Forces and Free Body Diagram Notes What is

Forces and Free Body Diagram Notes

What is a force and how is it illustrated? • A force is a push or a pull. • The metric unit: Newtons. • A force is usually shown as a vector, which includes both magnitude (size) and a direction. head tail

Vector vs Scalar These measurements have a size and direction: Vector Quantities • Postion (displacement) – 10 meters South • Velocity – 10 m/sec South Force! 10 Newtons South Also: Acceleration These have only a size: Scalar Quantities • Distance – 10 meters • Speed – 10 m/sec • Also: Temperature, Time, etc.

Types of Forces Contact Forces • • • Friction Tension Normal Force Air Resistance Applied Force Spooky At a Distance Forces • Gravity • Electrical • Magnetic

Force of Gravity • AKA: weight • Caused by the gravitational pull of a planet (LARGE object). • To calculate weight: – multiply mass times acceleration of gravity Fg = m x g – Basically, that’s just F=ma! • On Earth, g = 9. 8 m/s 2

Weight and Mass are NOT the Same • Mass: – Amount of matter an object has – The same everywhere – Measured in kilograms (kg) • Weight: – Force of gravity pulling on the object – Different on different planets – Measured in Newtons (N)

Mass and Weight: Example m = 10 kg Fg = 98 N g = 9. 8 G = weight m/s 2 10 kg m = 10 kg Fg = 16 N weight g = 1. 6 m/s 2

What is a free-body diagram? diagram • A free-body diagram illustrates the relative magnitude and direction of all forces acting upon an object. FN • Dog sitting on chair dog Fg

Normal Force • Normal Force (FN): the force the chair exerts on the dog. Any time the object is on a surface there will be a normal force! FN Fg

Net Force • Net Force: sum of all forces acting on an object. Net force is zero if all forces cancel. We also say “No net force” or EQUILIBRIUM! So, if there is “No net force” the object is: • Not moving or • Moving with constant velocity If all forces don’t cancel, We say there is a “Net force” But, if there is a “Net force” the object is: • Accelerating

Example: FN Net Force? FNet = 0 Fg No Net Force! The dog is not moving

Example: tennis ball falling into commons Net Force? FNet ≠ 0 Fg • Accelerating NOT ZERO!

Tension • Tension: force exerted by a string or rope supporting an object. FT Fg

Air Resistance • Air Resistance: Force opposing the motion of an object falling in air. Fair Fg

Air Resistance • If the frog is accelerating as it falls. Fair FNet ≠ 0 Fg

Air Resistance • If the frog is falling at a constant velocity. Fair FNet = 0 Fg

Friction • Friction: Force opposing the motion of an object sliding against a surface. Ff Fa Applied Force (FA): Someone applying a force on the object.

Friction • The force of friction between the desk and the floor is 100 N. To keep the desk moving at a constant velocity you need to push with a force of : Ff Fa