Definition This is the force of friction in action when a force is applied to an object at rest. This is the force of friction in action when a force is applied to an object that is moving. Weight is the result of gravity. This means an object with a mass of 1 kg would be attracted towards the centre of Earth by a force of 10 N. We feel forces like this as weight. One joule equals the amount of work that is done when 1 N of force moves an object over a distance of 1 m.
A normal is a dotted line drawn perpendicular to the surface of the refracting material, at the point of entry of the light. When light travels from air into a denser medium like water or glass, it will refract towards the normal.
There are 2 types of forces, contact forces and act at a distance force. Every day you are using forces. Force is basically push and pull. When you push and pull you are applying a force to an object. The normal force on an object at rest on a flat surface is equal to the gravitational force on that object. The sum of all forces on an object is the Fnet.
No Net Force. Net Force Equals Zero. Since this surface is slanted at a bit of an angle, the normal force will also point at a bit of an angle. Since the weight is a force, its SI unit is the newton. Its value is 9. That is to say, the acceleration of gravity on the surface of the earth at sea level is 9.
When discussing the acceleration of gravity, it was mentioned that the value of g is dependent upon location. The Newton is the SI unit for weight, and 1 Newton equals 0. The normal force is the support force exerted upon an object that is in contact with another stable object. For example, if a book is resting upon a surface, then the surface is exerting an upward force upon the book in order to support the weight of the book.
On occasions, a normal force is exerted horizontally between two objects that are in contact with each other. For instance, if a person leans against a wall, the wall pushes horizontally on the person.
The friction force is the force exerted by a surface as an object moves across it or makes an effort to move across it. There are at least two types of friction force - sliding and static friction. Though it is not always the case, the friction force often opposes the motion of an object. For example, if a book slides across the surface of a desk, then the desk exerts a friction force in the opposite direction of its motion.
Friction results from the two surfaces being pressed together closely, causing intermolecular attractive forces between molecules of different surfaces.
As such, friction depends upon the nature of the two surfaces and upon the degree to which they are pressed together. The maximum amount of friction force that a surface can exert upon an object can be calculated using the formula below:. The friction force is discussed in more detail later on this page. The air resistance is a special type of frictional force that acts upon objects as they travel through the air.
The force of air resistance is often observed to oppose the motion of an object. This force will frequently be neglected due to its negligible magnitude and due to the fact that it is mathematically difficult to predict its value.
It is most noticeable for objects that travel at high speeds e. Air resistance will be discussed in more detail in Lesson 3. The tension force is the force that is transmitted through a string, rope, cable or wire when it is pulled tight by forces acting from opposite ends. The tension force is directed along the length of the wire and pulls equally on the objects on the opposite ends of the wire. The spring force is the force exerted by a compressed or stretched spring upon any object that is attached to it.
An object that compresses or stretches a spring is always acted upon by a force that restores the object to its rest or equilibrium position. For most springs specifically, for those that are said to obey " Hooke's Law " , the magnitude of the force is directly proportional to the amount of stretch or compression of the spring.
A few further comments should be added about the single force that is a source of much confusion to many students of physics - the force of gravity.
As mentioned above , the force of gravity acting upon an object is sometimes referred to as the weight of the object. Many students of physics confuse weight with mass. The mass of an object refers to the amount of matter that is contained by the object; the weight of an object is the force of gravity acting upon that object. Mass is related to how much stuff is there and weight is related to the pull of the Earth or any other planet upon that stuff. The mass of an object measured in kg will be the same no matter where in the universe that object is located.
Mass is never altered by location, the pull of gravity, speed or even the existence of other forces. For example, a 2-kg object will have a mass of 2 kg whether it is located on Earth, the moon, or Jupiter; its mass will be 2 kg whether it is moving or not at least for purposes of our study ; and its mass will be 2 kg whether it is being pushed upon or not.
On the other hand, the weight of an object measured in Newton will vary according to where in the universe the object is. Weight depends upon which planet is exerting the force and the distance the object is from the planet. Weight, being equivalent to the force of gravity, is dependent upon the value of g - the gravitational field strength.
On earth's surface g is 9. On the moon's surface, g is 1. Go to another planet, and there will be another g value. Furthermore, the g value is inversely proportional to the distance from the center of the planet. So if we were to measure g at a distance of km above the earth's surface, then we would find the g value to be less than 9. The nature of the force of gravity will be discussed in more detail in a later unit of The Physics Classroom.
Always be cautious of the distinction between mass and weight. Be aware that weight and mass are different physical quantities, although they are closely related. Mass is an intrinsic property of an object: It is a quantity of matter.
The quantity or amount of matter of an object is determined by the numbers of atoms and molecules of various types it contains. Because these numbers do not vary, in Newtonian physics, mass does not vary; therefore, its response to an applied force does not vary.
In contrast, weight is the gravitational force acting on an object, so it does vary depending on gravity. For example, a person closer to the center of Earth, at a low elevation such as New Orleans, weighs slightly more than a person who is located in the higher elevation of Denver, even though they may have the same mass.
It is tempting to equate mass to weight, because most of our examples take place on Earth, where the weight of an object varies only a little with the location of the object. In addition, it is difficult to count and identify all of the atoms and molecules in an object, so mass is rarely determined in this manner.
Operationally, the masses of objects are determined by comparison with the standard kilogram, as we discussed in Units and Measurement. But by comparing an object on Earth with one on the Moon, we can easily see a variation in weight but not in mass.
For instance, on Earth, a 5. However, the mass of the object is still 5. A farmer is lifting some moderately heavy rocks from a field to plant crops. He lifts a stone that weighs What force does he apply if the stone accelerates at a rate of [latex] 1. We were given the weight of the stone, which we use in finding the net force on the stone. No forces act in the horizontal direction, so we can concentrate on vertical forces, as shown in the following free-body diagram.
We label the acceleration to the side; technically, it is not part of the free-body diagram, but it helps to remind us that the object accelerates upward so the net force is upward. Can you avoid the boulder field and land safely just before your fuel runs out, as Neil Armstrong did in ? This version of the classic video game accurately simulates the real motion of the lunar lander, with the correct mass, thrust, fuel consumption rate, and lunar gravity.
The real lunar lander is hard to control. Use this interactive simulation to move the Sun, Earth, Moon, and space station to see the effects on their gravitational forces and orbital paths. Visualize the sizes and distances between different heavenly bodies, and turn off gravity to see what would happen without it.
What is the relationship between weight and mass? Which is an intrinsic, unchanging property of a body? How much does a kg astronaut weight in space, far from any celestial body? What is her mass at this location? The astronaut is truly weightless in the location described, because there is no large body planet or star nearby to exert a gravitational force.
Her mass is 70 kg regardless of where she is located. When you stand on Earth, your feet push against it with a force equal to your weight. The force you exert a contact force equal in magnitude to your weight is small.
Earth is extremely massive by comparison.
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