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Chapter 5 -- Part 1
The effect of forces on a body is what we have considered so far.
The shape of the body will be examined.
The shape and size of the body change when a force is applied.
The body can be stretched, compressed, bent, or twisted depending on how the force is applied.
The body is distorted beyond its elastic limit if the force is large.
A bigger force will break the body.
The damaging effects of forces on bones and tissue will be examined.
Every part of the body is pulled apart by the applied force.
The cohesive force that holds the material together is resisting this force.
When the applied force exceeds the cohesive force, the material breaks.
The compression is elastic initially, but a large force will cause permanent damage.
A bar is stretched due to applied force.
Some materials are listed in Table 5.1.
The strength of these materials is shown.
An analogy can be drawn between a spring and a material.
When the stretching force is removed, work can be done by the stretched spring.
Expanding Eq can be seen as a sign of this.
By analogy with the spring.
Knowledge of the maximum energy that parts of the body can safely absorb allows us to estimate the risk of injury.
The bone is elastic until it breaks.
Consider the broken leg bones that have a combined length of 90 cm and an average area of 6 cm2.
The total energy absorbed by the bones of one leg is from Eq.
The two legs have more energy than this value.
The leg bones may break if all this energy is absorbed.
If the joints of the body bend and the energy of the fall is redistributed, it is possible to jump safely from a height greater than 56 cm.
There is a chance of injury in a fall from a small height.
A large force is put on an object in a collision.
The force starts at zero, increases to some maximum value, and then decreases to zero again.
It is difficult to determine the magnitude of the force during a collision because it takes place in a short period of time.
The collision force is larger in a fast collision than it is in a slower collision.
The injurious effects of collision were calculated in the preceding section.
The concept of impulsive force can be used to perform similar calculations.
The damage is caused by the force that causes it.
It depends on the type of collision.
The collision time is very short if the objects are hard.
The impulsive force is reduced if one of the objects is soft and yields during the collision.
It is less damaging to fall into soft sand than it is on a hard concrete surface.
Estimate of the collision duration is a difficult part of the problem.
If the impact surface is hard, such as concrete, and the person falls with his/her joints locked, the collision time is estimated to be about 10-2 seconds.
If the person bends his/her knees, the collision time is longer.
The force per unit area that may cause a bone break is shown in Table 5.1.
If the person falls on his/her heels, the impact may be 2 cm2.
We obtained this result from energy considerations.
It is reasonable to assume a 2- cm2 impact area.
The area may be smaller or larger depending on the nature of the landing.
There are more examples of calculating the injurious effect of impulsive forces.
The impact force can be calculated from the distance the center of mass of the body travels during the collision.
There is a bag in the dashboard.
In a crash, the bag expands and protects the passenger.
If contact with the hard surfaces of the car is to be avoided, the forward motion of the passenger must be stopped in 30 cm of motion.
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