4.3 Identifying Forces in Physics Problems

Identifying Forces

• Determine all forces acting on an object.

• Avoid including non-existent forces.

Common Forces and Notations

• General Force: $\vec{F}$

• Weight: $\vec{w}$

• Spring Force: $\vec{F}_{sp}$

• Tension: $\vec{T}$

• Normal Force: $\vec{n}$

• Static Friction: $\vec{f}_s$

• Kinetic Friction: $\vec{f}_k$

• Drag: $\vec{D}$

• Thrust: $\vec{F}_{thrust}$

• Other specific labels (e.g., $\vec{F}_{hand}$) may be used.

Tactic for Identifying Forces

1. Identify the object of interest: The object whose motion is being studied.

2. Draw a picture: Show the object and all touching objects (ropes, springs, surfaces).

3. Draw a closed curve: Around only the object of interest.

4. Locate contact points: Points where other objects touch the object of interest on the curve boundary.

5. Name and label each contact force: At least one force per contact point; use subscripts if needed.

6. Name and label each long-range force: For now, weight ($\vec{w}$) is the only one considered.

Example Applications

• Bungee Jumper:

  • Contact Forces: Tension ($\vec{T}$) from the cord.

  • Long-Range Forces: Weight ($\vec{w}$).

• Skier (Particle Model):

  • Contact Forces: Tension ($\vec{T}$) from the rope, Normal force ($\vec{n}$) and Kinetic Friction ($\vec{f}_k$) from the ground.

  • Long-Range Forces: Weight ($\vec{w}$).

• Rocket:

  • Contact Forces: Drag ($\vec{D}$) from air resistance, Thrust ($\vec{F}_{thrust}$).

  • Long-Range Forces: Weight ($\vec{w}$).