# Newtons Second Law But first Review Newtons First

• Slides: 19

Newton’s Second Law

But first… Review: Newton’s First Law If an object experiences NO net force…. Resting objects remain at rest. ¢ Moving objects move at a constant velocity. ¢

Newton’s First Law Also known as the Law of Inertia: An object’s tendency to keep on doing what it’s already doing. ¢

Newton’s First Law Inertia: An object’s tendency to keep on doing what it’s already doing. An object at rest will remain at rest…

Law of Inertia: An object in motion will continue in motion…

Inertia ¢ ¢ ¢ Inertia is a term used to measure the ability of an object to resist a change in its state of motion. An object with a lot of inertia takes a lot of force to start or stop; an object with a small amount of inertia requires a small amount of force to start or stop. The word “inertia” comes from the Latin word inertus, which can be translated to mean “lazy. ”

Inertia ¢ ¢ Inertia is a term used to measure the ability of an object to resist a change in its state of motion. Mass is a measure of inertia. The higher the mass of an object is, the more it resists changes to its motion:

Law of inertia: (click here) ¢ The law of inertia and YOU!

Equilibrium ¢ ¢ ¢ The condition of zero acceleration is called equilibrium. In equilibrium, all forces cancel out leaving zero net force. Objects that are standing still are in equilibrium because their acceleration is zero. Objects that are moving at constant speed and direction are also in equilibrium. A static problem usually means there is no motion.

Calculate force ¢ ¢ A woman is holding two dogs on a leash. If each dog pulls with a force of 80 newtons, how much force does the woman have to exert to keep the dogs from moving?

Newton’s Second Law The acceleration of an object is: ¢ Directly proportional to the net force acting on it, and… ¢ Inversely proportional to its mass

What does that mean? The acceleration of an object is: ¢ Directly proportional to the net external force acting on it (The stronger the force applied to an object, the greater the acceleration will be. ) ¢ Inversely proportional to its mass (The heavier the object, the less it will accelerate for a given force. )

Newton's Second Law ¢ If you apply more force to an object, it accelerates at a higher rate.

Newton's Second Law ¢ If an object has more mass, it accelerates at a lower rate because it has more inertia.

Practice: Calculating acceleration ¢ ¢ A cart rolls down a ramp. The cart has a mass of 500 grams (0. 5 kg). Using a spring scale, you measure a net force of 2 newtons pulling the car down. Calculate the acceleration of the cart.

Calculating acceleration ¢ ¢ ¢ Three people are pulling on a wagon applying forces of 100 N, 150 N, and 200 N. The wagon has a mass of 25 kilograms. Determine the acceleration and the direction the wagon moves.

Calculating force ¢ ¢ ¢ An airplane needs to accelerate at 5 m/sec 2 to reach take-off speed before reaching the end of the runway. The mass of the airplane is 5, 000 kilograms. How much force is needed from the engine?

More on weight…. Weight (as you know) is a force. ¢ Weight is the force exerted on an object by gravity. ¢ The magnitude of weight is found by Newton’s Second Law: F=mxa Weight = mass x (9. 8 m/s 2) ¢