Examples

Example: Predicting the final momentum & velocity using the Momentum Principle

Henrik Zetterberg is passing a hockey puck at a Red Wings practice. From video of the pass, you can determine the stick was in contact with the puck for $0.05s$. You estimate the force with which “Zäta” passes the puck is about a tenth of his weight, so $100N$. Determine how fast the puck leaves Zäta’s stick.

Facts

• The puck experiences several forces including
  • the gravitational force (directly downward)
  • the force of the stick
  • the force due to the ice (upward)
  • some frictional forces and air resistance

Lacking

• The mass of an NHL regulation hockey puck is unknown but can be found online ($m_{puck} = 0.17kg$).

Approximations & Assumptions

• Over the time interval the puck is in contact with the stick, the frictional forces are negligible.
• The puck is contact with the stick for $0.05s$.
• The force the stick exerts on the puck is roughly constant over the $0.05s$ time interval.
• The force the stick exerts is $100N$, and can be considered to act in a single direction.
• The puck starts from rest.

Representations

• The free-body diagram for this situation is given by the diagram below.

• The final momentum of the puck is given by the update form of the Momentum Principle: ${\overset{\rightarrow}{p}}_{f} = {\overset{\rightarrow}{p}}_{i} + {\overset{\rightarrow}{F}}_{net}\Delta t$.

Solution

Given the approximations and assumptions above, you can write the update form of the momentum principle for this question,

because the puck starts from rest. So that,

which we can consider in one dimension,