In physics, three laws that form the basis of Newtonian mechanics, describing the motion of objects. (1) Unless acted upon by an unbalanced force, a body at rest stays at rest, and a moving body continues moving at the same speed in the same straight line. (2) An unbalanced force applied to a body gives it an acceleration proportional to the force (and in the direction of the force) and inversely proportional to the mass of the body. (3) When a body A exerts a force on a body B, B exerts an equal and opposite force on A; that is, to every action there is an equal and opposite reaction.
The first law As an example, if a car is travelling at a certain speed in a certain direction, it will continue to travel at that speed in the same direction unless it is acted upon by an unbalanced force such as friction in the brake mechanism, which will slow down the car. A person in the car will continue to move forward (in accordance with the first law) unless acted upon by a force; for example, the restraining force of a seat belt.
The second law This can be demonstrated using a ticker timer and a trolley with mass that can be varied. If the mass is kept constant and the amount of force applied in pulling the trolley varies, then the dots on the ticker timer tape become further apart; the trolley is changing velocity and therefore accelerating. The acceleration (a) is proportional to the force (F) applied. This can be expressed as acceleration/force = a constant, or a/F = a constant.
If the force applied in pulling the trolley is kept the same (constant), and the mass placed on the trolley is varied (from low to high), then the dots on the ticker timer tape become closer together as the mass gets larger; the acceleration decreases as mass increases. This can be expressed as acceleration being inversely proportional to mass (m), or a ∝ 1/m.
The two equations can be combined to give an overall equation, expressing Newton's second law of motion. This is: force = mass × acceleration, or F = ma.
The third law As an example, a book placed on a table will remain at rest. The force of gravity acting on the book pulls the book towards the ground. The table opposes the force (weight) exerted on the book by gravity. The table exerts the same amount of force in an upward direction. Hence the book remains at rest on the table.
Acceleration and velocity
Balanced and unbalanced forces
Gravitational potential energy
Newton's second law
Newton's third law
Meaning of ‘pressure’
Speed, velocity and acceleration
Free Fall and Air Resistance
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