that    1 Newton is  1 kg-m/s²

So                       (force) x (time) 

                     =  (Newton) x (second) 

                     =  (kg-m/s²) x (second)  =  kg-m/s .

and  Momentum = (mass) x (speed)  =  (kg) x (m/s)

The units of impulse are the same as the units of momentum !

So when you give an object some impulse, you give it exactly
that much momentum.