Lecture 05-3: Solving for Unknown Forces
Sections 5.8-5.10 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.
Parallel and perpendicular components of dp/dt:
Dot Product:
Example:
At a certain instant, a planet has a momentum < 9.86e+28, −3.91e+28, 0 > kg m/s, and the net
force on the planet is < 5.12e+21, 4.76e+21, 0 > N.
1. What are the parallel and perpendicular components of the net force on the
planet?
2. Is the planet slowing down or speeding up?
Sections 5.8-5.10 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.
Parallel and perpendicular components of dp/dt:
Dot Product:
Example:
At a certain instant, a planet has a momentum < 9.86e+28, −3.91e+28, 0 > kg m/s, and the net
force on the planet is < 5.12e+21, 4.76e+21, 0 > N.
1. What are the parallel and perpendicular components of the net force on the
planet?
2. Is the planet slowing down or speeding up?
