Chapter 5

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.

The average speed.

There is a collision protective device.

The strength of body tissue is estimated.

The average stopping force and force per cm2 are at a 105 km impact speed.

The passenger would probably be hurt by the force.

The possibility that the bag may be triggered during normal driving has been considered in the design of this safety system.

If the bag were to remain expanded, it would make it harder for the driver to control the vehicle, so the bag is only expanded for a short time to cushion the impact.

Neck bones can be fractured by a moderate force.

If the impact is sudden, as in a rear-end collision, the body is accelerated in the forward direction by the back of the seat, and the neck is suddenly pulled back at full speed.

People who jumped out of airplanes with parachutes that failed to open and then survived because they landed on soft snow have been reported.

The body made a 1-m deep depression in the snow on impact.
The impact force that acts on the body during a landing can be calculated to verify the credibility of these reports.

We discussed the damaging effects of large impulsive forces in the beginning of the chapter.

The impact of feet with the ground in walking and running is one of the smaller repetitive forces that our bodies are subject to.
There is still a question as to the extent of the smaller repetitive forces that are encountered in exercise and sport.
Osteoarthritis is often caused by repetitive impact.

Osteoarthritis is a joint disease caused by wear and tear on the components of the joint.

The joint loses strength and flexibility as a result of wear and tear.
The underlying bone may begin to erode.
Osteoarthritis is a major cause of disability.
The most common affected joint is the knee.
The majority of men and women are affected by this condition after the age of 65.

A number of studies have been done to determine the link between exercise and arthritis.

The conclusion is that joint injury is related to osteoarthritis.
People who engage in high impact injury prone sports are more likely to have osteoarthritis.
There appears to be little risk associated with running 20 to 40 km a week.

An injured joint is more likely to wear and tear over time.

Un-lubricated bones have a higher coefficient of friction.
The ability of the joint to lubricate is usually compromised by a joint injury.
A simple picture would lead one to believe that the progress of osteoarthritis would be more rapid in the joints of people who are regular runners than in a control group of non- runners.
This doesn't seem to be the case.
Osteoarthritis seems to progress at the same rate in both groups, indicating that the joints have the ability to repair themselves.
The conclusions are subject to further study.

A 50 kilo runner trips and falls on his hand.

The length of the arm is 1 m and the area of the bone is 4 cm2.

The area of impact is 4 cm2 and the duration is 10-2 seconds.

The area of impact is 1 cm2.

Assume that the object is hard, that the area of contact with the skull is 1 cm2, and that the impact time is 10 to 3 seconds.

If you use the data provided in the text, you can figure out the mass of the head and the area of the neck.

A boxer is punching a bag.

His fist hits the bag at a speed of 7 m/s.
His hand came to a stop as a result of hitting the bag.
If the moving part of his hand is less than 5 kilo, you can calculate the rebound velocity and energy of the bag.