Question

1. Plastic and aluminum are both excellent in terms of strength in tension and in compression, and they are less dense than steel -- so why do you think that they aren’t used in making rebar?

2. A moment and a force are not the same thing – why not? Are there any similarities?

Answer 1

FIRST:

A property of steel used in reinforced concrete is that it has a similar coefficient of thermal expansion as concrete:

• Concrete: 14.5⋅10−6 m/mK
• Steel: 12.0⋅10−6 m/mK

Compare this with:

• Aluminum: 22.2⋅10−6 m/mK
• Plastic:65.5 10-6 m/mK

So a likely result of putting aluminum or plastic inside a concrete beam would be the concrete cracking when the aluminum or plastic expands more in the summer.

SECOND:

force and torque are not the same things because force causes translational motion and torque causes rotational motion.

Force: The tendency of force is to cause linear motion, it is simply considered as a force.Pushing a box on the floor is an example of force.

Moment: Whenever the force causes a rotational tendency, it is known as the moment. Since moment gives rotating tendencies, therefore, there is always a center about which moment acts. Imagine opening a nut with a wrench. The center about which the moment is applied is the center of nut.

the similarity is that the moment is also caused by a force acting with a tangential component to the center.