**Newton’s law of motion and law of inertia**

**different types of forces**.

**Kinematics**in this topic we have to study about dynamics means cause of motion means if an object is moving with constant velocity why ? if an object moving with constant acceleration why ? if an object is at rest why ? all the reason or cause we will study in dynamics and Newton’s laws of motion to understand Newton’s laws of motion it is very important to understand “force” in day to day life we listen force word so first try to understand force so lets talk about force.

**Newton’s laws of motion:**

**1st Law (Laws of inertia)**it state that if a body is at rest or moving it will continue in its same state unless any external force is not applied

**only external force can change its initial physical state this is also called laws of inertia if net force on a body is zero acceleration of body will be zero change in velocity is zero or velocity is constant means magnitude and direction is constant**

From above three picture state clearly Newton’s first laws of motion state if velocity is not constant then acceleration is there and hence net force will not be zero if net force is zero it does not mean that velocity is zero velocity will be constant and body will be in motion.

**2nd Laws :**rate of change of momentum is equal to the force applied on the body by external agent that is ∆p/∆t = (p₂-p₁)/(t₂-t₁) now for very very small time we write dp/dt =d(mv)/dt now differentiate with respect to time dp/dt = mdv/dt +vdm/dt here mass is taken as constant for this laws so dm/dt = 0 hence equation becomes dp/dt = mdv/dt = ma

**Newton’s third law :**if we apply force on a body then that body also apply same force in opposite direction.

See the picture above block is on ground apply a normal contact force on earth N and earth also apply same normal reaction force on block N contact force has two important point first its always acts normal to contact surface and second its direction compress the object this is important point for contact forces.

Newton’s third law of motion is action reaction pair of forces every action has reaction this is law of nature both the forces action and reaction nature should be same.

important point where is reaction of mg force normal N force is not reaction force because mg force is gravitational force and N force is electromagnetic force hence it can’t be reaction force because both the forces nature should be same see below some question.

Free body diagram state that show all the forces acting on a body according to their direction to analyse acceleration forces easily on the body

from above FBD we can write Newton’s laws of motion second equation Fnet = ma along x axis see below

50 – N1 = 2a (1) this is for 2kg block

N1 = 3a (2) this is for 3kg block now put the value of N1 in equation (1) then we can write 50-3a = 2a or a = 10 m/s*s this is acceleration of both block move with same acceleration now N1 = 3*10 = 30N this is normal force between block.

**Important concept** alternate solution for above question since both block moves with same acceleration we can add two masses and take as single mass that is 2kg +3kg = 5kg single block external force will be same 50N now we can write a single equation Fnet = ma 50 = 5a so a = 10m/s*s and this acceleration will provide contact force N1 = ma = 3*10 = 30N

**Newton’s law of motion and law of inertia**thanks for reading and sharing.

dated 21st Jul 2018

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