Understanding our Solar System -Isaac Newton


Kepler's laws of planetary orbits are underpinned by Isaac Newtons laws of motion and gravity.

It was Kepler who suggested a force of attraction must be acting in order to keep planets orbiting around the Sun. Newton defined this pulling force as gravity. He defined the gravitational force with the formula on the right.

The formula tells us that the strength of the gravitational force acting between two bodies increases as their mass increases, while the force decreases as the distance between them increases.
Newton went on to propose three laws of motion.

Newtons first Law

Every mass, or body, be it at rest or moving, will not alter its present state unless acted upon by an external net force. This is also known as the Law of Inertia.

Simply put, this law states that an object can not stop, start to move or slow down, unless a force acts on it. This is well understood by astronauts living in sapce.

View the video on the right. Notice how objects continue to move with constant speed and direction or stand still unless acted upon by a net outside force.

 

Newton's Second Law
The greater the force acting on an object the greater the change in its speed or direction. The change in motion of the object is in the direction of the force acting on the object.

Simply put, If we push an object, it will move in the direction of the push. If an object is already moving and a push is applied perpendicular to its motion, it will change the direction it is moving towards the perpendicular.

 

Newton's Third Law

For every action there is an equal and opposite reaction.

This law is the basis for jet engines propeller planes and space rockets.

 

 
1) Gravity is a force that acts between  
2) The Sun is a massive object. Its gravitational force is
3) Gravity depends on the
4) A planet exerts a gravitational force on a space craft. What happens to the gravitational force experienced by the space craft as it moves away from the planet?

Below are some challenging questions.

View the video on the right. Newton's laws are well demonstrated in the weightless environment of space.

Here, two astronauts push each other. Both move back with force.

a) What is the action and reaction.

b) One of the astronauts now has extra weight attached.
i) What happens to the speed with which he moves backwards?

ii) Consider the formula F= ma,
He moves away with Explain.

c) When we jump off the ground we push against the Earth. As a result the Earth gets pushed in the opposite direction.
i) Does the Earth move with the same force?

ii) Dows the Earth move away with the same acceleration as the person? Explain.

 
Continue with Hubble