- Newton made a wide range of study on the motion of bodies.
- Prior to Galileo, people believed "rest is the natural state of any body".
- Galileo stated that. "rest, uniform motion and inform circular motion are the natural states of a body".
- Finally, Newton concluded that "only rest and uniform motion are the natural states".
Study of Motion
In the study of motion of objects, generally we observe the following cases
- A body which is at rest initially continues to be in the same state.
- A body which is at rest (or moving with certain velocity) changes its velocity to some other value.
- A body which is moving with certain velocity continues its motion with the same velocity without any change.
Change of motion :
Change of motion occurs when
- a body changes its state from rest to motion
- magnitude of velocity changes
- direction of velocity changes
- both magnitude and direction changes
- a body is in uniform circular motion
- a body is moving in circular path with non uniform speed
- a body is moving along any curved path
some examples for change of motion :
- A cricket ball which is at rest is hit by a bat and attains a velocity of 100 m/s.
- A truck moving with a velocity of 30 kmph by the application of acceleration reaches a velocity of 70 kmph
- A car travelling with a velocity 60 kmph towards the North suddenly changes its direction towards the west, keeping its magnitude constant.
- A bike travelling with a velocity 60 kmph towards in one direction changes to 70 kmph in some other direction.
After observing these, one can question that, " What is the cause of change of motion? "
Newton studied and concluded that "Force is the cause of change of motion".
Force is a physical quantity which changes (or) tries to change the state of rest (or) uniform motion of a body.
- If a body of mass "m" acquires an acceleration a when it is subjected to a force F then F = ma
Various types of forces :
Various types of forces in the nature can be grouped into four categories,
- Gravitational Force
- Electromagnetic Force
- Strong Interaction
- Weak Interaction
Any two bodies attract each other by virtue of their masses. the force of attraction between two point masses is directly proportional to the product of masses and inversely proportional to the square of distance between them.
Electro magnetic force
In atoms the electrons are bound by the electromagnetic force acting an them due the protons. the atoms combine to form molecules due to the electromagnetic forces. A lot of atomic and molecular phenomena results to form electromagnetic forces.
some of the Important electromagnetic forces are :
- Contact Force
- Buoyant Force
- Viscous drag
Contact Force :
A force exists when two solid surfaces are in contact. this force (reaction) is called contact force.
Normal reaction (N) : The component of reaction (R) perpendicular to the plane of contact is called normal reaction (N).
Frictional force (f) : The component of reaction (R) parallel to the plane o contact is called frictional force (f). Hence the resultant of frictional force (f) and normal reaction (N) is equal to the reaction force (R).
- Friction always opposes the relatives motion of one surface over the other surface.
- Friction may oppose or support the motion of the body.
Tension is also one type of electromagnetic force which can be observed in strings, rubber, chords and spring.
When a force is applicable on the above structure along their length to make them straight (in the case of spring, when it gets compressed also) they are said to be in a state of tension.
For a string : It is inextensible. It undergoes tension when it is stretched. Tension acts in such a way that it pulls the object.
For rubber chord : It undergoes tension when it extends. It also pulls the objects when it is under tension. when rubber chord slakens, tension becomes zero.
For a spring : It undergoes tension both in extension as well s compression. In extension, spring pulls the objects, during compression it pushes the objects along the length.
Spring constant can be defined as the force required to elongate (or) compress the spring by one unit length.
Buoyant Force :
A substance which flows is called fluid. Liquids and gasses are considered as fluids.
The weight of ABCD is balanced by an upward force exerted by the remaining fluid. This upward force exerted by the fluid is called Buoyant force (B).
Note : If the part of the fluid is replaced by another object of same volume irrespective of material Buoyant force remains same when the fluid is at rest.
Viscous Drag :
when a body is in motion in the fluid then there develops an opposing force which opposes the motion of the body by the fluid. this force is called viscous drag. In mos of the cases when speed is small, viscous drag is proportional to the speed of the body.
Note : Viscous drag in the fluid is analogous to frictional force in case of solids. the origin of all the forces except gravitational pull is Electromagnetic force