Dynamics - The causes of Motion
Isaac Newton defined and described the motion of objects in three very simple but powerful Laws.
The First Law
This law is also known as the Law of Inertia. Inertia is a measure of the "laziness" of an object; its resistance to movement or to the change of motion.
Newton's Law of Inertia states that an "Objects at rest will stay at rest or will continue to move in the same direction and with the same speed, unless an external (unbalanced) force acts upon them".
Example:
If you are a passenger in a car cruising at a steady, comfortable speed of say 60 km/h and suddenly the driver hits the brakes -- what will happen?
You will continue to move forward at 60 km/h because you have the tendency to keep doing what you were doing before the brakes were applied, i.e. move at a constant speed of 60 Km/h. [Wear seatbelts next time you are in a car].
The Second Law
This law of motion looks at the force that will force objects to change their state of rest or motion. In soccer, the harder you kick the ball, the faster in moves (accelerates). The ball moves in the direction of the kick.
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If you kick a tennis ball with the same force as you would kick a soccer ball -- what happens?
The tennis ball will accelerate much faster -- why? Because the tennis ball has less mass than the soccer ball.
Newton's Second Law states that the stronger the force acting on a mass, the greater its acceleration. If the mass increases, the acceleration will decrease if the force stays constant.
The acceleration of an object is proportional to the unbalanced force and is inversely proportional to the mass of the object. We call this "unbalanced" force, the net force Fnet .
In mathematical terms:
Net Force = mass X acceleration
Using Symbols...... Fnet = m x a
| Quantity | Symbol | Unit |
| Net Force | Fnet | Newton [N] |
| Mass | m | Kilogram [Kg] |
| Acceleration | a | m/s2 |
The Newton -- a unit of force
1 Newton [N] is approximately the force of gravity ("the weight") acting on a medium sized apple falling under the influence of gravity alone.
1 Newton is also defined as the force that accelerates a 1 Kg mass at 1 m/s2 .
Make sure that the units are always in Newtons, Kilograms, and meters-per-second-squared when this formula is used.
Notice that the Newton can also be defined as the Kg/m/s2
Example:
What is the net force required to make a 300 g soccer ball accelerate at 2.0 m/s2 ?
Solution:
Given: m = 300 g = 0.30 Kg
a = 2.0 m/s2
Find: Fnet
Solution: Fnet = m X a
Fnet = 0.30 Kg X 2.0 m/s2
= 0.60 N
The Third Law
This law of motion looks is also known as the action-reaction Law.
"For every action, there exists an equal but opposite reaction".
Examples: When rowing a boat one notices that the boat moves in the opposite direction of the motion of the oars. In skating, you push back on the skates to move forward. The recoil action of a rifle: as the bullet leaves the muzzle of the gun, the gun kicks back in the opposite direction.
The slow-motion film strip shows a nail being hammered by a hammer.
Among other principles (see Energy Transformations for example), this animation illustrates Newtons Third Law of Motion. Why?
As the hammer strikes the nail with a force Faction [down], the nail strikes back with an opposite Freaction [up].
A free body diagram for the above situation is?
