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Lesson 8 - Effects of Inertia on Car Collisions
Imagine that you are riding your bike home from school and you hit a deep rut in the road.
 Your front wheel dips suddenly causing the bike to come to an abrupt halt, and you are
sent forward over your handle bars.
 Why do you keep moving after your bike has come to a sudden stop?
 Would your injuries be worse if you were traveling faster before you hit that bump in
the road?
Learning Outcomes
After completing this lesson you will be able to:
 Define inertia in terms of an object at rest or in uniform motion
 Discuss how seatbelts acting with air bags help to protect passengers in car accidents
 Describe the relationship between the speed of a car and the distance traveled by an
unrestrained passenger in a car crash
Keywords
 Inertia
 Velocity
 Acceleration
 Gravity
 Inclined plane
Introduction
Inertia - the tendency of an object to resist changes in its’ state of motion.
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The dominant thought prior to Newton's day was that it was the natural tendency of objects to come to
a rest position.

Moving objects, so it was believed, would eventually stop moving; a force was necessary to
keep an object moving.

But if left to itself, a moving object would eventually come to rest and an object at rest would
stay at rest;

Thus, the idea which dominated people's thinking for nearly 2000 years prior to Newton was
that it was the natural tendency of all objects to assume a rest position.
Example 1: You are a passenger in a car traveling south at 45 km/h. Another car darts out
into the intersection and you do not have enough time to stop.

When the cars collide, you as a passenger are still travelling with the momentum of the
moving car, and will move forward in the car at a speed of 45 km/h until YOU collide
with the dashboard, which will cause you to stop.

Alternatively, you may be thrown from the car through the window at the same speed
the car was traveling before the collision occurred.

Thus, in every accident on the road there are actually two collisions;

what collides with the car, causing it to change direction or to stop

what the passenger then collides with, causing her/him to stop
What we do know, is that the distance a passenger is thrown form a moving car is
proportional in some way to the velocity of the car before collision.

Simply put, this means that the faster the car is traveling, the farther a passenger may
be thrown in the case of a car collision.

We can determine the distance a thrown passenger will travel in this type of scenario
of we know the velocity of the vehicle (before collision) and the interval of time the
passenger travels through the air.
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Inertia Review
Inertia is the resistance to changes in motion.
Inertia is proportional to mass.
Big masses resist changing their motion more than smaller masses.
Objects at rest remain at rest unless acted on by a net force.
A lot of inertia! The large train resists changing its
Very little inertia. The small baby carriage has
motion.
very little resistance to changes in motion.
Since the train is so huge, it is difficult to change
Since the baby carriage is so small, it is very
its speed. In fact, a large net force is required to
easy to change its speed or direction. A small net
change its speed or direction.
force is required to change its speed or direction.
Objects in motion remain in motion in a straight line (unless acted upon by an outside force).
A lot of inertia!
Very little inertia
Since the train is so huge, it is difficult to stop it
Since the soccer ball is so small, it is very easy to
once it is moving. It is difficult to change its
stop it once it is moving. A small net force is
speed. In fact, a large net force is required to
required to change its speed.
change its speed.
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