Lecture 05-1: Solving For Unknown Forces
Sections 5.1-5.4 in Matters & Interactions
Outline:
• Problem-solving strategy
• Applying the Momentum Principle and solve for unknown Forces
• Special case: constant velocity
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
Example:
A load of 190 kg is supported motionless above the ground by two ropes. Rope 1
exerts a force of ⟨−300, 500, 0⟩ N on the load. What is the force exerted by rope 2
on the load? Start from a fundamental principle.
Sections 5.1-5.4 in Matters & Interactions
Outline:
• Problem-solving strategy
• Applying the Momentum Principle and solve for unknown Forces
• Special case: constant velocity
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
Example:
A load of 190 kg is supported motionless above the ground by two ropes. Rope 1
exerts a force of ⟨−300, 500, 0⟩ N on the load. What is the force exerted by rope 2
on the load? Start from a fundamental principle.
