Newton s First Law of Motion

Newton s First Law of Motion

First we need to define the word FORCE: The cause of motion (what causes objects to move) Two types of forces Pushes Pulls Slide from www.science-class.net

Forces may be balanced or unbalanced Balanced forces all forces acting on an object are equal There is NO MOTION Unbalanced forces one or more forces acting on an object are stronger than others There is MOTION A NET FORCE

Objects at Rest Objects at rest tend to stay at rest unless acted upon by a force. [push or pull] Newton described this tendency as inertia. Inertia can be described as the tendency of an object to keep doing whatever s it s doing.

Mass & Inertia Which vehicle has more inertia? Mass is the amount of matter in an object. The more MASS an object has, the more INERTIA the object has. Bigger objects are harder to start & stop Slide from www.science-class.net

What about objects that are already in motion? Newton stated that objects in motion tend to stay in motion until acted upon by a force (or hits it.)

Newton s 1 st Law (also known as the law of inertia) A moving object moves in a straight line with constant speed unless a force acts on it. The tendency of an object at rest to remain at rest and an object in motion to remain in motion unless acted upon by an unbalanced force. Objects do not change their motion unless a force acts on them

The truck is in motion. What is the force that causes it to stop? The push of the stopped car. The car is at rest. What is the force that causes it to move? The push of the truck.

Newton s Laws of Motion

Newton s 1 st Law of Motion (law of inertia)

Newton s 1 st Law of Motion An object in motion tends to stay in motion and an object at rest tends to stay at rest, unless the object is acted upon by an outside force. The Law of Inertia-due to an object s mass

We feel the effects of Newton's First Law every day, but usually don't notice them because other forces interfere. In space, the First Law is much more obvious. Objects will follow their natural path until they are stopped by an outside force. On Earth, the atmosphere will eventually slow down all moving objects, but in a vacuum (basically an empty space with no air or atmosphere), like space, it will be more obvious that objects obey Newton's Laws.

Some Examples The person is standing on the floor. The only forces acting on the person are the force due to gravity pulling down & the normal force pushing up. The net force is zero and the person remains still.

Another Example One of the most common places people feel the First Law is in a fast moving vehicle, such as a car or a bus, that comes to a stop. An outside force stops the vehicle, but the passengers, who have been moving at a high speed, are not stopped and continue to move at the same speed.

Balanced vs. Unbalanced Forces

Some Problems An astronaut in outer space away from gravitational or frictional forces throws a rock. The rock will..

And now the answer continue to move in a straight line at a constant speed. The rock s tendency to do this is called INERTIA.

A rock is being whirled at the end of a string in a clockwise direction. If the string breaks, the path of the rock is

It will follow an inertial path so it will follow path C. Once the rock leaves the string, there are no unbalanced forces to affect its motion.

A Weighty Problem I spend most Sunday afternoons at rest on the sofa, watching football and consuming large quantities of food. What effect (if any) does this practice have upon my inertia? Explain. My inertia will most definitely increase. My mass will increase because of this practice and if mass increases, then inertia increases.

An 2-kg object is moving horizontally with a speed of 4 m/s. How much net force is required to keep the object moving with the same speed and in the same direction? How much force? 0 N An object in motion will maintain its state of motion. The presence of an unbalanced force will change the velocity of an object.

Is a force required? If you were in a weightless environment in space, would it require a force to set an object in motion? Yes, because even in outer space, an object has mass. If an object has mass then the object is going to resist changes in its motion. A force must be applied to set the object in motion. Newton s Laws rule Everywhere!

A Final Problem Why isn't the girl hurt when the nail is driven into the block of wood?

And the answer is. Due to the large mass of the books, the force of the hammer is sufficiently resisted (inertia). This is demonstrated by the fact that the blow of the hammer is not felt by the girl.