Newton's Laws of Motion
In the late 17th century, Sir Isaac Newton postulated that acceleration of an object is dependent upon two variables - the net force acting upon the object and the mass of the object.
It has been found later that the acceleration of an object, and inversely on the mass of the object. As the net force increases so the will the object acceleration. However, as the mass of the object increases, it acceleration will decrease.
mathematically, they are related by the equation below:
F = m* a
Sir Issac Newton put forth a variety of laws which explain why objects move (or don't move) as the do. these laws have become popularly known as Newton's Laws of motion .
The relationship F= m*a describes Newton's second Law of motion
"For a constant mass object, the acceleration of an object as produced by a net force, is directly proportional to the magnitude to the net force, in the same direction as the net force, and inversely proportional to the mass of the object".
Worked example 3.2
A strong horse pulls a sledge from rest with a constant force of 650 N. the average friction acting on the sledge is 250 N when the sledge is in motion.
The sledge and its rider have a mass of 200 kg.
(a) i. Determine the value of the net force acting on a sledge.
ii. Hence Calculat the acceleration experienced by the sledge
(b) what is the speed of the sledge 5.0 s into its journey?
Solution:
(a) i. Fnet = 650 - 250 = 400 N
ii. F net= m *a
400 = 200 *a
a = 400 / 200 = 2 m/s2
acceleration experienced by the sledge is 2 m/s2
(b) a = v final - v initial / time taken
v = 2 * 5 = 10 m/s
speed of the sledge 5.0 s into its journey is 10 m/s
Body in equilibrium
Logically speaking a body not experiencing any net force, will therefore not speed up or slow down or change direction. Hence the body will not accelerate when it experiences zero net force.
This has two possible consequences:
1.If the body is not moving originally,it will stay at rest , unless acted upon by a net force. The figure below shows that Ala' stalled car will not move unless the net force is applied to move it. he needs his friend to help push his car.
Ala' car will only move if his friends applied force is greater than the friction between the wheeles and the ground.
2. It will continue to move at the same speed, in the same direction, if it is originally moving. for Example. a comet in outer space travels in a straight line at constant speed, unless a net force changes it motion.
we say that the body is in equilibrium if it is not acted upon by a net force.
Newton's first Law of motion relates to a body in equilibrium
*An object at rest tends to stay at rest and an object in motion tends to stay in motion at the same speed and on the same direction unless acted upon by a net force.
Inertia
All bodies resist being set into motion if they are originally at rest, and resist to stop once in motion, unless a net force is applied to change their state of rest or motion.
This property of a body to resist a change in its state of rest or motion is called inertia. if the body has a large mass, its inertia is large. This is easily observed, as a large amount of net force is required to change the state of rest or motion of a huge body.
Traffic accident usually happens when a big v speeds and can't stop. A very large force is required to stop its motion.
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