Consider a cannonball launched along the path with length units in feet and time units in seconds. When does it return to the height from which it was launched? (The coordinate system is right-handed, with $z$ the vertical axis.) Answer: $t=$ What is its velocity then? Answer: $\langle$,,$\rangle$ Momentum is mass times speed, usually measured in $\frac{\text{kg m}}{\text{s}}$, but here we can measure it in $\frac{\text{kg ft}}{\text{s}}$. If the cannonball has mass 35 kg, what is its momentum upon impact with the ground? Answer: $\frac{\text{kg ft}}{\text{s}}$