1.2 Newton’s First Law of Motion
Newton’s first law of motion states “A body at rest or a body in
uniform motion will remain in their state of motion provided that
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the net external force acting on the body is zero.” A body at rest
or a body in uniform motion may be considered in linear
equilibrium. A ball can continuously roll at constant speed on a
smooth frictionless surface because there is no net external force
that acts on it. But over a rough and bumpy surface, the ball may
experience a decrease of speed or may even stop because of Figure1.1 A man is thrown
friction. A man in a stopped car can be thrown forward when the forward when a car slows
car suddenly stops (Figure 1) or thrown backward when the car down.
suddenly starts.
How large the net force is needed to change the state of motion of an object depends on its inertia.
Inertia is an object's tendency to resist changes in its initial state of motion. The greater its inertia,
the difficult it is to change an object’s state of motion, whether it is initially at rest or moving with
constant velocity. The inertia of an object can be measured by its mass. The greater is its mass,
the greater is its inertia. Massive objects require greater net force to accelerate it.
To maintain the object’s state of motion, the condition that it must satisfy is that the net force
acting on it must be zero. That is, ∑ 𝐹⃗ = 0. In three-dimensional coordinate system, there can
be forces acting on the object along the x-axis, y-axis, and z-axis, thus
∑ ⃗⃗⃗⃗
𝐹𝑥 = 0 ⃗⃗⃗⃗
∑𝐹 𝑦 =0 ∑ ⃗⃗⃗⃗
𝐹𝑧 = 0
The net force is akin to the resultant force when two or more force
vectors are added together. There are several methods to calculate
the resultant. These are component method, law of sine and cosine
method, polygon (tip-to-tail) method, and parallelogram method.
Resultants can also be expressed using unit vector notation 𝒊⃗ , 𝒋⃗, ⃗⃗⃗⃗⃗
𝒌.
Figure 1.2. Cartesian coordinates
in three dimensions
Newton’s first law of motion states “A body at rest or a body in
uniform motion will remain in their state of motion provided that
https://mammothmemory.net/physics
the net external force acting on the body is zero.” A body at rest
or a body in uniform motion may be considered in linear
equilibrium. A ball can continuously roll at constant speed on a
smooth frictionless surface because there is no net external force
that acts on it. But over a rough and bumpy surface, the ball may
experience a decrease of speed or may even stop because of Figure1.1 A man is thrown
friction. A man in a stopped car can be thrown forward when the forward when a car slows
car suddenly stops (Figure 1) or thrown backward when the car down.
suddenly starts.
How large the net force is needed to change the state of motion of an object depends on its inertia.
Inertia is an object's tendency to resist changes in its initial state of motion. The greater its inertia,
the difficult it is to change an object’s state of motion, whether it is initially at rest or moving with
constant velocity. The inertia of an object can be measured by its mass. The greater is its mass,
the greater is its inertia. Massive objects require greater net force to accelerate it.
To maintain the object’s state of motion, the condition that it must satisfy is that the net force
acting on it must be zero. That is, ∑ 𝐹⃗ = 0. In three-dimensional coordinate system, there can
be forces acting on the object along the x-axis, y-axis, and z-axis, thus
∑ ⃗⃗⃗⃗
𝐹𝑥 = 0 ⃗⃗⃗⃗
∑𝐹 𝑦 =0 ∑ ⃗⃗⃗⃗
𝐹𝑧 = 0
The net force is akin to the resultant force when two or more force
vectors are added together. There are several methods to calculate
the resultant. These are component method, law of sine and cosine
method, polygon (tip-to-tail) method, and parallelogram method.
Resultants can also be expressed using unit vector notation 𝒊⃗ , 𝒋⃗, ⃗⃗⃗⃗⃗
𝒌.
Figure 1.2. Cartesian coordinates
in three dimensions
