11Aforcediags

Force Diagrams


 * A __**force diagram**__ helps us understand the different forces acting on an object and hence helps us calculate the resultant force.
 * It consists of a central point with vector arrows drawn from that point to represent the different forces.

Force Diagrams for a body on a slope (inclined plane)

A common question in mechanics is to study the forces acting on a body on an __**inclined plane**__ (slope).

Weight:
 * Gravity creates a force on the body acting vertically downwards called Weight.
 * The weight force is calculated by m × g
 * m = mass,
 * g = acceleration due to gravity,
 * g = 9.8 m/s 2

Normal Force (Reaction Force):
 * The Normal Force (or Reaction Force) is created by the surface pushing back against the body.
 * It always acts perpendicular to the slope.
 * This is the force that stops the body falling through the surface and down to the centre of the Earth.

Friction:
 * Friction is a force on the body that resists the body sliding along the surface.
 * It always acts parallel to the surface and in the opposite direction to the direction of motion.
 * Some questions will ask you to ignore friction by describing the surface as **smooth**.

Tension:
 * Tension is a force that acts through a string or rope that is pulling on the object.
 * In this course we will always assume the string is light enough that we can ignore the weight of the string in our calculations.
 * Strings can only provide a pulling force. They can't push.


 * If a string connects two objects then the string exerts a Tension force on both objects.
 * The force exerted on the two objects by the string will be **__equal__ in magnitude** but pulling __away__ from each object in the direction of the string.


 * If a string passes over a **__smooth__ pulley**, the pulley changes the direction of the string (and hence the force) but does not change the magnitude of the Tension force.

Force Diagrams


 * In a force diagram, it is important to draw and label __every__ force acting on the objects. Direction of motion can be shown with an arrow.


 * In this case, the the two objects would be analysed seperately.

Resultant Force


 * The __Resultant force__ is the vector sum of all the forces acting on a body.

Equilibrium


 * If the Resultant force is zero then the body is in __equilibrium__
 * The object will have an acceleration = zero.
 * This is Newton's First Law of Motion.

Adding Forces

Forces can be added in two ways: Either .
 * 1) Resolved into their components and their components added, or
 * 2) Adding magnitude and direction using geometry.