Newton’s First Law Level 1 Physics

CHAPTER 4 ESSENTIAL QUESTIONS What are Newton’s Laws of Motion? How do forces cause motion? How can the Law of Universal Gravitation be applied? How can one weigh the earth? What effect does friction have on the motion of an object?

OBJECTIVES FOR NEWTON’S LAWS

Force Facts Basic definition: a push or a pull Can we do a better job? Probably. An interaction between bodies. Forces can be classified by two types; 1. Contact Force i.e. Normal, Tension, Friction 2. Field Force (at-a-distance) i.e. Gravitational, Magnetic, electric

Newton’s First Law The Law of Inertia The natural tendency for all objects is motion Newton’s First Law (N.F.L.) An object remains at rest or in a state of motion at constant speed along a straight line unless compelled to change that state by a net force (SF). Forces are balanced Object at rest vo = 0 m/s Object in motion v0 0 m/s/s a = 0 m/s/s a = 0 m/s/s Object in motion Same speed/direction Stays at rest

Inertia Inertia – Tendency to resist motion The more massive an object is, the more inertia it will have It is harder to get boulder in moving (also harder to stop the boulder from moving) than it is to bet a pebble moving Inertia is a quantity that is dependent upon the mass of the object

Free Body Diagrams (FBD) A pictorial representation of all of the forces acting on an object N Normal force (N): Surface force that is always drawn perpendicular to the surface T f Weight (mg): Always drawn Straight down from center mg T Tension (T): Force in ropes/chains drawn away from object Friction (f): Drawn opposing motion mg

FBD for slopes N f mg

Example A 10-kg box is being pulled across the table to the right at a constant speed with a force of 50N. Calculate the Force of Friction Calculate the Force Normal N F f mg

Example Suppose the same box is now pulled at an angle of 30 degrees above the horizontal. Calculate the Force of Friction Calculate the Force Normal F N Fy f 30 Fx mg

A Final Note