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### Course: Class 11 Physics (India)>Unit 9

Lesson 2: Newton's first law: Mass and inertia

# Newton's first law review

Review the key concepts, equations, and skills for Newton's first law of motion, including the difference between mass and weight

## Key terms

TermMeaning
MassThe amount of matter in an object, independent of its size or any forces acting on the object. Both a property of a physical object and how resistant an object is to acceleration. SI units of $\text{kg}$.
InertiaThe tendency of an object to remain at rest or remain in motion. This is measured by its mass.
WeightThe force of gravity on an object. Depends on the value of $g$.
External forceA force acting on an object from the outside, as opposed to forces acting within the object.
${F}_{\text{net}}$The net force, which is the vector sum of all external forces.

## Newton’s first law of motion

An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by a net external force.

## Equations

EquationSymbol breakdownMeaning in words
${F}_{g}=W=mg$${F}_{g}$ is force due to gravity, $W$ is weight, $m$ is mass, and $g$ is the gravitational field strength (acceleration) which is $9.8\frac{\text{m}}{{\text{s}}^{2}}$ on Earth's surface.The weight of an object is a long range force due to gravity. It is directly proportional to its mass and gravitational acceleration $g$.

## Common mistakes and misconceptions

Some students confuse mass and weight. Mass is how much matter an object is made from; it is constant. The weight of an object is the force exerted by gravity on that object, so it depends on the local $g$ value. For example, the value of $g$ on the Moon is much lower than on Earth, so an object weighs less there, but the mass does not change.

For more on weight and mass, read our article what is weight?.
To check your understanding and work toward mastering these concepts, check out the exercise on using Newton's first law to find net force and acceleration.

## Want to join the conversation?

• why is the gravity of the black hole ineffective in the following case?
according to hawkings there is some amount of emitted radiation which is generally due to the particles(like matter-antimatter that exists in pairs, and which is slightly away, it escapes in the form of radiation).
But the gravity at the center of the blackhole is quite high so how is it possible for such a small particle to overcome the gravitational force??
• No one knows for sure, but scientists suspect that the radiation particles come out the other side where there is no gravitational force. Technically speaking though, no one has ever been in a blackhole in the first place so no one knows.
• What do you mean by inertial frame of reference ? Are there different kinds of inertial frame of reference?
• In classical physics and special relativity, an inertial frame of reference is a frame of reference not undergoing any acceleration.
(1 vote)
• what is the major difference (main idea) between mass an weight?
• mass is how much matter something is made up of and weight is how hard the earth is pulling on an object. The amount of matter in an object would be the same wether it was on the moon or on earth, but the earth pulls down on objects less than the earth does so the same object with the same masses on the earth and the moon would have a larger weight on earth and a smaller weight on the moon
• what are forces acting within the object?
• These are internal forces- often denoted by 'U'- these have no contribution whatsoever to the Fnet, by any chance. Internal forces become a critical concept in Thermodynamics- but not in Newtonian Physics, so far.
(1 vote)
• so galialo newton and that other one were the first ones to know about force.
• Most likely, they were the first ones that almost completely understood force relative to the technology at that time. There may of been some unknown individual before them, but they were the ones to publish well-known and acknowledged books about their findings that were very accurate and still referred to today.
• how does not the mass depend on the size of object? and what is inertial reference frame?
• the larger the size of the body the more particles it contain thus the mass increases.
• What is this “s” squared variable? Can someone define it, please?
• seconds squared
(1 vote)
• what does vector mean
(1 vote)
• In mathematics and physics, a vector is a mathematical object that has both magnitude (size or length) and direction. It is often represented as an arrow, where the length of the arrow represents the magnitude, and the direction of the arrow represents the direction.

Vectors are used to describe quantities that have both magnitude and direction, such as velocity, force, displacement, and acceleration. They can be added, subtracted, and multiplied by scalars (single numbers) to perform various mathematical operations.

For example, if you are driving a car, the velocity of the car can be represented as a vector. It would have a magnitude (speed) and a direction (e.g., north, east, etc.). Vectors are a fundamental concept in many areas of science, engineering, and mathematics.