Force and Motion

Force and Motion

The Greek Philosopher Aristotle stated that motion was maintained by forces. A force makes something move. In force and motion, there are a few scientific terms that a Science student must know and understand. The terms have different definitions as well as different units that are applied in calculations.

Force

Force is what causes something to move, stop or change direction. Its unit of measurement is Newton (N). Force is equal to mass multiplied by acceleration.In physics, a force is a push or pull that can cause an object with mass to change its velocity. Force has both magnitude and direction, making it a vectorquantity. Newton's second law states that an object with a constant mass will accelerate in proportion to the net force acting upon and in inverse proportion to its mass. Equivalently, the net force on an object equals the rate at which its momentum changes.

Force = mass x Acceleration

Newton's Law of Motion

The motion of an aircraft through the air can be explained and described by physical principals discovered over 300 years ago by Sir Isaac Newton. Newton worked in many areas of mathematics and physics. He developed the theories of gravitation in 1666, when he was only 23 years old. Some twenty years later, in 1686, he presented his three laws of motion in the "Principia Mathematica Philosophiae Naturalis." The laws are shown below, and the application of these laws to aerodynamics are given on separate slides.

Newton's First Law of Motion:

If there is no resultant force on a stationary object, it will remain at rest. If it is already moving, it will keep on moving at a constant velocity. The bigger the mass, the higher the inertia and therefore needs a bigger force to overcome its inertia.

For example: When the ball is already moving, the ball will keep moving at a constant velocity.

Newton's Second Law of Motion:

The rate of chage of momentum in an object is directly proportional to the resultant force acting upon it. When the mass is constant, this law is often stated as "Force equals to mass times acceleration (F=ma)" ; the net force on an object is equal to the mass of the object multiplied by its acceleration.

For example: When a ball is pulled to a certain height and is realeased, the ball will hit the other balls. The last ball will swing up to about the same height as the first ball.

Newton's Third Law of Motion:

If object exerts a force on object B, then object B exerts an equal but opposite force on object A. Therefore, there is an equal and opposite reaction for every action.

For example: When the cardboard on a glass try is pulled towards the right, the toy car is seen moving in the oppositedirection. This is because the action force F which causes the cardboard to move towards the right is equal to the reaction force F' (friction force) which causes the toy car to move towards the left.

Inertia

Inertia is the relectance of an object to move once it is at rest, or the reluctance to stop once it is in uniform velocity. In other words, inertia is the tendency of an object to remain at rest, or if moving with uniform velocity, to continue its motion in a stright line. Newton's first lawof motion, which is also called the law of inertia describes the characteristics of inertia. Newton's first law of motion states that every object will continue in its state of uniform velocity or at rest unless an external forces acts upon it.

For example,

a) When a car stops suddenly, the driver and the passengers surge forwards. This is due to their inertia to keep them moving.

b) When a car starts to move, the driver and the passengers move backwards because their inertia acting to maintain their position at rest.

Mass is a measurement for the amount of inertia. A body of greater mass will have a greater inertia. It has a greater tendency to maintain its state of motion.

Momentum

It is difficult to stop a bullet which travels at a very high speed or a lorry which has a large mass. The momentum of an object is defined as the product of its mass and velocity.

Momentum = Mass x Velocity

p = mv

Momentum is a measure of motion. It is a vector quantity. Its direction is the same as the velocity and its unit is kg m/s or N s.

Gravity

Do you know why a ripe duran fruit always falls down and not flies up? According to Sir Isaac Newton, an attractive force exists between any two objects. The durian fruit falls down because there is a gravitational force between the durian fruit and the Earth. Since the mass of the Earth is too massive, we only see the durian fruit falls towards the Earth. The Earth's Gravitational Force tends to pull everything towards the centre.

By Newton's second law of motion, the gravitational force

F = mg

where m is the mass of the object and g is the acceleration due to gravity. The value of g is a constant, which is equal to 9.81 m/s/s, or approximately 10 m/s/s.

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