Forces and Free-Body Diagrams

1. Forces and Free-Body Diagrams Common Forces Gravity (Fg) • Acts between every two masses in the known universe • Always attractive • On Earth, always acts down on an object (to- wards Earth’s centre)

2. Near the surface of a planetoid for which gravitational field strength is known, weight is calculated with Normal Force (FN) • Exerted on an object by the surface on which it rests • NOT always equal in magnitude to gravity

3. Tension (FT) • A pulling force • Is often exerted by a rope, a string or a cable • Can also be exerted by a rigid object Compression (Fc) • A pushing force • Exerted on/by rigid bodies like girder, ties rods and rafters

4. Can't be exerted on/by ropes, strings, etc. Static Friction (Fs) • Is exerted between surfaces that are in contact with each other and NOT sliding (unless “applied force” is zero) • Will only be great enough to prevent the object(s) from sliding, until applied force is greater than maximum static friction force. Kinetic Friction (Fk) • Is exerted between surfaces that are in contact with each other and ARE sliding relative to each other.

5. Is always less than the maximum force of static friction • Friction ALWAYS opposes the sliding of one object relative to another. Free-Body Diagrams • Used to show ALL the forces acting on an object • Drawn before adding force vectors (to find net force) so that none are missed

6. NEVER include net force!! Examples – Draw FBDs for the following: • A car skidding to a stop (brakes are “locked”) FN Fk Fg

7. A snowboarder going downhill Fk FN Fgy Fg Fgx

8. Batman climbing up a rope (in contact with a wall) with constant speed FT FN Fg

9. When adding force vectors in two dimensions, we add the “x” and “y” components of each force Example: Determine the net force acting on Selma’s barge while in the Sydney harbour, with one tug pulling it with a force of 12500 N [E] and another with a force of 9400 N [S 40º E]. Be sure to draw an FBD. N E Barge 12500 N 40º 9400 N

10. 18542 N θ 7201 N Fnet