Chapter 4 - Forces in Action

Chapter 4 - Forces in Action

1. Force, Mass and Weight

   • Mass - Physical property depending on the amount of matter an object has
   • A net force (N) acting on the object makes it accelerate in the direction of the force
   • Mass is constant, but weight can vary. Weight is also a force

2. Centre of Mass

   • C.O.M - A point through which any externally applied force produces straight line motion but no rotation
   • The total weight of an object, acts through a point called the centre of gravity, coinciding with the C.O.M
   • To find the C.O.G, hang the object freely, e.g with a clamp stand. Using a plumb line, draw lines vertically down from multiple points, until they overlap at the C.O.G

3. Free Body Diagrams

   • Each force vector is represented by a labelled arrow, which is drawn to the same scale
     • Component of the weight down the slope is responsible for acceleration down the slope
       • Normal contact force also = mgcosΘ
         • For a trolley on a ramp, using a=f/m, where f=mgsinΘ, acceleration is independent of the mass (as m cancels out)

4. Drag and terminal velocity

   • An object moving through a fluid (Air or liquid) experiences a Drag force, a frictional force opposing the motion of the object
     • The magnitude depends on speed, shape, texture of the object and the medium
     • Greater Cross sectional Area results in greater drag
       • Drag is directly proportional to speed squared
     • At the instant he falls, no air resistance. Force = W, [a=g]
       • Speed and therefore Drag increases, meaning net force decreases [a=(mg-D)/m]
         • At T.V, Drag is equal and opposite to weight. No Acc, constant speed. [a=0]

5. Moments and Equilibrium

   • The turning effect of a force about some axis or pivot
     • Moment = Force x Perpendicular distance of the line of action of force from the axis or pivot
       • Moment = Fx (Units Nm)
         • It must be perpendicular distance
       • Principle of moments = For a body in rotational equilibrium, the sum of anticlockwise moments about any point is equal to the sum of clockwise moments about the same point

6. Couples and Torques

   • A couple is a pair of equal and opposite forces acting on a body but not in a straight line
     • The moment of a couple is known as a torque
       • Torque of a couple = One of the forces X perpendicular seperation between the forces = fd

7. Triangle of forces

   • Arrows are drawn end to end, to represent the three forces
     • The triangle is closed because the net force is zero and so the object is in equilibrium

8. Density and pressure

   • Density is the amount of mass per unit volume
     • For density you need to know the mass and volume. This can be measured, e.g with a ruler or measuring cylinder for liquids, along with a measuring scale. For irregular solids, volume is worked out through displacement
   • Pressure is the normal force per unit area (Units Nm^-2, or Pascals (pa))

9. P=Hpg and Archimedes principle

   • Fluid pressure does not depend on C.S.A, and it also shows that presure is directly proportional to depth
   • Upthrust is the upward buoyant force exerted on a body immersed in fluid, due to differences in pressure on the top and bottom of an object
     • Archimedes principle - The upthrust exerted on a body immersed in a fluid, either fully or partially submerged, os equal to the weight of the fluid that the body displaces
       • For a floating object, Upthrust = Weight of the object. If it sinks, Weight>Upthrust

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