Collisions

What Is a Collision?

A black and white cartoon picture of two people bouncing into one another front on.

A collision is an event that occurs when two things bump into one another and energy changes in form or moves from one thing to the other. We are involved in hundreds of collisions each day, from car accidents to dropping a pencil on the floor; we even collide with air particles as we move around.

What Is Energy?

A black and white cartoon picture of a man holding his arms straight out to his sides and holding a ball in his right hand

Energy causes things to happen, and is defined in physics as the ability to do work. The two most basic types of energy are:

Potential energy—the energy something has based on its shape and position. For example, if you hold a ball up in the air it has potential energy. If you stretch or twist a rubber band, it gains potential energy—something happens when you let go!

Kinetic energy—the energy something has because it is moving. As you drop a ball to the ground the potential energy is converted to kinetic energy.

What Happens when a Collision Occurs?

When two things collide with each other, they exchange energy. Energy can’t be created or destroyed, but it can be converted from one form to another, and it can also be transferred from one object to another. When the ball hits the ground kinetic energy is changed to sound and heat energy, and causes the ball to change its shape.

A famous scientist called Sir Isaac Newton developed three laws that explain why and how objects move. These three laws have become known as Newton's laws of motion.

Newton’s First Law of Motion

A black and white cartoon picture of a girl kicking a soccer ball over the head of a sitting dog.

An object at rest will stay at rest and an object in motion will stay in motion, unless the object is acted upon by an outside force.

This means that if you leave a ball on a table overnight, the ball will be in the same place where you left it when you return in the morning unless something or someone has made it move. If you roll the ball, it should continue moving at the same speed in a straight line until a force makes it change. A force called friction slows things down. Friction between the ball and the table will slow the ball down to a stop if it doesn’t change direction first as it falls off the table—when the force we call gravity pulls it towards the centre of the Earth. When you kick a ball it makes a curved path as gravity constantly pulls it downwards.

Newton’s Second Law of Motion

A black and white cartoon picture of a man pushing with all his might at the back of a truck

The acceleration of an object is directly proportional to the force acting on it and is inversely proportional to its mass. The direction of the acceleration is in the direction of the entire applied force.

This means that a heavier object requires a bigger force (push or pull) to change its speed or its direction than a lighter object. For example, it is much easier to push start a small car than a big truck.

The second part of this law also tells us that if you push or pull a stationary object in a particular direction it will travel in that direction. If you push or pull a moving object it will change its direction towards the direction of the push or pull.

Newton’s Third Law of Motion

A black and white cartoon picture of a man on roller blades moving backwards away from the rear of a truck

For every action, there is an equal and opposite reaction.

This means that whenever one object pushes on a second object, the second object pushes back on the first object by the same amount. For example, if you try and push a big truck while wearing a pair of roller blades then you will move backwards. When you push the truck your body applies a force to the truck and the truck in turn applies a force to your body. Since the truck is a lot heavier than you it doesn’t move very far, but you move backwards a long way because you accelerate more (and that’s what the second law says!)

Isaac Newton—the Man

Isaac Newton was born in Lincolnshire, England on December 25, 1642, (though by today’s calendar he was actually born on January 4, 1643).

As a student Newton was very good at constructing his own mechanical devices. Once he completed school, Newton’s uncle sent him off to Trinity College at Cambridge where he excelled in mathematics and science. The College was closed down during the plague (1665) so Newton went home to the family farm. It was there he thought out the theory of gravitation when he (perhaps) watched an apple fall from a tree.

During Newton’s lifetime he came up with many more scientific theories that are still used to this day. This included calculus, the theory of light, the colour spectrum, and a new type of telescope as well as the three laws of motion.

Sir Isaac Newton died at the age of 85 in London and was buried in Westminster Abbey along side England’s most esteemed revolutionaries. He is still considered by many to be one of the most brilliant and influential minds in the history of science.

It’s Smart to Be a Dummy

A black and white cartoon picture of a test crash dummy strapped into an jet set. He is thinking 'Sigh, another day another disaster'.

The first crash test dummy was created in 1949 for the United States Air Force to use for research into aircraft ejection seats. Nicknamed Sierra Sam, this dummy represented a large male who was bigger that 95 percent of the adult male population.

These early dummies were very simple and basically showed scientists the effectiveness of seat belts. Previously, human volunteers had been used but due to the increasing degree of danger involved with each test it was no longer considered safe for human subjects.

These days the Hybrid III dummy is designed to represent a an average sized man who is 178 cm tall and weighs 77 kg. He has a specially designed chest, head, neck and knees that simulate human parts in a crash, and vinyl skin that is equipped with sophisticated electronic tools that measure the acceleration and deflection that body parts experience during a crash.

This Hybrid III is the most commonly used crash test dummy but there are other members of the Hybrid III family. There is a very large adult male, a very small adult female and two children aged 3 and 6.

Think About It

Newton was once asked what the secret behind his genius was and he replied by saying that he thought out his theories, not by inspiration or sudden insight, but by continuously thinking very hard about the subject until he had worked something out.

What Happens if You Don’t Wear a Seat Belt?

A black and white cartoon picture of a woman sitting in a car and wearing a seat belt. She is smiling and holding two thumbs up.

Seat belts are designed to distribute the force of an accident to the strongest parts of the human body (the chest and pelvis) and to keep it away from our more delicate parts such as the head or abdomen.

The seat belt restrains the wearer so that they move with the car, and stop with the car, during an impact.

When an occupant of a vehicle doesn’t wear a seat belt during a collision, that person keeps on moving even though the car stops suddenly (see Newton’s first law). They move until coming into contact with a solid object such as a windscreen or steering wheel. This action would certainly result in serious injury or even death.

Seat belts play other safety roles as well.

They confine other passengers of the vehicle who, if unrestrained, could become missiles and cause critical injury to other occupants.

Research has shown that using a seat belt decreases the risk of driver crash deaths by 45% and the risk of serious injury by 50%. So it seems that you are much more likely to die in a car crash if you don’t wear a seatbelt.