Sir Isaac Newton discovered one of the most important relationships in physics: the link between the force of an object, its mass, and its acceleration. 3.1 Force, Mass and Acceleration Newton's first law of motion- The Law of Inertia

An object at rest will remain at rest unless acted on by an unbalanced force. An object in motion will continue with constant speed and direction, unless acted on by an unbalanced force.

In other words, Unless you apply force, things tend to keep on doing what they were doing in the first place. Sir Isaac Newton (1642-1727), an English physicist and mathematician, is one of the most brilliant scientists in history. Before the age of 30, he formulated the basic laws of mechanics, discovered the universal laws of gravitation, and invented calculus! The acceleration of an object is directly proportional to the force acting on it and inversely proportional to its mass.

In other words, force causes an object to accelerate, while the object's mass resists acceleration. Newton's Second Law of Motion Whenever one object exerts a force on another, the second object exerts an equal and opposite force on the first.

In other words, for every action there is an equal and opposite reaction. If you push on the wall, you feel the wall pushing back on your hand. Newton's Third Law of Motion A force. Every object continues in a state of rest, or of motion, unless force is applied to change things. This is a fancy way of saying that things tend to keep doing what they are already doing. There is no way the cart with the heavy box is going to move unless a force is applied. Of course, the force applied has to be strong enough to actually make the cart move.

Once the cart is set into motion, it will remain in motion, unless another force is applied to stop it. You need force to start things moving and also to make any change in their motion once they are going. What do you need to change motion? A force is what we call a push or a pull, or any action that has the ability to change motion. This definition does not, however, mean that forces always change motion! If you push down on a table, it probably will not move. However, if the legs were to break, the table could move.

*Force is an action that has the ability to change motion. * What is a force? There are two units of force that are commonly used: pounds and newtons. Scientists prefer to use newtons. The newton is a smaller unit than the pound. There are 4.448 newtons in one pound. A person weighing 100 pounds would weigh 444.8 newtons.

Unit Equivalents

1 newton 0.228 pounds

1 pound 4.448 newtons

Example:

A person stands on a scale and measures a weight of 100 pounds. How much does the person weigh in newtons?

Solution:

100lbs x 4.448 N/lbs= 444.8lbs Pounds to Newtons The metric unit of force, the newton, relates force and motion.

One newton equals 1 kilogram multiplied by 1 meter per second squared. This means that a force of one newton causes a 1 kilogram mass to have an acceleration of 1 m/sec2. In

talking about force, “newton” is easier to say than

“1 kilogram · m/sec2.” Where did the newton come from? Force is a push or pulling action that can change motion. Mass is the amount of

“stuff” or matter in an object. Mass is a basic property of objects. Mass resists the

action of forces by making objects harder to accelerate. Defining mass.... Comparing weight and mass! The weight of a person can be described in pounds or newtons. On Earth, a child

weighs 30 pounds or about 134 newtons. In other words, the force acting on the

child, due to the influence of Earth’s gravity, is 134 kg·m/sec2.

A child that weighs 30 pounds on Earth has a mass of about 14 kilograms because

on Earth 2.2 pounds equals 1 kilogram. Because mass is an amount of matter,

mass is independent of the force of gravity. Therefore, the mass of a person is the

same everywhere in the universe. However, the weight of a person on Earth is

different from what it would be on the moon **Your mass is the same everywhere in the universe, but your weight is different** Newton’s first law is also called the law of inertia. Inertia is defined as the

property of an object to resist changing its state of motion. An object with a lot of

inertia takes a lot of force to start or stop. Big trucks have more inertia than small

cars, and bicycles have even less inertia Law of Inertia Inertia is a property of mass!! The amount of inertia an object has depends on its mass. Mass is a measure of the

inertia of an object. Mass is what we usually think of when we use words like

“heavy” or “light.” A heavy object has a large mass while an object described as

“light as a feather” has a small mass. We can also define mass as the amount of

matter an object has. Newton’s second law is one of the most famous equations in physics. It says that:

• Force causes acceleration.

• Mass resists acceleration.

• The acceleration you get is equal to the ratio of force over mass. Mass is measured in kilograms. 1 kilogram has a weight of about 2.2 pounds. Force causes acceleration When forces on an object are balanced, the net force is zero, and we say that the

object is in equilibrium. In equilibrium there is no change in motion. An object at

rest stays at rest, and an object already moving keeps moving at the same speed. The motion of an object depends on the total of all forces acting on the object. We

call the total of all forces the net force. The second law refers to net force The diagram above illustrates the difference between balanced and unbalanced

forces. Imagine a giant box being pulled on both sides by tractors. If the tractors

are equal, the forces are equal, the box is in equilibrium and does not move. If

tractor A is 10 times stronger than tractor B, the forces are not in equilibrium. The

net force points toward tractor A, so the box accelerates toward tractor A.

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# 3.1 Force, Mass and Acceleration

Newton's Laws of Motion

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