Unit Dynamics Module Newton s Three Laws page 1 of 2 Newton s First Law Dynamics allows you to understand why objects move the way they do Newton s first law states that in the absence of a net force a body at rest stays at rest and a moving body continues moving in a straight line with constant speed Inertia is the resistance of an object to changes in its state of motion Friction is a ubiquitous force in daily life that brings moving objects to a halt Everyday experience with friction can make Newton s first law seem counterintuitive Newton s first law applies only in inertial reference frames Kinematics allows you to describe the motion of objects for example the path of a golf ball after it is hit Dynamics allows you to understand why objects such as golf balls move the way they do Experiments by Galileo Galilei in the late 1600 s set the stage for a more complete exploration of motion by Isaac Newton Newton first published the ideas of dynamics in Philosophiae Naturalis Principia Mathematica in 1687 revolutionizing our understanding of the natural behavior of objects A simplified statement of Newton s first law says that an isolated body maintains uniform motion An isolated body is one that is not being pushed or pulled in other words it experiences no net force Uniform motion refers to motion that is steady or unchanging A more complete statement of Newton s first law says that in the absence of a net force a body at rest stays at rest and a moving body continues moving in a straight line with constant speed The absence of a net force means that the vector sum of the forces on an object is zero Newton s first law is also known as the law of inertia Newton s first law may seem counterintuitive at first If a driver applies a force to a car it moves with a constant velocity and if a driver stops applying that force it will slow to a stop In both cases the net force not just the force of the driver is important Friction is a ubiquitous force that resists the relative motion between two surfaces In this example friction must be considered since it always resists the motion of the car www thinkwell com info thinkwell com Copyright 2001 Thinkwell Corp All Rights Reserved 1785 doc rev 03 29 2001 Unit Dynamics Module Newton s Three Laws page 2 of 2 Newton s First Law A table is set with plates glasses and flowers The tablecloth is yanked out from underneath the settings The objects remain sitting on the table demonstrating Newton s first law in the absence of a net force objects at rest remain at rest There is a small frictional force between the objects and the tablecloth but not enough to move them A marble is rolled around a C shaped track When it reaches the end of the track it does not continue moving in a circle but instead moves in a straight line away from the track As the ball moves around the track the track exerts a force on the marble and makes its path circular Once the track ends the force is removed and there is no net force on the marble The marble s motion is a straight line with constant speed demonstrating Newton s first law www thinkwell com info thinkwell com Copyright 2001 Thinkwell Corp All Rights Reserved 1785 doc rev 03 29 2001
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