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Unit of force is Newton, denoted by the letter N. In the International System of Units (SI), newton is defined as the force required to give a mass of one kilogram and an acceleration of one meter per second squared. Thus,
→ 1 N = 1 kg x 1 m/s2
Force is defined as the product of mass and acceleration. It was first discovered by Sir Issac Newton whose further research established the Newton’s Second Law of Motion. Force is a vector quantity that has both magnitude and direction and consists of a unit just like pressure. The unit of force is expressed in CGS, FPS and other international units.
- The base units related to force are: unit of length (metre, m), unit of mass (kilogram, kg) and unit of acceleration (m/s2).
- Hence, the unit of force in SI units is 1 Newton (1N) which is equivalent to 1 kg/ms2.
- In addition to the newton, other units of force include the pound (lb) in the US customary system and the dyne (dyn) in the centimeter-gram-second (CGS) system.
- The pound is defined as the force required to accelerate a mass of one pound at a rate of 32.174 feet per second squared, and the dyne is defined as the force required to accelerate a mass of one gram at a rate of one centimetre per second squared.
- However, newton is the preferred unit of force in most scientific applications because of its consistency with the SI system of units.
Read More: Force and Motion
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What is Force?
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Force is a physical quantity which helps to describe the influence of one object on another, causing it to accelerate or deform. It is a vector quantity, meaning that it has both magnitude and direction. Force is measured in newtons (N), which is the amount of force required to accelerate a mass of one kilogram by one meter per second squared.
What is Force?
Force is a push or pull on an object which results when one object come in contact with another. Newton is the derived SI unit of force. One Newton is defined as the force needed to accelerate 1 kg of mass at a rate of 1 m/s2. The mass accelerates in the same direction of the applied force. From Netwon’s Second law of Motion, force can be mathematically represented as:
| Force (F) = Mass (m) x Acceleration (a) → F = ma |
This can be expressed as F = mass x (length/time2) in terms of the seven fundamental dimensions of the physical world.
The dimensional formula for force can be written as: F = [M][L]/[T2]
= [M L T-2]
Units and Measurements Detailed Video Explanation
SI Unit of Force
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SI Unit of Force, Newton, is the universally accepted system of measurement. The SI Unit of force is denoted by the letter N written in uppercase. Mathematically this can be represented from Newton’s second law of motion as: F = ma
Thus, the SI Unit of Force is:
| SI Unit of Force, 1 N= 1kg/ ms2 |
Derivation of SI Unit of Force
From Newton’s second law, force (F) is the rate of change of momentum (P). The momentum is a product of mass (m) and velocity (v). Therefore,
Force = Rate of change of Momentum
= P/t, where t is the time
Substituting for the momentum we get,
F = mv/t
Substituting for units we get
N= kg. m/s.s
=kgm/s2
Here unit of force= N= kg/ms2
unit of mass = kg
unit of velocity = m/s
unit of time = s
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| Important Concepts Related to Force | ||
|---|---|---|
| Angular Momentum | Unit of Momentum | Mass and Momentum |
| Inertia and Mass | Momentum and Inertia | Moment of Force |
| Fundamental Forces | Types of Forces | Conservative Force |
Other Units of Force
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The unit of force is classified into two systems: Gravitational System and Absolute System.
- When the force is dependent on the gravitational forces, the unit of force is expressed in gravitational system.
- When the force is independent of the gravitational forces, the unit of force is expressed in absolute sustem.
- Hence there are other units of force apart from Newton.
Newton is the SI unit of force, whereas in the FPS system, when the mass is measured in pounds, the unit of force is called Poundal, pdl and pound-force is used to express force dependent on gravitational forces. Let us briefly look at soem of the important units of force in the international systems.
Dyne (Dyn)
Dyne is a unit of force in the CGS system. 1 dyne is the force applied to move an object of the mass 1 g moving with an acceleration of 1 cm/s2. This unit is generally used when working with small objects. The conversion from dye to SI unit of force is
So, 1 g⋅cm/s2 = 1 dyn
1 dyn = 10-5 N
Therefore, the conversion from dyne to SI unit of force is 1 N= 100,000 dynes
Poundal (pdl)
When an object of mass 1 pound is subjected to an acceleration of 1 foot/second2, the force applied is 1 poundal. This is a unit of the FPS (foot-pound-second) system. It is rarely used for scientific measurements.
Unit of Force = pound⋅foot/second2
1 lb⋅ft/s2 = 1 pdl
The conversion from pdl to SI unit of force is, 1 pdl = 0.1382 N.
Gram Force (gf)
This is another unit of force in the CGS system. 1 gram force is defined as the force acting on an object of mass 1 g due to gravitational acceleration. Another term for gram force is pond (p).
1 p = 1 gf
The conversion from gram force to SI unit of force is1 gf = 0.00980665 N
Pound Force (lbf)
Another unit of force in the FPS system is Pound Force. 1 pound-force is defined as the force acting on an object of mass 1 avoirdupois pound due to Earth’s gravity. This is a non-technical term of force and is known as the weight of an object.
1 lbf = 4.448222 N
Kip or Kipf
1 kip is defined as 1000 pounds force. It is mainly used by civil engineers, where pound-force is too small a measurement for heavy loads.
1 kip = 1000 lbf
Ton Force
1 ton-force is the force acting on an object of mass 1 ton due to Earth’s gravity.
1 tf = 9806.65 N
Ounce Force (ozf)
1 ounce force is the unit of force acting on an object of mass 1 ounce due to the gravitational acceleration of the Earth.
1 ozf = 1/16 lbf
The conversion from ounce force to SI unit of force is1 ozf = 0.278014 N.
Sthene
1 sthene is defined as the force applied to an object of mass 1 ton to give it an acceleration of 1 m/s2. This is a unit of the MTS (metre-tonne-second) system.
1 sthene = 1 ton⋅m/s2
The conversion from sthene to SI unit of force is1 sthene = 1000 N
Kilogram Force (kgf)
1 kilogram-force is the unit of force acting on an object of mass 1 kg due to Earth’s gravitational field.
1 kgf = 9.80665 N
The conversion from kgf to SI unit of force is 1 kgf = 1000 gf
Read More: Difference between Force and Momentum
Types of Force
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There are two types of forces:
- Contact Force: Contact force is a type of force that occurs between objects which are in physical contact with one another. When two objects touch each other, the atoms and molecules on their surfaces interact with each other, resulting in a force that can either push or pull the objects apart or together.
- Non-Contact Force: Non-contact force acts between two objects without the need for physical contact between them. This means that the objects do not need to be in direct contact with each other in order for the force to be applied.
Effects of Force
Here are some of the main effects of forces:
- Change in motion: A force can cause an object to start moving, stop moving, or change its direction of motion.
- Change in shape: Forces can cause objects to change shape. For example, if you push a spring down, it will compress and change the shape it has.
- Deformation: Forces can cause deformation or damage to objects.
- Friction: Forces can cause friction between two objects, which can either slow them down or prevent them from sliding against each other.
- Energy transfer: Forces can transfer energy from one object to another. For example, when you throw a ball, the force you apply transfers kinetic energy to the ball, causing it to move.
- Acceleration: Forces can cause acceleration, which is the rate of change of an object's velocity. A force can cause an object to speed up, or slow down.
Solved QuestionsQues: What is the push or pull of an object that causes it to accelerate called? What is the unit? Ans: Force is the push or pull experienced by an object that causes it to accelerate. the unit of force is Newton, or kg/ms2 Ques: In a tug of war, if one team pulls with a force of 100 N and the other team pulls with 80 N, what is the net force? Ans: Force by team 1= 100 N The force applied by team 2= 80 N However these forces are in the opposite direction in the game of tug of war. Hence, Net Force= 100-80= 20 N Ques: What is the force required to accelerate a car of 2000 kg at 3 km/s2? Ans: Mass of the car= 2000 kg Acceleration= 3m/s2 Therefore, F= mass x acceleration = 2000 x 3 = 6000 N Read More: Laws of Motion MCQs |
Things to Remember
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- There are many units of force, like dyne, kip, poundal.
- The most commonly used unit of force for scientific measurements is Newton (N).
- Newton is also the SI Unit of force.
- 1 N = 1 kg⋅m/s2
- 1 Newton force is defined as the force acting on an object of mass 1 kg moving with an acceleration of 1 m/s2.
- 1 dyne = 10-5 N
- The weight of an object is the force acting on it due to gravity.
- W = m⋅g (where m = mass of object, g = gravitational acceleration)
Previous Year Questions
- Calculate the tension in the string. [AMUEEE 2013]
- If the coefficient of static friction between the shoes of a runner… [UPSEE 2018]
- Assertion: Frictional forces are conservative forces. Reason… [AIIMS 2019]
- Which direction will the pendulum tilt? [NEET 2019]
- Calculate the ratio of rate of work done on the two balls. [KEAM]
- Which of the options do not represent a force? [KEAM]
- Calculate the impulse from the position-time graph. [AMUEEE 2012]
- Calculate the net force acting on the stone.
- Which of the following statements is true for the particle?
Sample Questions
Ques. Define 1 Newton force. (1 Mark)
Ans. The resultant force acting on an object of 1 kg mass moving with an acceleration of 1 m/s2 is defined as 1 Newton force.
1 N = 1 kg⋅m/s2
Ques. Which of the following is not a unit of force? (1 Mark)
(a)kg⋅m/s2
(b)N/m2
(c)Kip
Ans. Dyne, kg⋅m/s2 and Kip are units of force. N/m2 isn’t a unit of force, Newton (N) is.
Ques. 9000 dyn = ____ N (1 Mark)
(a)0.09
(b)9 x 10-5
(c)9 x 108
(d)0.009
Ans. 1 dyn = 10-5 N
9000 dyn = 9000 x 10-5 N = 0.09 N
Ques. What is the force applied over a 10 Kg cork floating on the surface of water? (2 Marks)
Ans. As we know, the weight of anybody always acts downward due to the involvement of gravity. Over the water surface, the buoyant force is also there that acts against the cork. This buoyancy force of water and force of the cork cancel out. So, the net force is zero.
Ques. Are the units of force, Kilogram Force and Newton the same? (2 Marks)
Ans. No, Kilogram Force and Newton are two different units of force. 1 kilogram-force (kgf) is defined as the force acting on an object of mass 1 kg due to Earth’s gravity (9.8 m/s2). While 1 Newton (N) is the force acting on an object of mass 1 kg moving with an acceleration of 1 m/s2.
1 kgf = 9.80665 N
Ques. Evaluate the Direction & Magnitude of the Net Force of a Stone of Mass 6 Kg Falling Out of the Window of a Stationary Bus. (3 Marks)
Ans. Data are given, the mass of the stone = 6 KG
Acceleration of the stone, a = g = 10 m/s²
According to newton's second law, F = ma
As such, the net force that acts on the stone Fnet = ma = 6 * 10 = 60 N
Therefore, the acceleration is downward, as a result, is 60 N downward direction due to gravitational force.
Ques. What is weight? Find out the weight of an object whose mass is 10 kg. (3 Marks)
Ans. In our day-to-day life, the line between the mass and weight of an object is often blurred. Mostly, we refer to the mass of an object as its weight. But the weight of an object is the force acting on it due to gravity.
Weight = mass x gravitational acceleration
= 10 kg x 9.8 m/s2
= 98 kg⋅m/s2
= 98 N
Thus, the weight of an object whose mass is 10 kg on Earth is 98 N.
Ques. A railway car of mass 20 tonnes moves with an initial speed of 54km/hr. On applying brakes, a constant negative acceleration of 0.3m/s2 is produced.
(a) What is the breaking force acting on the car?
(b) In what time will it stop?
(c) What distance will be covered by the car before it finally stops?Force
Ans. m = 20 tonnes = 20 x 1000kg u = 54 km/hr = 15 m/s
a = -0.3 m/s2 = 9 = 0
(a) F = ma
F = 20000 x (-0.3)
F = -6000N
(b) v = u+at
v-u = at
t = 50s
(c) (0)2 - (15)2 = 2(-0.3)s
S = 375m
Ques. When a body is in motion, in which direction does friction act? (2 marks)
Ans. When a body is in motion, friction acts in the direction opposite to the motion. This is also referred to as kinetic friction, which is the force that opposes the relative motion between two surfaces in contact when they are moving past each other.
For instance, if a book is sliding on a table to the right, the frictional force acting on the book will be to the left, opposing its motion. The amount of kinetic friction depends on the type of surfaces in contact, the force pressing the surfaces together, and other factors.
Ques. Determine the magnitude of the force exerted by a horse when pulling a cart of 600 kg, with acceleration at the rate of 1.2 m/s2. (2 marks)
Ans. Mass of cart = 600 kg,
And Acceleration = 1.2 m/s2.
By applying Newton’s second law of motion, the magnitude of the force is going to be:
F = ma = 600 x 1.2
Hence, F = 720 N
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