Forces and FreeBody Diagrams Common Forces Gravity Fg
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 (towards Earth’s centre)
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
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
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.
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
NEVER include net force!! Examples – Draw FBDs for the following: A car skidding to a stop (brakes are “locked”) FN Fk Fg
A snowboarder going downhill Fk FN Fgy Fg Fgx
Batman climbing up a rope (the rope is angled slightly towards the wall and his feet are in contact with the building) with constant speed FT Fs FN Fg
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 N to draw an FBD. E Barge 12500 N 40º 9400 N
18542 N θ Fnet 7201 N
- Slides: 11