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.