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Forces

Forces
by

Zachary McAllister

on 12 March 2014

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Transcript of Forces

Forces
What are forces?
A force is a push or a pull. Technically, force is the action that has the ability to change motion. You need force to start something moving or change it's motion if it is already moving.
How are forces created?
Forces are created in many ways. For example, Lebron's muscles create a force when he passes a basketball. On a windy day, the movement of air can create forces. Earth's gravity creates a force called weight that pulls on everything around you.
Units of Force
The Pound: The pound (lb) is a unit of force used in the U.S. When you measure weight in pounds on a scale, you are measuring the force of gravity acting on an object.

The pound is based on the Roman unit libra, which means "balance". This is why the abbreviation for pounds is lb. The word pound comes from the Latin pondus, meaning "weight".
Newtons
Scientists prefer to measure forces in newtons. The newton (N) is a metric unit of force. The newton is defined by how much a force can change the motion of an object. A force of 1 newton is the exact amount of force needed to cause a mass of 1 kg to speed up (or slow down) by 1 m/s each second.
Sir Isaac Newton
Converting pounds to newtons
The newton is a smaller unit of force than the pound. One pound of force equals 4.448 newtons. For example, a 100-pound fish weighs 444.8 newtons.
100 pounds = 444.8 newtons
Force is a vector
Just like velocity and acceleration, force is also a vector. This means it has both an (amount) strength and a direction. Force vectors can be positive or negative just like velocity and acceleration.
Negative and Positive
Force Vectors
Forces may be assigned negative and positive values to tell their directions. Suppose a person pushes a box with a force of 10 newtons to the right. The force vector is +10N. A person pushing with the same force in the opposite direction would create a force vector of -10N.
+10N
Drawing a Force Vector
It is sometimes helpful to show both the strength and direction of a force vector as an arrow on a graph. The length represents the strength and the arrow points in the direction of the force.
Drawing a Force Vector
Look at the pictures below. Draw a vector graph on a simple number line that represents each force vector. Let the directions up and right be (+) and down and left (-). Remember to make a good scale that is consistent and easy to interpret.
Man pushing box with 50N of force to the left
Man lifting 800N of weight straight up
How forces act
There are two ways that objects can affect each other through forces. One way is through direct contact. People engaging in a tug-o-war is a good example of direct contact forces.
How forces act
If Earth were to suddenly disappear, the Moon would sail off into space by itself. The Moon doesn't fly off because Earth exerts a gravitational force that keeps the Moon in place. Earth is kept in its place by gravity from the Sun. Gravity is what we call a force that acts from a distance.
The force field
The force of gravity actually acts in two steps. First, the mass of Earth creates a gravitational field that fills the space around Earth with potential energy. Second, the gravitational field of Earth creates the force on the Moon. The gravitational force is carried from Earth to the Moon by a force field.
Contact Forces
Tension is the pulling force that is carried by a rope. Tension always acts along the direction of the rope. A rope carrying tension will be stretched tight and the same force will be experienced on either end of the rope.
Contact Forces
Springs are used to make or control forces. A spring creates a force when it is stretched or compressed away from its original size/shape. The force from a spring always acts to return the spring to its resting size/shape.
Spring Force
Non-contact Forces
Gravity is a force that acts at a distance or is non-contact. The force of gravity on an object is called weight. At Earth's surface, gravity exerts a force of 9.8 newtons on every kilogram of mass. Weight is a force that depends on mass and gravity. Remember, your mass is constant throughout the universe, but your weight can change!
Calculating Weight
Calculating your weight is quite easy. First, convert your body weight to kilograms. Next, multiply your mass by the gravity constant to get the amount of force gravity is exerting on your body.
Friction
Friction is a force that resists motion. Friction is found everywhere in our world. You feel the effects of friction when you walk, swim, ride in a car, or sitting in a chair. Friction can act when an object is in motion or at rest.
Causes of Friction
Anytime you have two surfaces in contact with one another, you will have friction. Even if the two surfaces are smooth (like glass) to our eyes, they will still have tiny hills and valleys on their surfaces. These hills and valleys grind against one another anytime the surfaces slide across each other.
Types of Friction
Static Friction

Sliding Friction

Rolling Friction

Air Friction

Viscous Friction
Different amounts of friction
It is easier for me to push a box across a smooth floor than it is to push it across a porous or rough floor. This is because friction depends on materials, roughness, how clean the surfaces are, and other factors. No single formula can accurately describe all types of friction.
The force between surfaces
In the example where I slid the paper across the table, I showed how adding more weight on top of the paper increases the amount of friction. The only two surfaces that were touching (direct contact) were the paper and the table. When I placed the calculator and stapler on top of the paper, the paper and table were pressed together with more force, so the friction increased.
Some friction behaves
differently
On the previous slide, I told you that when someone slides a box across a floor the force of friction is approximately proportional the force squeezing the two surfaces together. If I double the weight of the box, the friction will double.

This rule is NOT true if one or both of the surfaces are wet, or if they are soft. For example, rubber is soft compared to pavement. The friction between rubber and pavement depends on how much rubber is contacting the road. Wide tires have more friction than narrow tires.
Reducing friction
It is impossible to eliminate friction, but it can be reduced. Below are a few examples of ways we reduce friction.
Using Friction
There are many occasions when friction is useful. For example, the brakes on a car create friction between the brake pads and the rotors of the wheel. Below are other ways we use friction.
Friction & Energy
Energy moves through the action of forces. Energy also changes into different forms. Friction changes energy of motion into heat energy. You can feel this by simply rubbing your hands together for a few seconds.
Heat in Machines
Friction is always present in any machine with moving parts. In small machines, like a sewing machine, the friction force created is small and the amount of heat produced by friction may be small. Large machines, however, have more problems with heat caused by friction. In many machines, oil is pumped around moving parts. The oil reduces the friction so less heat is generated and it also absorbs some of the heat and carries it away from the moving parts.
Friction causes wear
Another way friction changes energy is by wearing away moving parts. You have probably noticed how a well-worn device has smooth or rounded surfaces. This is due to friction wearing away the surfaces as they slide past each other. Think about stones in a river, the friction caused by water smooths the surface of the stones and gives them a round shape.
Forces & Equilibrium
We almost never feel only one force. For example, while walking, friction and weight are two forces that both act on you at the same time. It is the total of all forces acting on your body that determines how you move.
Adding Forces
The sum of all the forces acting upon an object is called the net force. The word net means total. Net force also means that the direction of each force is considered when multiple forces are added. Consider the airplane below, what forces are acting upon it?
Adding Forces
Four forces act on the plane: weight, drag (air friction), the thrust of the engines, and the lift force caused by the flow of air over the wings. The picture below shows how these forces would be represented in a free-body diagram.
Adding Forces
A pilot must always be aware of these four forces and know how to change them in order to speed up, slow down, lift off, and land. For example, to speed up there must be a net force in the forward direction. The thrust must be greater than the drag. What is the pilot of the plane below trying to accomplish?
Adding Forces
To calculate the net force on an object, you must add the forces in each direction separately. Remember to define positive and negative directions. Usually, we assign directions up and to the right as positive (+), and we assign down and to the left as negative (-).
In this example, the net force acting on the plane is + 5,000N.

The net force left to right is +20,000 -20,000 = 0N.

The net force up and down is
+55,000N -50,000N = +5,000N.
Net force is either zero
or not zero
When many forces act on the same object either: The net force is zero, or the net force is NOT zero. The pictures below show an example of each case.
Equilibrium
When the net force on an object is zero, we say the object is in equilibrium.
EQUILIBRIUM DOES NOT MEAN THERE ARE NO FORCES!!!!
Equilibrium means all the forces cancel each other out leaving a zero net force. For example, with a zero net force an object at rest will remain at rest.
+900N
-900N
+1000N
-1000N
When net force is
not zero
If the net force is not zero, then the motion of an object will change. An object at rest will start moving. An object that is moving may change its velocity. In other words, unbalanced forces cause acceleration.
Normal Forces
Imagine a book sitting on a table. Gravity pulls the book downward with the force of the book's weight. The book is at rest, so the net force must be zero. But what force is balancing the weight of the book? The table exerts an upward force on the book called the normal force. In this class, the word normal means perpendicular. The force the table exerts is perpendicular to the table's surface.
When normal force
is created
A normal force is created whenever an object is in contact with a surface. The normal force has equal strength to the force pressing the object into the surface, which is often the object's weight. The normal force has opposite direction to the force pressing the object into the surface.
How normal force
is created
The normal force is very similar to the force exerted by a spring. When a book sits on a table, it squeezes the atoms in the table together by a tiny amount. The atoms resist this squeezing and try to return the table to its natural thickness. The greater the table is compressed, the larger the force it creates.
The Free-Body Diagram!!!
How can you keep track of many forces with different directions? You draw a free-body diagram. A free-body diagram contains only a single object, like a book or a table. All connections or supports are taken away and replaced by the forces they exert on the object. An accurate free-body diagram includes every force acting on an object, including weight, friction, and normal forces.
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