(Electronics) Crossing a Wheatstone Bridge

Strain gauges are sensors commonly used on bridges to determine if there is too much stress on a particular area. For example, if the cables supporting the Golden Gate Bridge need to be replaced or reinforced.

The gauge is made up of a strip of conductive metal. Its electrical resistance will increase if the gauge is stretched (smaller cross-section and longer length), and decrease if the gauge is compressed (larger cross-section and shorter length). Knowing the relationship between strain and electrical resistance, i.e. gauge factor, allows us to measure the stress.

Classic strain gauges use a Wheatstone bridge circuit setup like shown above. Under normal stress, we might want a reference output voltage of 0V. Assuming R2 = R3 = R4 = 10 Ω, what would the electrical resistance of the strain gauge (R1) have to be for that to occur? If the cable stretches (thus stretching the metal on the gauge), is the output voltage then a positive or negative value (assuming top output lead is positive, and the left excitation lead is at a higher potential than the right)?

(Physics) Moment of Inertia

Which of the below has the smallest moment of inertia I about its center?

Can't figure it out?... Guess! Then CLICK above to learn from the solution!

The Flying Dachschund...

You live in a house with a really sturdy ceiling fan. One day, your mischievous little dachshund somehow manages to jump up and grab onto a ceiling fan blade. Spinning for dear life, the poor little dog starts speeding up in a counterclockwise direction along a circular path as shown below.

The direction of its resultant acceleration is...

Kinetic Energy of Two Pieces


An explosion splits an object initially at rest into two pieces of unequal mass. Which piece has the greater kinetic energy?

(After 5 min, if you don't know, guess! Then learn from the solution for next time!)

(Physics) Velocity of Simple Harmonic Motion

A block of mass m is attached to a spring with spring constant k is free to slide along a horizontal frictionless surface. At t = 0 the block-spring system is stretched an amount x0 > 0 from the equilibrium position and is released from rest. What is the x -component of the velocity of the block when it first comes back to the equilibrium?

(After 5 min, if you don't know, guess! Then learn from the solution for next time!)