Newton's First Law of Motion

The Law of Inertia: Objects at rest stay at rest, objects in motion stay in motion unless acted upon by an external force.

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Explore the Law of Inertia
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Galileo Galilei
1564 - 1642
Italian astronomer, physicist, and engineer who pioneered the experimental scientific method. Through his inclined plane experiments, Galileo discovered that objects maintain constant velocity without external forces - laying the groundwork for Newton's First Law. His telescopic observations revolutionized astronomy, while his studies of motion transformed physics.
Newton's First Law of Motion
An object remains in a state of rest or of uniform motion in a straight line unless compelled to change that state by an applied force. This tendency of objects to resist changes in their state of motion is called inertia, which is why this law is also known as the Law of Inertia.
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Sudden Braking
When a car brakes suddenly, passengers lurch forward. Their bodies tend to continue moving at the original speed due to inertia. Seatbelts provide the force needed to decelerate with the car.
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Bus Starting
When a stationary bus suddenly starts moving, standing passengers fall backward. Their bodies resist the change from rest to motion, maintaining their state of rest while the bus floor moves forward.
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Sharp Turns
During sharp turns at high speed, passengers are pushed to one side. Their bodies tend to continue in a straight line while the vehicle changes direction, demonstrating directional inertia.
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Galileo's Discovery
A marble on a frictionless inclined plane will roll down and up to the same height. If one side becomes horizontal, the marble continues forever, showing objects in motion stay in motion without friction.