In Physics, we use the word force a lot. A force is any push or pull that can affect the motion of an object. An object's motion can change, or not change, depending on the sum of forces acting on it. A force can have the following effects on an object:
A force can cause an object at rest to start moving
A force can cause an object to speed up
A force can cause an object to slow down
A force can cause an object to change direction
A force can combine with other forces to keep an object's motion from changing.
When there is more force on one side of an object than on the opposite side, the forces are unbalanced. The object's motion will always change when there are unbalanced forces. When the forces on each side of an object are matched by equal sized forces on the opposite, we say that the forces are balanced. An object with balanced forces will not change its motion. If the object is moving, it will continue moving at the same speed and direction. If the object is not moving, the object will not start moving.
Objects have balanced forces if the forces on each side of the object are the same as the forces on the opposite side. Such an object will either be moving at a constant velocity or not moving at all.
Here are some examples of objects with balanced forces:
These pictures are called 'free body diagrams' A free body diagram is a way to draw objects in order to examine the forces. In a free body diagram, the box in the middle represents an object (regaurdless of what the object looks like in real life). The arrows on the outside of the box point in the directions that forces are pushing or pulling on the object. The larger the force, the longer the arrow is. The smaller the force the shorter the arrow is.
The forces on an object are balanced if the total length of the arrow(s) on one side is equal to the total length on the other side.
As you can see from the images above, the unit commonly used for force is Newtons (N). One Newton is the amount of force needed to move a 1 kg object 1 m/s faster every 1 second.
An important fact about forces is that forces obey Newton's 3rd Law. Newton's third law says tha for every action there is an equal and opposite reaction. Everytime there is a force of any kind, there must also be another force at work in the opposite direction. This opposite and equal force is called the force pair. Every force has one.
There are several categories of forces that often influence the motion of things in the universe. These forces include:
Gravity, Tension, Friction, Normal, and Applied
Gravity is a force that attracts things with mass to other things with mass. The direction of gravitational force is toward the center of the mass causing the force. Gravitational force only exists if there are two objects with mass. Both objects will experience the same force. Object 1 will be pulled toward object 2, and object 2 will be pulled toward object 1. These 2 gravitational forces are equal and opposite. They are force pairs. The strength of the gravitational force between two objects can be calculated with this equation:
Gravitational force is fairly weak unless the mass of one of the objects is very large. For us, the earth is the main source of gravitational force. Nothing else in our surroundings is large enough to exert a noticeable force of gravity. Because the gravitational force we experience comes almost entirely from the earth, the direction of that force is toward the center of the earth. Anywhere on or near earth, the force of gravity is straight down toward the center of the earth.
There is a simpler equation we can use whenever we need to know the force of gravity on an object near a planet's surface. Take the equation above, put in the earth's radius (6378137 meters) for 'r' and the earth's mass (5.97219 x 1024kg) for one of the m's.
Now if we multiply and divide, we'll get an equation that just involves the object's mass. If you do the math, you will get 9.7 or 9.8 depending on how many decimal places you use. Since the distance to the center of the earth is not constant everywhere, this value will change a little depending on where you are on earth. While some Physics courses will use the number 9.8, we will use 10. It is much simpler to remember and to do math with.
With this equation, finding the gravitational force on an object is simple. Just multiply an object's mass by 10 and you will find the force of gravity.
The value for 'g' is about 10 anywhere near the earth. It is different on different planets. Smaller planets have a smaller 'g'. Larger planets have a larger 'g'. This value for 'g' happens to represent how fast an object would accelerate if it were dropped without any other forces pulling it up.
The difference between kilograms and Newtons is this: kilograms measure mass, but Newtons measure force. Mass is a measure of how much matter makes up an object. Mass would not change if you moved the object to a different gravitational situation. For instance, imagine you took a book with a mass of 2 kg and moved it from the earth to the moon. The mass of the book would not change. It would be 2 kg on the earth and 2 kg on the moon. The gravitational force would change. The earth is much larger than the moon and so pulls harder on objects near its surface than does the moon. The box would experience a larger gravitational force on earth than it would from the moon.
Another word for gravitational force is weight. Mass is measured in kilograms. Weight is measured in Newtons. Weight is also measured in pounds. A pound is equal to 22 Newtons. If you were to travel to different planets in the solar system, your mass would not change. Your weight would change from one planet to another. You would feel heavier on larger planets and lighter on smaller planets. Enter your weight on earth below to see what you would weigh on other planets.
or
Mass in kilograms kgOn Earth, g = 10 m/s/s
F = mg
F = (0)(10)
F = 0
On Earth, the force of gravity on you is 0 N
The force on gravity on you would be 0 N
Tension force is the force caused by a rope, string, chain, or similar device pulling on an object. The direction of the force is always toward the middle of the rope.
Notice from the picture that the forces on each end of the rope are both directed toward the middle of the rope. The hand experiences a force upwards toward the middle of the rope. The block experiences a force downwards toward the middle of the rope. These forces are equal and opposite. They are force pairs.
When forces are balanced, we can often figure out the strength of the tension by looking at what forces it is balancing. For instance, look at these examples.
Look at these two examples:
First Example
In this example, a 50 kg box is hanging from the ceiling. What forces are acting on it?
Second Example:
In this example, a 50 kg box is hanging from the ceiling. What forces are acting on it?
On the example on the right, the same 50 kg box is now hanging from two ropes. The force of gravity is still 500 N. However, there are two tension forces that balance the force of gravity. As long as these ropes are at the same angle and same distance from the center of the box, they will split the force equally. Each rope provides a tension force of 250 N.
It is important to understand that in these examples, the force of gravity and the force of tension are not force pairs. They may be equal and opposite in some situations, but they don't have to be. The actual force pair for the tension force pulling up on the box is the tension force of the same rope pulling down on the ceiling. Those two forces will be equal in all situations. The force pair for the force of gravity pulling down on the box is the the force of gravity from the box pulling up on the earth.
Normal force is the force caused by a solid surface preventing something from moving through it. For instance, the ground stops you from falling into it. A wall prevents you from walking through it. A chair prevents you from falling all the way to the ground. Each of these are examples of a normal force.
The direction of a normal force is always at 90 degrees to the surface that is causing the normal force.
Consider these examples:
The strength of the normal force can be figured out by looking at the other forces that are acting on the object. Look at the following two examples:
First Example: A 50 Kg box is resting on the ground
Second Example: A 50 Kg box is resting on the ground. Someone is pulling up on a string connected to a box. He pulls with a force of 100 Newtons. The box does not move.
With normal forces, like any force, there is always a force pair. The normal force is two ways. For instance, in the first example, the 50 kg box has a normal force from the ground pushing it upward with 500 N of force. At the same time, the floor experiences 500 N of normal force from the box pushing down on it. Both objects involved in the normal force experience equal and opposite forces. They are force pairs.
An applied force is when one object pushes or pulls another object by physically contacting it. Applied force is a fairly broad category that includes pushing, hitting, kicking, bumping, and lifting an object. The strength of an applied force can be found the same way as the other types of force. As long as the forces are balanced, the strength of the applied force can be found by balancing out the other forces present on an object.
When one obect applies a force to another object, it also experiences the same force in return. If you punch a bag with 100 N of force, your fist feels 100 N of force pushing it backward. They are force pairs.
Friction forces are forces that oppose the motion of objects. Friction forces include the effects of surface on surface sliding, water viscosity, and air resistance. If the object is moving, the friction force will be in the direction opposite the objects motion. If the object is not moving, friction force is in the direction the object would move if friction weren't present.
The strength of friction forces can be found in a couple of ways. One way is to balance the other forces present on the object. This approach only works when the forces are balanced and we know what how strong the other forces are.
When forces are balanced the object is either at rest or moving at a constant velocity. Either way, friction may be balancing out another force to keep the object's forces balanced. If an object is at rest, friction would be just as strong as any force that is trying to get the object to move. Look at the following example:
Mr Stickman pushes with 200 Newtons of force on a 100 kg box. The box does not move.
Mr Stickman pushes with 200 Newtons of force on a 100 kg box. The box is moving at a constant velocity."
The box is being pushed and is moving at a constant velocity. What forces are acting on the box?
Lets review what we have seen in these two exmaples. While the box is still, the friction force will simply be the same as the force Mr Stickman is pushing with. Once the box starts to move, there is a force at which Mr Stickman can move the box with a constant velocity. The forces are still balanced. In this case, that force is 300 N. If Mr Stickman pushes any harder, the forces will not be balanced. The box will speed up. If he starts pushing with less than 300 Newtons, the box will slow down and eventually stop.
The second way of finding the strength of a friction force is by equation. The equation is a little different depending on whether the object is moving or not moving. Here are the two equations:
The equation on the left is for static friction. Static friction occurs when an object is not moving. The equation on the right is for kinetic friction. Kinetic friction occurs when an object is moving. In either case, you can see that the force of friction depends on two things. Those two things are the normal force and the coefficient of friction. The coefficient of friction is just a number that is really small for slippery surfaces and bigger for more rough surfaces. For example, a coefficient of .01 means that the surface is slippery. A coefficient of 1.0 means that the surface is very rough.
The equation can be used any time we know the coefficient of friction and also know or can find the normal force. Here is an example:
Mrs Stickman is pushing a 80 kg box. The coefficient of kinetic friction between the box and the ground is 0.2. How much force does Mrs Stickman need to use to keep the box moving at a constant velocity?
Even with friction force, there are always force pairs. The surface causing the friction also experiences a friction force from the moving object.
Forces are not always balanced. Unbalanced forces occur whenever the force on one side of an object is greater than or less than the forces on the opposite side.
Here are some examples of objects with unbalanced forces:
The object in this picture has unbalanced forces acting on it. Its vertical forces are balanced. The two arrows on the top are canceled out by the larger arrow on the bottom.
The horizontal forces are not balanced. The force pushing the object to the right is clearly larger than the sum of the forces pushing left.
The measurement of how much more force is in one direction the opposite direction is called 'net force'. If forces are not balanced on an object, it must have a net force that is not zero. There must be more force in one direction than another. Here are some examples:
We'll start with the vertical forces. The upward forces = 15 + 15 = 30 N
The downward forces = 30N
Since both the upward and downward forces are 30 N, the vertical forces are equal.
Now, lets look at the horizontal forces. The force to the right is 45 N. The forces to the left = 5 + 5 = 10 N.
To find the horizontal forces: Net force = Force to the Right - Force to the Left
Net Force = 45 - 10
Net Force = 35 N
Net force effects the acceleration of an object. Anytime the net force is not zero, the object will change its motion. It will either speed up, slow down, or change direction. A non-zero net force means a non-zero accelertion. The acceleration of an object will always be in the direction of its net force.
The amount of acceleration an object will experience can be found if we know the net force. The equation is:
If you can find the net force on the object, dividing that force by the mass tells you the amount of acceleration the object will experience. The direction of the net force will tell you the direction of the acceleration
Vertically, the forces are balanced. There will be no acceleration up or down. If the box is already moving up or down, it will continue at that speed. If the box is not moving up or down, it will not start moving in those directions.
Horizontally, the forces are not balanced. There is more force to the right than there is to the left. There will be acceleration to the right. To find out how much acceleration, we use the equation:
Fnet = mass x Acceleration
Fnet = 5 x Acceleration
Looking at the arrow, we can see that the net force in the horizontal directions equals 45 - 5 - 5 = 35 N. So,
35 = 5 x Acceleration
Acceleration = 35/5 = 7 m/s2