Lecture 05-2: Changing Momentum
Sections 5.5-5.7 in Matters & Interactions
Tips for applying the Momentum Principle
1. Describe the problem.
2. Draw a picture.
3. Define the system.
4. Identify objects in the surroundings that interact with the system.
5. Draw a free-body diagram showing the forces.
6. Write the Momentum Principle. Then, write it in component form.
7. Substitute for the net force using your free-body diagram; use the motion to determine
the rate of change in momentum.
8. Solve for the unknown(s) algebraically.
Review of direction cosines:
The magnitude of the force on a volleyball by an athlete’s hand at this
instant is 15 N. Express the force by the hand as a vector.
Example:
At a certain instant, while pushing a 5.0-kg sled on level ice, an athlete
exerts a force on the sled at an angle 20 degrees below the horizontal
as shown below. The sled's rate of change of momentum at this instant
is < 94, 0, 0 > kg m/s2. Assume the friction force by the ice on the sled
is negligible.
1. What is the magnitude of the force on the sled by the person?
2. What is the force on the sled by the ice?
Sections 5.5-5.7 in Matters & Interactions
Tips for applying the Momentum Principle
1. Describe the problem.
2. Draw a picture.
3. Define the system.
4. Identify objects in the surroundings that interact with the system.
5. Draw a free-body diagram showing the forces.
6. Write the Momentum Principle. Then, write it in component form.
7. Substitute for the net force using your free-body diagram; use the motion to determine
the rate of change in momentum.
8. Solve for the unknown(s) algebraically.
Review of direction cosines:
The magnitude of the force on a volleyball by an athlete’s hand at this
instant is 15 N. Express the force by the hand as a vector.
Example:
At a certain instant, while pushing a 5.0-kg sled on level ice, an athlete
exerts a force on the sled at an angle 20 degrees below the horizontal
as shown below. The sled's rate of change of momentum at this instant
is < 94, 0, 0 > kg m/s2. Assume the friction force by the ice on the sled
is negligible.
1. What is the magnitude of the force on the sled by the person?
2. What is the force on the sled by the ice?
