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By observing the motion of objects on an inclined plane Galileo deduced that an external unbalanced force is required to set an object in motion (from its state of rest) but no external unbalanced force is needed to sustain the uniform motion of an object in a straight line.
During the seventeen century, the English scientist and mathematician Sir Isaac Newton further investigated the ideas put forth by Galileo and formulated three laws of motion that explain the relationship between the motion of objects and the forces acting on them. Newton’s three laws of motion, together with his theory of universal gravitation are regarded as one of the greatest scientific works ever written.
In this unit, learners will explore Newton’s first law of motion and some of its important application.
Newton's first law of motion
Newton's first law of motion
Newton's first law of motion states that an object at rest will continue to be in a state of rest or an object in motion will continue to be in a state of uniform motion along a straight line unless acted upon by some external unbalanced force. This law is made up of three parts.
The first part of the law describes the behaviour of stationary objects. Let's analyze the following images!
New Home, New Adventures
New Home, New Adventures
Suppose you're moving into a new home. Disassembling and loading the furniture onto a truck would be the most time-consuming. The furniture cannot move on its own into your new house. What action must you take to move them?
It is obvious that an unbalanced force is required to put a stationary object in motion. But why do moving objects seem to slow down on earth? This is due to the net force of friction acting on the objects.
Objects experience friction when they move or attempt to move over a solid surface or through a fluid such as water or air. Friction acts in a direction so as to oppose the motion of moving objects.
The second part of the law holds to a good approximation for objects under uniform motion in a straight line. Let's look at the following situations to understand this concept.
If friction disappeared, would the ball slow down?
If friction disappeared, would the ball slow down?
Before Galileo, Aristotle, the ancient Greek philosopher and scientist, adopted the view that the natural state of a body was the state of rest and in the absence of an external force all moving objects would come to rest.
Galileo, however, stated that a body moving with uniform speed in a straight line needs no force to keep it in motion provided there are no external impediments. In other words, a moving body will continue moving in a straight line forever provided there are no external opposing forces.
You have most likely experienced that an external agent in the form of a force is necessary to change the direction of a moving body, otherwise it would simply keep moving in a straight line. An illustration of this approach is given below:
Change in Direction of Motion
Change in Direction of Motion
If it weren't for gravity's existence, the ball would continue to move in a straight line.
Check your understanding
Check your understanding
Questions
Questions
1. According to Newton's first law of motion, a state of rest and a state of uniform motion are completely equivalent. Explain. In the absence of a net force, object is either at rest and has zero velocity or moves with a steady speed in a straight line. It is also worth noting that, an object moving with constant speed in a straight line needs no net force to sustain its velocity provided there are no external forces opposing its motion.
2. A stationary object has no forces acting on it.
a). True
b). False
3. Describe an experiment to investigate the effect of force on the position of an object.
Summary
Summary
According to Newton's first law of motion, an object continues to be in a state of rest or of uniform motion in a straight line unless compelled by some external unbalanced force to act otherwise. When an aircraft is parked on a level road it continues to stay where it is unless we apply a force to move it along the road. The weight of the aircraft is balanced by the push of ground.
Once the engine is switched on, the aircraft accelerates. The air resistance increases as its speed increases. If the forward thrust from the engine is equal to the backward push from the air resistance , the forces cancel each other out and so the air craft moves at a constant speed in a straight line.
Check your understanding
Check your understanding
Questions
Questions
State whether there is a resultant force in each of the following situations.
1. The car going along the road at constant speed.
2. A motor cycle takes 8s to increase its velocity from 10 m/s to 30 m/s.
3. A book lying on a table.
Answers
Answers
1. The resultant force=0. The forward thrust due to the engine is balanced by the backward force due to air resistance.
2. There is a resultant force. Speed of the motor cycle goes by 2.5 m/s every second.
3. The resultant force=0. The book is at rest on the table because the upward push (normal force) exerted by the table on the book is equal to the gravitational force (weight) exerted by the earth on the book.
Saucy Scientists
Saucy Scientists
We've all been a part of the struggle of handling 'stuck tom sauce'. A strong whack on a near - empty glass bottle of tom sauce would achieve the required amount of force to dislodge the sauce from the bottom. Turn the bottle upside down and thrust downwards at high speed. Swiftly stopping the bottle should splatter the sauce all over your pizza.
Newton's first law of motion is often called the law of inertia. Newton attributed the term "inertia" to mean "the characteristic of an object due to which the object on its own is unable to change its state of rest or state of uniform in a straight line. The downward force applied accelerates both the bottle and the sauce. The sauce continues its downward motion due to inertia even after the bottle has come to an abrupt halt.
Check your understanding
Check your understanding
Questions
Questions
Explain the following statements in terms of Newton's first law of motion.
1. The apples from an apple tree fall when its branches are shaken.
2. A passenger falls backwards when a bus suddenly accelerates from rest.
3. Which ball offers more effective resistance.
a). golf ball
b). bowling ball
4. Which one of the following statements is correct.
a). all objects with mass have inertia
b). only stationary objects have inertia
c). only objects in uniform motion have inertia d).Only accelerating and decelerating objects have inertia
Answers
Answers
1.On shaking, the branches come into motion but the apples tend to remain at rest due to inertia of rest. Hence, the apples get detached and fall.
2.When the bus accelerates from rest, the passenger in the bus tends to fall backward. This is due to inertia of rest experienced by the passenger that tends to oppose the forward motion of the bus. As the bus suddenly starts moving from rest, the lower part of his body which is in contact with the bus shares the same state of motion as that of bus while the upper part of his body continues to maintain its state of rest due to inertia of rest. As a result, he falls backwards.
3. Quantitatively, the inertia of an object is measured by its mass. The bowling ball being heavier than the golf ball has larger inertia to resist the attempt to change its state of motion.
4. a)
Activity
Activity
Check your understanding
Check your understanding
Questions
Questions
There are two objects P and Q of same mass. Object P is moving at a constant speed while object Q is moving with a constant acceleration of 4 m/s2. Which of the following statements is correct ?
1. inertia of P < inertia of Q
2. inertia of P = inertia of Q
3. inertia of P > inertia of Q
4. inertia depends on the shape and size of the objects.
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