Module 3: Braking Systems

To have kinetic energy an object must be

• in motion.

• at an elevated position.

• travelling at constant velocity.

• accelerating.

Answer explanation

An object must be in motion to have kinetic energy and it must be under constant velocity or undergoing acceleration. The object's position in space is not relevant.

The adjacent figure shows the fluid principle of

• Pascal.

• Archimedes.

• transmissibility.

• equilibrium.

Answer explanation

Pascal's principle states that any externally applied pressure is transmitted through all parts of an enclosed fluid, undiminished in every direction.

Given that the modulus of elasticity of a 0.85 m long brake cable is 190 GPa, calculate the extension if a load of 0.5 KN is applied to the 3 mm diameter cable.

• 0.316 mm

• 2.966 x 10^-4 mm

• 0.2248 mm

• 2.248 x 10^-4 mm

The yield point on a stress-strain diagram marks

• an increase in stress relative to strain.

• the beginning of the elastic phase.

• an increase in strain relative to stress

• the toughness of a material.

Answer explanation

The yield point indicates the point from which strain increases without a corresponding increase in stress.

True stress is

• mostly larger than engineering stress.

• the elastic modulus divided by strain.

• without units.

• also known as strength.

Answer explanation

True stress is always greater than engineering stress due to the reduction in the cross-sectional area.

B is wrong because true stress is not the Young's modulus divided by strain.

C is wrong because stress is measured in megapascals,

D is wrong because strength is measured as the maximum stress a material can withstand.

Which of these statements about friction is untrue?

• Friction between two surfaces depends on the

  nature of the surfaces in contact.

• Friction is dependent on the area of the surfaces in

  contact.

• Friction is directly proportional to the reaction

  between the surfaces.

• Friction always opposes motion.

Answer explanation

Friction is not dependent on the area of the surfaces in contact.

If the velocity ratio of a 100% efficient machine is 20 and the load is restricted to a movement of 20 mm, how far will the effort move?

• 0 mm

• 1 mm

• 20 mm

• 400 mm

As the carbon content in steel increases

• ductility and weldability increase, while strength

  and hardness decrease.

• strength and ductility increase, while hardness and

  weldability decrease.

• strength and hardness increase, while ductility and

  weldability decrease.

• weldability and hardness increase, while ductility

  and strength decrease.

Brittle materials when compression tested often fail

• through fatigue.

• due to torsion.

• due to bending stresses.

• in shear at an angle of 45° to the axis of stress.

Answer explanation

In compression brittle materials fail in shear at an angle of 45°. to the axis of stress.

Simple compression tests do not induce torsion or bending stresses and are conducted until failure, so fatigue is not relevant.

“True stress’ is calculated through

• use of the original cross-sectional area.

• final cross-sectional area.

• changing cross-sectional areas.

• cross-sectional area at necking.

Answer explanation

True stress-strain data is obtained by taking into account changes in cross-sectional area.

Engineering or apparent stress uses the original cross-sectional area.