Newtonian Law

Newtonian Law

To completely understand physics and the motion of objects in relativity then it is paramount that you learn the three laws that Newton realised.  They are simple but will make your understanding of what is happening in other topics so much easier. 

Before I delve into Newtons Laws it is important that the concepts that underpin his laws are considered.  You should be aware that any description of motion uses the co-ordinate system to describe direction and position.  The Cartesian co-ordinate system is the more typical one used with the directions x,y&x.

y is the vertical direction while x & z are horizontal directions 90 degrees from each other.  When talking about motion the co-ordinate system will also be used with a clock.  'at time t' and 'at some particular instant on the co-ordinate system' are phrases that are used in context with motion in more technical journals.  They mean at a point in time the point of the object on the co-ordinate system.  By using both the co-ordinate system and a clock a frame of reference is created.

The description of a body's motion depends on the frame of reference from which the motion is observed.

A long time ago a man called Sir Isaac Newton had the genius ideas about the laws of gravity and the laws of physics.  Previous to this absolute understanding there was a fundamental knowledge that it was there but......what was it?  He managed to establish quantitative relationships that accounted for the changes in the motion of an object in terms of the forces acting on that object.

What is a force? in ley-mans terms it is the push and pull acting on an object. But in science it is a vector quantity that is characterised by both a direction and a magnitude.  For an object to move the force must be continual or friction and resistance will bring the object to a resting position.  Friction will be discussed on a different page later.

Newtons First Law

This law relates to how an object reacts when a force is applied.  An object will not move until a force is exerted that is strong enough to overcome friction force.  To start with imagine an object in a vacuum with no gravity, as it would be in space, it will be in continuous motion until a force is applied, it will  travel in the direction of that force until another force changes it. The duck in your bath, will float one direction until you push it in another or it hits the side of the bath and stops.  The same happens with objects, they will move in one direction until they stop due to frictional resistance force decelerating them or until they hit an unmovable object.

For example, a book on the table.  If you put force continually on the book it will slide across the table assuming that the force applied is enough to over come friction.  If the book is given a large force that is not continuous the friction between the table and the book will eventually stop the book moving.

Watch the video below to gain more examples.

So to recap the video if an object is at rest it will stay at rest, the force of the object on the surface it is resting on is counteracted by the force of the surface reacting upwards.  Once a force is applied the object will continue in the same plane as the force acted upon at a constant velocity until another force acts to change its velocity/direction/motion.

  Newtons Second Law

This is the law that you are probably more familiar with because of the equation that it relates to.  

where 
         F = the force applied, 
         m = the mass of the object
         a = the acceleration created by the force.  

Because the equation is balanced the force has to be proportional.  

An increase in either the mass or acceleration will cause an increase in the force.  It goes without saying then that an increase in Force will create an increase in either mass, acceleration or both.

It is possible to predict either of the variables by rearranging the equation.  When wanting to know how an object is going to accelerate when a force is applied the force is known and so is the mass.  Therefore the equation becomes:

Acceleration is the differential of velocity, which in turn is the differential of displacement, so the equation can be classed as a second order differential.  This shows that the Force = the rate of change in momentum, namely velocity.  The greater the force the faster the object accelerates, as long as the mass does not increase proportionally to the Force.

Remember, acceleration is the increase over time in the  speed of the object, velocity is a constant speed.

Watch this video to get a better idea of the second law.

So to review the video clip.  The Second Law shows the interaction between objects and forces.  As the force increase the movement of an object increases in acceleration, as long as the mass of the object does not alter.  With the equation F=ma  it is possible to estimate the the acceleration of an object by rearranging the equation  to a=f/m.  It is also possible to calculate the force on an object if the acceleration is known. Through these principles the acceleraometer on your vehicle will show how fast the vehicle is moving.  There are many places the acceleration is measured from on a vehicle.  Sometimes it is off the wheel and other times it can be done from within the engine.  

Newtons Third Law

This law is possibly the most recognisable, but still worth talking about. the reason why when stood we don't fall through the ground.  When we push a wall there is no movement (unless the wall is weak and unstable) 

Forces act in pairs, each force acting on a different object in the opposite direction. Consider the floor you stand on. There is a force acting down on the ground, but there is also an equal and opposite reaction from the floor acting back up on your feet.  The same happens with a table, the plate on the table acts with a force on the surface.  The surface acts with equal force back on the plate. 

Consider the force reaction between the road and the tyres of a car.  As the wheel moves and puts a force onto the road surface, the surface reacts back on the tyre.  This force that is created is 'Friction' and is what creates motion.

another way to imagine this is a boat on water. The force of displacement that the hull creates is equal to the force the water pushes back up on the hull which keeps the boat afloat. 

Watch the following video clip which explains the theory again for you.

 

So to reiterate the law. every force has a reaction force pushing in the opposite direction against it.  This is the case for all forces. It is how a plane stays in the air, it is how a boat floats on water, and how objects react with each other.

Your Turn

Below is a link to test what you have learnt.  Please take the test, it would be beneficial to you if you did not look for answers.  You will require a calculator.

Newtonian Laws Quiz

Thankyou