American scientists have developed the first material that repairs itself. The material is a form of plastic that has been engineered to fill breaks in its surface.
Plastics are used today in everything from airplane wings to hundreds of object found in the home. Scientists wanted to find a way to make objects made of plastic last longer. The researchers hope their new discovery can be used to make objects that are difficult or impossible to replace.
The surface of plastic objects breaks over time. Very small breaks develop every time a plastic object is used. The researchers wanted to find out how to stop plastic from developing small breaks that weaken and destroy it. They found the answer in the chemical structure（构造）of plastic itself.
Plastic is made of monomers（单体）. These monomers link together to form polymers（聚合物）. Polymers give plastic its ability to be shaped and its strength. The research team found a way to make plastic that contains very small balloons filled with monomer liquid（液体）. When the new plastic cracks（裂开）, the monomer liquid is let out and flows into the crack. Thus the break is repaired. The repaired plastic has seventy – five percent of the strength of undamaged plastic.
The new self – repairing plastic is not yet ready for production. But it has several possible uses. One could be in space vehicles where parts cannot be repaired or replaced. Another possible use might be in bridges. And it could be used in people, to replace bone joints that have become broken or damaged.
72．Which of the following can be the best title for the article?
A．Ever-Lasting Material Invented.
B．The Chemical Structure of Plastic.
C．Plastic that Repairs Itself.
D．Small Balloons that Makes Plastic Stronger.
73．We know from the passage that the repair is done as .
A．the cracked plastic parts are changed
B．the plastic is made
C．the liquid is put into the plastic cracks
D．the plastic cracks
74．The scientist are planning to use the new material in space vehicles, bridges and people because .
A．the plastic parts in them are required to last longer
B．the plastic parts in them are easily broken or damaged
C．the new material cannot be used elsewhere
D．they want to experiment in these places first
75．What can we know about the new material mentioned in the passage?
A．It is a kind of liquid that is commonly used.
B．It is more difficult to make than common plastics.
C．Its surface does not crack so easily as other ones.
D．It makes it difficult or impossible to replace plastic objects.
，重力加速度为g。求恒力F作用的时间。
，式中σ、τ应为（）。