Newton’s Laws of Motion First Law – Inertia Unit 3.2

Newton’s 1st Law of Motion Physicists study everyday motion as examples to determine exactly what effect forces have on the motion of an object.

Newton’s 1st Law of Motion The first step is to examine the motion of the object before forces are applied. A common misconception is that an object with no force acting on it will always be at rest.

Newton’s 1st Law of Motion To test whether this is true consider sliding a block on different surfaces.

Newton’s 1st Law of Motion First imagine a block on deep, thick carpet. If you apply a force by pushing a block, it will begin to slide but once you remove the force, the block will come to a stop.

Newton’s 1st Law of Motion Now, if you were to push the same block across a smooth, waxed floor with the same force, the block will slide much farther before coming to rest.

Newton’s 1st Law of Motion In the 1630’s, Galileo concluded that “it is not the nature of an object to stop once it is set in motion, rather, it is an object’s nature to maintain its state of motion.”

Newton’s 1st Law of Motion This concept was further developed by Newton in 1687 and has come to be known as Newton’s first law of motion. The tendency of an object not to accelerate is called inertia.

Newton’s 1st Law of Motion First law states that an object at rest remains at rest and an object in motion continues in motion with constant velocity (that is constant speed in a straight line) unless the object experiences a net external force.

Newton’s 1st Law of Motion Acceleration is determined by net external forces. So according to Newton’s first law, a car at constant velocity has a net external force of zero. However many forces are acting on a car in motion.

Newton’s 1st Law of Motion Fforward ~ forward force of road on tires Fresistance ~ friction between road & tires and air resistance Fgravity ~ downward force Fground on car ~ upward force of the road on car

Newton’s 1st Law of Motion To understand how a car can maintain constant velocity with so many forces, we need to distinguish between external force and net external force.

Newton’s 1st Law of Motion An external force is a single force that acts on an object as a result of the interaction between an object and its environment. The net external force is the vector sum of all forces acting on an object.

Newton’s 1st Law of Motion When all external forces acting on an object are known, the net external force can be found using the methods for finding resultant vectors.

Newton’s 1st Law of Motion This means that an object’s acceleration is determined by the combination of all acting forces.

Newton’s 1st Law of Motion The greater the mass of a body, the less the body accelerates under an applied force. Similarly a light object will undergo a larger acceleration than a heavy object under the same force.

Newton’s 1st Law of Motion Therefore mass, which is the measure of matter in an object is also the measure of inertia.

Newton’s 1st Law of Motion Objects in motion tend to stay in motion. Example: A passenger in a vehicle not wearing a seat beat will continue to move forward.

Newton’s 1st Law of Motion The purpose of a seat belt is to prevent serious injury by holding a passenger firmly in place. A seat belt may also lock when a car rapidly slows down or turns a corner.

Newton’s 1st Law of Motion Objects that are either at rest or moving with a constant velocity are said to be in equilibrium.

Newton’s 1st Law of Motion Newton’s first law describes objects in equilibrium since the net external force acting on a body must be equal to zero. ∑ Fx = 0

https://science360.gov/obj/video/642db496- d506-432e-85b4-4e38f75d9142/newtons- three-laws-motion