Law of Motion and Force | Physics, Types, Facts, Simple explanation |
Newton's laws of motion are of central importance in classical mechanics (physics). A large number of laws and results may be derived by Newton's laws. The first two laws related to the type of motion of the system that result from a given set of forces, while second law relates force, acceleration and mass.
Force
Force is a push or pull which can change the position of a body as required. It is a vector quantity following all the rules of vector.
Forces are of two types:
1. Balanced Forces If there are many forces acting on a body, but resultant of all of them is zero, then the forces are called balanced forces.
2. Unbalanced Forces If the resultant of all the forces is not zero, then the forces are called unbalanced forces.
Newton's Law of Motion
There are three laws of motion
First Law of motion
"Everybody remains in its state of rest or state of motion, until an external force is applied on it." This law is also known as law of inertia.
Inertia Inertia is a property by virtue of which, a body tries to retain its state. Inertia are of two types
1. Inertia of Rest If a body is in rest, then it will remain in rest until an external force is applied on it.
2. Inertia of Motion If a body is in motion, then it will remain in motion until an external force is applied on it.
There are some examples of inertia in everyday life.
- A glass is broken into pieces when a stone strikes on it. but when a high speed bullet hits the glass it makes a clean hole.
- The persons sitting in the bus falls backward when bus starts to move suddenly.
- An athlete runs very fast before taking high jump.
- A man getting down of a moving bus falls forward.
Second Law of motion
"The force applied on a body is equal to the product of mass of the body and the acceleration produced in it.
F = ma
and also the rate of change of linear momentum (p = mv) of a body is proportional to force applied in direction of force
F = dp/dt
Third Law of motion
"Every action have equal and opposite reaction." Action and reaction always act on the different bodies.
Linear Momentum
The product of mass and velocity of a body is called linear momentum It is represented by p. its unit is kg-m/s. It is a vector quantity.
p = mv
Law of Conservation of Linear Momentum
If the external force on a system is zero, then the linear momentum of the system remains conserved.
If two particles of masses m1 and m2 move with velocities u1 and u2 before collision and after collision with velocities v1 and v2, then from law of conservation of momentum.
m1u1 + m2u2 = m1v1 + m2v2
Impulse
If a force acts on a body for a very short time Δt, then the product of force and time is called the impulse.
Impulse = Change in momentum
or, Impulse = F × Δt
Friction
When a body slides (or has sliding tendency) or rolls over another body or on a surface, then a force opposing the motion acts between those surfaces of the body which are in contact, this force is called friction.
Two basic types of friction is kinetic friction and static friction.
Kinetic friction
Kinetic friction is defined as a force that acts between moving surfaces. A body moving on a surface experiences a force in the opposite direction of its motion. The magnitude of the force will depend on the coefficient of kinetic friction between the two substances.
Friction is simply defined as the force that holds a sliding object back. Kinetic friction is a part of everything and it interferes with the motion of two or more objects. The force acts in the opposite direction to the way an object is supposed to slide.
If a car has to stop, we apply the brakes and that's where the friction comes in.
When walking, when one wants to stop suddenly, friction is again to thank.
But when we have to stop in the middle of the puddle, things get tough because there is less friction and no one can get that much help.
kinetic friction formula
The coefficient of kinetic friction is denoted by the Greek letter "mu" (μ) with a subscript "k". The kinetic frictional force is μk times the normal force on a body. It is expressed in Newton (N). The kinetic friction equation can be written as:
Force of Kinetic Friction = (Coefficient of Kinetic Friction) (Normal Force)
FK = μk η
where,
Fk = force of kinetic friction
μk = coefficient of kinetic friction
η = normal force (Greek letter "eta")
Static friction
Static friction is a force that keeps an object stationary. The static friction definition can be written as:
Friction is experienced when individuals attempt to move a stationary object across a surface without actually triggering any relative motion between the body and the surface it is on.
It can be explained as the force of friction that precisely balances the applied force for the duration of the steady state of the body.
Static friction force is a self-regulating force, i.e. static friction will be equal and opposite to the force applied at all times.
Expert TIPS
- If a body of mass m strikes horizontally a wall with velocity v and bounces back with the same velocity then the change in momentum = 2mv.
- Liquid of density d coming out of a pipe of cross-sectional area A with velocity v exerts the force Adv².
- If n bullets each of mass m and muzzle velocity v, are fired from gun, then the average force acting on the gun is mnv.
- Motion of gun and bullet ie, when a bullet is fired from a gun also get a motion in reverse direction with speed v = m×u/M, where m is mass of bullet, u is speed of bullet. M is mass of gun and v is velocity of gun in reverse direction.
Weight of a man in a lift (w)
- When lift is at rest w - mg
- When lift is moving upward with acceleration a, then w = m(g + a)
- When lift is moving downward with acceleration a, then w = m(g - a)
- When the rope of lift breaks then lift will move downward with acceleration g, then w = 0.
Moment of Force
It is the measure of the tendency of a force to cause a body to rotate about a specific point or axis. It is also defined as the product of force applied and perpendicular distance from the pivot.
ie, τ = F × d
It has two directions of rotation i.e., clockwise and anticlockwise.
Laws of Moments
When an object is balanced (in rotational equilibrium) the sum of the clockwise moments is equal to the sum of the anti-clockwise moments.
Force1 × Its distance from pivot = Force2 × Distance from the pivot
ie, F1 d1 = F2 d2
Parallel Forces
It is a system of forces whose routes of action are parallel. There are two types of parallel forces
Like Parallel Forces
Forces which are parallel and acting in same direction are called like parallel forces. Resultant force of two like parallel forces is the sum of parallel forces.
Unlike Parallel Forces
Forces which are parallel and acting in opposite direction are called unlike parallel forces. Resultant force of two unlike parallel forces is the difference of two forces.
Couple
It consist of two equal and opposite forces whose line of action do not coincides. The length between these two line of forces is known as arm of couple.
Lever
It is a simple machine which has a board or bar that rest on turning point. This turning point is known as fulcrum.
Mechanical Advantage It is the measure of the force amplification achieved by using a tool, mechanical device or machine system.
Mechanical advantage = Weight w / Line of effort P
There are three types of lever
1. Lever of First Group In this group, fulcrum is in between and the applied force and load are at opposite ends like scissors, sea saw etc.
Mechanical advantag (MA) = Load/line of Effort = AF/BF
(AF > BF)
2. Lever of Second Group In this lever, the load is between the effort and the fulcrum e.g., wheel barrow whose wheel's axle is the fulcrum, handles are effort and load is placed between them.
= w/P = AF/BF
3. Lever of Third Group In this type of lever, effort is placed between the load and the fulcrum eg., hammer.
= w/P = AF/BF
To Measure Accurate Weight by Faulty Balance
1. When both scales of a balance are unequal, then weighing the object on both scales, find the mass of w1 and w2, then accurate mass w = w1 + w2 / 2.
When arms of the balance are not equal and object on both scales having masses w1 and w2, then
w = √ w1 × w2
Related Questions : (Answer in comment box.)
Q1. Two spheres of masses 3 kg and 4 kg are attached to ends of a string which passes over a frictionless pulley. Take g 9.8 m/s². The relative acceleration of the system is
(a) 5 ms-²
(b) 1.4 ms-²
(c) 3 ms-²
(d) 3.8 ms-²
Q2. A person of 60 kg enters a lift going up with an acceleration 2 ms-². The vertical upward force acting on the person will be (g = 10 ms-²).
(a) 720 N
(b) 680 N
(c) 900 N
(d) 1600 N
Q3. A person of 60 kg descends in a lift with an acceleration 2 ms-². The cable of lift suddenly breaks down. The weight of the person inside the lift is.
(a) 60 g
(b) 28 g
(c) zero
(d) 68 g
Q4. A man is sitting on the upper birth of the coach of a train, which is to be stopped on a nearby station. Man drops an apple on the open hand of his friend sitting just below the man about 2 m away. Apple will drop
(a) exact in the hand of his friend
(b) little away from the hand, in the direction of motion of trains
(c) little away from the hand, opposite to the direction of motion of train
(d) None of the above
Q5. A rocket works on the principle of conservation of
(a) angular momentum
(b) energy
(c) linear momentum
(d) speed
Q6. A solid block is lying on a smooth horizontal table. A bullet hits and gets embedded in it. The physical quantity conserved is
(a) kinetic energy alone
(b) momentum alone
(c) both kinetic energy and momentum
(d) power alone
Q7. When brakes are applied, it becomes difficult to moves a cycle because
(a) sliding friction is greater than rolling friction
(b) rolling friction is greater than sliding friction
(c) rolling friction is absent
(d) sliding friction is absent
Q8. The weight of a person will be greater when the lift
(a) accelerates downwards
(c) accelerates upwards
(b) fall with uniform velocity
(d) retards upwards
Q9. A person of 50 kg revolves around earth in a satellite. In this situation, his weight will be
(a) 50 kg wt
(c) 5 × 9.8 N
(b) 50 N
(d) zero
Q10. On a stationary sail boat, the fan attached to the boat projects/blows air at sails. The boat will
(a) move with air blown
(b) move opposite to air blown
(c) not move
(d) spin around
|Law of Motion and Force|
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