3.2 Forces in action

• force (N)= mass (kg) x acceleration ($ms^{-2}$)
• A force of 1N will give a 1kg mass an acceleration of 1$ms^{-2}$ in the direction of the force
• weight is a force based on the gravitational pull of the planet
• mass the the amount of matter an object is made of
• Weight (N) = mass(kg) x gravitational field strength (N/kg)
• A newtonmeter shows the gravitational force acting on an object, in Newtons
• Weight is the gravitational force acting on an object through its centre of mass
• Friction is the force that arises when two surfaces rub against each other
• Drag is a resistive force on an object travelling through a fluid
• Tension is the force within a stretched cable or rope
• Upthrust is the upward buoyancy of an object in a fluid
• normal contact force is the force that arises when objects rest against each other
• A fluid is a liquid or gas
• magnitude of drag depends on speed, cross sectional area, texture, and density of the fluid
• drag = speed$^2$
• Terminal velocity is reached when air resistance equals weight in the opposite direction
• The centre of gravity is the point where the entire weight of an object appears to act
• The centre of mass is the point at which any externally applied force produces a straight line motion but no rotation
• experiment to find the centre of gravity : pins in an object and use a plumbline to draw lines - centre is the point of intersection
• Conditions for equilibrium

  • resultant force of zero - horizontally and vertically
  • no rotation
• When an object is in equilibrium, the forces acting on it form a closed triangle
• The moment of a force is the turning effect of a force about some axis or point
• The SI unit for a moment of a force is Nm
• moment = force x perpendicular distance to the line of action of the force
• The Principle of Moments

  For a body in rotational equilibrium, the sum of the anti clockwise moments about any point its equal to the sum of the clockwise moments about that same points
• Principle of Moments

  $\Sigma M_{clockwise}=$$\Sigma M_{anticlockwise}$
• Applying a pair of equal but opposite forces parallel to each other along different lines causes a rotation without any translational motion
• A couple is pair of equal but opposite forces parallel to each other that act along different lines
• Torque of a couple = one of the forces x the perpendicular separation between the forces
• Density = mass / volume
• To find the volume of an irregular object, put the object in a container with a known volume of liquid and calculate the increase of volume
• Pressure is the normal force exerted per unit of cross sectional area
• Pressure

  $\rho =$$F/A$
• Gases and liquids exert pressure on surfaces as there is a constant movement of particles which collide with the surface
• Pressure exerted by a vertical column of liquid = height of the column x density of the liquid x the value of acceleration under free fall
• Pressure doesn't depend on the cross sectional area
• density is directly proportional to height of a vertical column of liquid so pressure increases with depth
• Denser liquids will exert greater pressure
• Upthrust is the resultant force due to pressure of an object submerged in a fluid
• Objects submerged in fluids exert pressure on the fluid which exerts pressure on the object which causes an upward force