Forces and Newton’s Laws Quick Guide

Forces Forces are responsible for all motion Forces cause change in motion, or accelerations

Forces Free body diagrams Represent the forces acting on the object at that moment. What happened before, or may happen next do not matter. Forces vectors must be labelled (ex: the accepted labels for gravitational force are mg, FG, or W for weight). Vectors should have lengths that correspond to their values as they relate to one another.

Forces Common forces include: Tension, Applied, Gravitational, Friction, Normal Remember the normal is always perpendicular to the surface. Net force: sum of all forces acting on an object. Must treat horizontal and vertical forces independently. Net force is never drawn on a FBD.

Newton’s Laws First Law (Law of Inertia) Inertia – tendency of an object to maintain its motion An object’s motion is unchanged unless acted on by an unbalanced net force.

Newton’s Laws F=ma a = Σ𝐹 𝑚 Second Law An object’s acceleration is dependent on the net force acting on the object and the object’s mass. F=ma a = Σ𝐹 𝑚 A larger net force on the same mass will result in a greater acceleration. A larger mass with the same net force will experience a smaller acceleration.

Newton’s Laws Third Law Force occur in action reaction pairs When objects interact they apply an equal force in the opposite direction on one another.

Newton’s Laws Third Law Misconception Forces are the SAME, it’s the effect of the forces that is different The sun and the Earth’s pull on each other is EQUAL Affects the Earth more because the earth has less inertia

Summary Sketch a FBD for all situations involving force. Keep horizontal and vertical forces independent. Use the FBD to write force equations for horizontal and vertical forces. If the net force is 0, then the object is in equilibrium. Force equations are set equal to 0. If the net force is > 0, then the object has an acceleration. Force equation is set equal to ma.