A ball on a 1.4-meter long string is being whirled in mid-air in a horizontal circle at a constant speed v. The tension in the string is 100 N. The mass of the ball is 3.70 kg.

Question: What are the forces acting on the system?

Answer: Use DID TASC

Step 1: Draw diagram and coordinate system.

Step 2: Isolate the system.

Step 3: Draw all forces acting on the system:

• Gravity, mg, directed down;
• Tension, pointing away from the ball along the string.

Anything else?

1. Normal force, directed vertically up.
2. Centripetal force, directed toward the center of the circle.
3. Centripetal force, directed away from the center of the circle.
4. None of the above.