Chapter 12 Examples

Example 1 (HRW)

A uniform beam of length L and mass 1.8 kg rests on two digital scales.  A uniform block of mass M (2.7 kg) rests at a distance L/4 from the left end.  What do the scales read?

Key concepts: Scales read N, the normal reaction.  Designate Nl and Nr as the readings at the left and right end respectively.  Apply the condition for static equilibrium (St=0, SF=0)

Example 2 (HRW)

A ladder of length, L=12.0m and mass, m=45.0kg rests against a wall.  Center of mass is a third of the way up.  A fire fighter (M=72.0kg) climbs halfway up.  What forces are exerted on the ladder by the wall (frictionless) and the ground?

Key concepts: Frictionless wall implies that forces due to the wall are horizontal only.  Choose the foot of the ladder (on the ground) as the location about which all torques are computed.  This eliminates the horizontal and vertical forces due to the ground from the equation.  Apply the condition for static equilibrium (St=0, SF=0).