Forces and Free Body Diagrams
Kinematic Forces Force Description Applied Force (FA)
Result of a push, pull or thrust Force of Gravity (FG) Attraction between objects (ex. between an object and Earth) Normal Force (FN) Perpendicular to surface of object. Balancing force. Tension (FT) Exerted by materials that can strech (ex. rope, spring, cable) Friction Resists motion/attempted motion between objects in contact. Acts in opposite direction to motion/attempted motion Static Friction (FS) Prevents stationary object from starting to move Kinetic Friction (FK) Acts against object’s motion Air Resistance (FAIR) Frictional force opposing object’s motion through air
Example 1 Draw a FBD of the following scenarios:
A pen sitting on a table. A rope connected to a crane raising a piano vertically upward at a constant speed. A suitcase being rolled across the floor at a constant velocity with a force applied at 20° above the horizontal. A dresser that is being pulled to the right up a ramp into a delivery truck by a cable parallel to the ramp; the ramp is 14° above the horizontal. Remember the magnitude of the vector is represented by the length of the line
Example 2 Draw three FBDs depicting the forces of an object as it changes from free-fall to terminal velocity.
Fnet = Σ Facting on object
Net Force Often there is more than one force acting on an object. To describe the effect of all the forces acting on an object, physicists determine the net force. Sometimes the net force is referred to as the total or resultant force. Fnet = Σ Facting on object Where ΣF1 = F1 + F2 + F3 + … Where F1 , F2 ,F3 are all the forces acting on a body. Remember that an appropriate vector method will be required to solve this force problem. Note that if the net forces acting on an object equals zero, we say that the object is in static equilibrium
Example 3 Two tractors pull a large rock on a construction site with a net force of zero on the rock. Tractor 1 exerts a force of 1.2 x 104 N [E 120 N] on the rock, and tractor 2 exerts a force of 1.2 x 104 [E 120 S]. Calculate the force of friction acting on the rock. Assume the rock is at rest.
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