Newton's Laws
Newton's three laws of motion and applications
Newton's Three Laws of Motion
First Law (Inertia): An object remains at rest or moves at constant velocity unless acted on by a net force.
Second Law: The net force on an object equals its mass times its acceleration (F_net = ma).
Third Law: For every action force, there is an equal and opposite reaction force.
Example
Applying Newton's Second Law
A 1 200 kg car experiences an engine force of 4 000 N and a friction force of 1 000 N.
F_net = 4 000 − 1 000 = 3 000 N (forward)
a = F_net / m = 3 000 / 1 200 = 2.5 m·s⁻²
The car accelerates at 2.5 m·s⁻² in the forward direction.
Free-Body Diagrams
A free-body diagram shows all the forces acting on an object. Draw the object as a dot. Draw arrows from the dot in the direction of each force. Label each arrow with the type and magnitude of the force. Longer arrows = larger forces.
Note
Common Forces
Weight (W = mg, downward), Normal force (N, perpendicular to surface), Friction (f, opposing motion), Applied/Engine force (F, in direction of applied push/pull), Tension (T, along a string or rope). Use g = 9.8 m·s⁻² in South Africa.
Key Vocabulary
ForceA push or pull that can change an object's motion
InertiaThe tendency of an object to resist a change in its motion
Net forceThe overall force on an object after all forces are combined
FrictionA force opposing the motion between two surfaces in contact
Normal forceThe perpendicular force a surface exerts on an object
WeightThe gravitational force on an object (W = mg)
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