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Change in State of Motion

Last Updated : 08 Dec, 2021
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In science, a push or pull of an entity is identified as a Force. The interaction between two objects arises from the force. Force has both magnitude and direction. The strength of a force is articulated in magnitude. Force brings about an altar in the direction or state of motion of a body.

Characteristics of forces:

  • The net resultant force on an object is the sum of these two forces when two forces act in the same direction.
  • The net resultant force is the difference between these two forces when two forces act in opposite directions. The magnitude of the force describes its strength.
  • The force all the time has a direction in which it is applied and determines its strength or magnitude.
  • When the direction of the magnitude of the force has changed the effects of a force may alter.
  • By evaluating the net force acting on that object the effect of more than one force being applied on an entity is calculated.
  • The net force acting on the entity will be zero if two forces are acting upon each other having the same magnitudes (strength) and in conflicting directions.
  • Force can bring dissimilar effects to an object’s position, size and shape.
  • F = m × a, where F = Force, m = Mass of object and A = Acceleration
  • Newton (N) is the SI unit of force.

Force can alter the state of motion of an entity:

Motion of an object:

  • If an object is moving at a certain speed in a particular direction then it is said to be in motion.
  • The direction of the magnitude of the force is changed if the object is at rest. It means that it is not altering its position with respect to an observing point.
  • When the entity starts moving it means that its spot is being distorted with respect to a surveillance point.
  • To move an entity from one place to another, a force is necessary to bring that object in motion.
  • Not only this, a force applied to an object can change its speed, bring it to rest, or even change the direction of its motion.
  • It may bring alter in the speed of the motion in total and an amalgamation of these effects as well such as an alter in direction of motion.
  • Force can alter the state of motion of an entity.
  • Without the application of a force, any object cannot move by itself or change its state of motion on its own.
  • This change of state of motion will not take place every time with every kind of object. For instance, if someone tries to move forward a very weighty entity such as a wall, it would not at all.

Force can change the shape of an object:

The shape of an object can be altered if some force is applied to it. Depending upon the magnitude of the applied force and the rigidity of the object, the effect on its shape and size can be observed.

Push: A force exerted away from the body is called push, e.g: Hitting a ball, kicking a football.

Pull: A force exerted towards the body is called pull, e.g: drawing a bucket of water from a well, playing tug of war.


  • A push or a pull can be a force.
  • The interaction between objects can change the state of the objects.
  • The state of an object from rest to motion or vice versa can be changed by a force.
  • Two or more objects must interact with each other, to let a force approach into play.

Net force:

  • The resultant of all the forces acting on an object is known as the net force.
  • The body’s acceleration is along the direction of the net force.


  • In magnitude as well as the direction of the object vector quantities are expressed. E.g: Velocity, displacement, weight, momentum, force, acceleration, etc.
  • To find the resultant component acting on an object, vectors are used.
  • When several forces act on a body, they can be determined into one component. It is called the net force acting on the entity.

When the force acts at an angle to the horizontal, vectors are also used.

Application of Force:

  • A force is an attempt that changes the state of an entity at rest or in motion.
  • It can alter an entity’s velocity and direction.
  • The shape of an entity can also be altered by force.

State of Motion

The state of motion of an entity is defined by its velocity – the speed with a direction. Thus, inertia could be redefined as follows:

Inertia = tendency of an entity to oppose changes in its velocity.

An entity at rest has zero velocity – and (in the nonappearance of an unhinged force) will stay with a zero velocity; it will not alter its state of motion (i.e., velocity). Objects oppose changes in their velocity.

Types of Forces

Contact Force 

Touch or contact is necessary to do the majority of our daily actions. E.g Lifting, pulling, kicking, pushing, etc. Forces that require a touch or contact to be applied are known as contact forces. E.g: Muscular forces, frictional forces

  • Muscular force: The force that comes into engaging in recreation because of the action of muscles is called muscular force. For example:
    • In order to walk human beings use muscular force.
    • The expansion and contraction of the lungs are because of muscular force.
    • Movement of food along the food pipe.
  • Frictional force: Whenever the object moves on the surface this force is exerted by the surface over an object. Characteristics of the force of friction:
    • The force of friction forever acts in the contradictory direction of the motion of the entity.
    • It leads to the generation of heat as two surfaces come in contact with each other. For example, heat is produced as a result of friction between our hands, when we rub our hands together.
    • Frictional force also leads to wear and tear of the surfaces of entities that get in touch with each other. For example, the sole of shoes often gets worn out due to friction force that acts between them and the ground as we walk.
    • The relative motion between two surfaces is opposed by this force.
    • Acts between the surfaces of the two bodies in contact.

Air Resistance: An object experiences a force called air resistance, whenever it moves or flies in the air, it experiences a force called air resistance.

Non-contact forces

These forces do not need contact or have their effect without a touch. Example: magnetic force, electrostatic force, gravitational force.

  • Magnetic force:
    • The force of attraction or repulsion between two magnetic bodies thanks to their poles is understood as a magnetic force.
    • The force exerted by any magnetic object is named magnetic force.
    • We know that like magnetic poles forever drive back each other, that is, they shove each other away.
    • Also, opposite magnetic poles constantly attract each other, that is, they pull each other towards themselves.
  • Gravitational force:
    • The attractive force that a body experiences towards the center of the earth is called the force of gravity due to the earth.
    • Every entity attracts or exerts a force on every other entity, the property of the universe.
    • That acts upon all the objects that are present on or near the Earth’s surface is also called the force of gravity or gravity.
    • Gravity is a property shown by every entity present in space and not only the earth. Hence, all the planets, the moons, and even the sun have a gravitational force of their individual.

Electrostatic force: Electrostatic Force is the force of attraction or repulsion experienced by a charged body from another charged body in the same neighborhood.

Nuclear forces:

  • The nuclear force acts among all the particles in the nucleus. i.e., between two protons, between two neutrons, and between a neutron and a proton.
  • In all cases, it is an attractive force.
  • This force by overcoming the enormous repulsive force between positive protons keeps the nucleus intact.

Change in Motion


Acceleration is defined because of the rate of altering velocity with reference to time. Acceleration may be a vector quantity because it has both magnitude and direction. It is also the second derivative of position with reference to time, or it’s the primary derivative of velocity with reference to time.

Instantaneous Acceleration:

Instantaneous acceleration is defined as the ratio of alter in velocity during a given time period such that the time interval goes to zero. Acceleration Formula:

Acceleration formula is given as:

Acceleration = (final velocity – initial velocity)/time 

                    = (change in velocity)/(time) 

                    = \frac{v_{f} - v_{i}}{t}

                 a = \frac{\Delta v}{t}


a is the acceleration in m.s-2

vf  is the final velocity in m.s-1

vi is the initial velocity in m.s-1

t is the time interval in s

Δv is that the chicken feed within the velocity in m.s-1

Unit of Acceleration: The SI unit of acceleration is given as m/s2.

Uniform and Non-uniform acceleration:

It is possible in circular where speed remains constant but since the direction is changing hence the speed changes, and therefore the body is claimed to be accelerated.

Average acceleration:

The average acceleration over a period of time is defined because the total change in velocity within the given interval divided by the entire time taken for the change. For a given interval of time, it’s denoted as ā.


\bar{a} = \frac{v_{2} - v_{1}}{t_{2} - t_{1}} = \frac{\Delta v}{\Delta t}

Where v2 and v1 are the instantaneous velocities at time t2 and t1 and ā is that the average acceleration.


You must have noticed that always we hamper the speed of our bikes during heavy traffic when more bikes are obstructing us. So, a decrease in speed because the body moves far away from the start line is defined as Deceleration. Deceleration is the opposite of acceleration.

It is expressed as

Deceleration = (Final time – Initial time)/(Time taken)

Deceleration also is known as negative acceleration. Hence, it is denoted by (– a).

Deceleration Formula is given by it is the final velocity minus the initial velocity, with a negative sign in the result because the velocity is decreasing, if starting velocity, final velocity and time taken are given.

= \frac{v - u}{t}

If initial velocity, final velocity and distance travelled are given, deceleration is known by

a = \frac{v^{2} - u^{2}}{2s}


v = final velocity,

u = initial velocity,

t = time taken,

s = distance covered.

Deceleration Formula is employed to calculate the deceleration of the given body in motion. It is expressed in m/s2.

Sample Problems

Question 1: A boy weighing 56 kgf stands on a platform of dimensions 3.5 cm × 1.5 cm. What pressure in pascal does he exert?


Force = Weight = 56 kgf = (55\times 10)     N = 560N

Area = 3.5 cm \times 1.5 cm = \frac{35}{1000} m \times \frac{15}{1000} m = \frac{40000}{7}     m2

Pressure = \frac{Force}{Area} = \frac{560N}{\frac{40000}{7} m^2} = 320000 Pa = 3.2\times 10^{5} Pa

Question 2: A wheel of diameter 4 m can be rotated about an axis passing through its centre by a moment of force equal to 5.0 N m. What minimum force must be applied on its rim?


Diameter = 4 m

Radius = 2 m

Therefore, ⊥ distance = 2 m

Moment of force = 5.0 Nm

Moment of force = Force × ⊥ distance

5.0 Nm = F × ⊥ distance

5.0 Nm = F × 2 m

F = 2.5 N

Question 3: The moment of a force of 60N about a point is 3 Nm. Find the perpendicular distance of force from that point.


Force applied = 60 N

⊥ Distance from the point of rotation =?

Moment of force = Force × ⊥ distance

3 = 60 × ⊥ distance

Distance = 3/60 = 1/20 m = 20 cm

Question 4: Find the thrust required to exert a pressure of 40000 pascals on an area of 0.006 m2?


Pressure = \frac{Force}{Area}

Force = Pressure × Area

F = 40000 × 0.006 = 240 Newton

Question 5: A car moving with a uniform velocity of 55 Kmph is brought to rest in traveling a distance of 2.5 m. Compute the deceleration formed by brakes?


Given: Initial velocity u = 55 Kmph,

           Final velocity v = 0

           Distance covered s = 2.5m

We know that v2 = u2 + 2as

Deceleration a = \frac{v^{2}- u^{2}}{2s}

                        = - \frac{55000^{2}}{2\times 2.5}

a = -605x 106 m/s2

Question 6: A toy automobile accelerates from 2 m/s to 6 m/s in 4 s. What is its acceleration?


Given: Initial Velocity u = 2 m/s,

Final Velocity v = 6m/s,

Time taken t = 4s.

The acceleration is given by a = \frac{v-u}{t}

= \frac{6-2}{4}


= 1m/s2

Question 7: From a bridge, a stone is released into the river. It takes 5s for the stone to contact the river’s water surface. From the water level calculate the height of the bridge.


Because the stone was at rest, Initial Velocity, u = 0

t = 5s (t is Time taken)

Acceleration due to gravity, a = g = 9.8 m/s2

Distance covered by stone = Height of bridge = s

The distance covered is articulated by

s = ut + \frac{1}{2}gt^{2}

s = 0 + \frac{1}{2}× 9.8 × 5 = 245    m/s2

Therefore, s = 24.5 m/s2

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