Mechanical Reasoning Principles Flashcards

A fire apparatus wheel lug nut must be tightened to 450 ft-lbs of torque. A mechanic uses a wrench 18 inches long. How much force (in lbs) must be applied to the end of the wrench?

Answer: 300 lbs

Torque = Force × Arm Length. Force = Torque ÷ Arm. Convert 18 in to 1.5 ft: Force = 450 ÷ 1.5 = 300 lbs.

A fire truck uses an air brake system. When the driver presses the brake pedal, compressed air pressure is _____ to the service chambers to apply the brakes.

Answer: Directed (increased) in the brake chambers

Pressing the foot valve (brake pedal) on an air brake system directs compressed air into the service chambers, which pushes the brake shoes against the drums to slow the vehicle.

What is the mechanical purpose of a fire pump's relief valve?

Answer: Prevent excessive pressure buildup that could damage hoses or injure firefighters

A pressure relief valve automatically opens when system pressure exceeds a set limit, diverting water to prevent over-pressurization of hoses, appliances, and firefighters.

A firefighter needs to move a 500-lb object using a ramp. The ramp is 10 feet long and 2.5 feet high. What is the mechanical advantage of the ramp?

Answer: 4

Mechanical Advantage = Length of Ramp ÷ Height = 10 ÷ 2.5 = 4.

A fire engine's V-belt drives the alternator. If the engine crankshaft pulley is 8 inches in diameter and the alternator pulley is 2 inches in diameter, how much faster does the alternator spin than the crankshaft?

Answer: 4×

Speed ratio = Driver Diameter ÷ Driven Diameter = 8 ÷ 2 = 4. The alternator spins 4 times faster than the crankshaft.

Which simple machine principle is applied when a firefighter uses a wedge to prop open a door?

Answer: Inclined plane — a wedge is a double inclined plane

A wedge is a compound inclined plane — two inclined surfaces that convert downward force into sideways spreading force.