Force, Weight, Newtons, Velocity, Mass and Friction—Basic Principles of Mechanics
Mechanics is a branch of physics which deals with forces, mass, and motion.
This article explains the basic concepts.
Quantities Used in Mechanics
This is a property of a body and a measure of an objects resistance to motion. It is constant and has the same value no matter where an object is located on Earth, on another planet or in space. Mass in the SI system is measured in kilograms (kg). The international system of units, abbreviated to SI from the French "Système International d'Unités," is the units system used for engineering and scientific calculations. It is basically a standardization of the metric system.
This can be thought of as a "push" or "pull." A force can be active or reactive.
Force in the SI system of units is measured in newtons (N). A force of 1 newton is equivalent to a weight of about 3.5 ounces or 100 grams.
This is the speed of a body in a given direction and is measured in metres per second (m/s).
When a force is exerted on a mass, it accelerates. In other words, the velocity increases. This acceleration is greater for a greater force or for a smaller mass.
When a force is exerted on a structure, this is known as a load. Examples are the weight of a roof on the walls of a building, the force of wind on a roof, or the weight pulling down on the cable of a crane when hoisting.
Note: In US English, metres is spelled "meters"
In mechanics, vector or free-form diagrams are used to describe and sketch the forces in a system. A force is usually represented by an arrow and its direction of action is indicated by the direction of the arrowhead. Rectangles or circles can be used to represent masses.
Examples of Forces
- When you lift something off the ground, your arm is exerting a force upwards on the object. This is an example of an active force
- The Earth's gravity pulls down on an object and this force is called weight
- A bulldozer can exert a huge force, pushing material along the ground
- A huge force is produced by the engines of a rocket lifting it up into orbit
- When you push against a wall, the wall pushes back. If you try to compress a spring, the spring tries to expand. When you stand on the ground, it supports you. All these are examples of reactive forces. They don't exist without an active force. See (Newton's laws below)
- If the unlike poles of two magnets are brought together (N and S), the magnets will attract each other. However, if two like poles are moved close together (N and N or S and S), the magnets will repel
A Very Large Force
Types of Forces
This can be thought of as the force applied to an object which may eventually cause it to move. For example when you push or pull a lever, slide a piece of furniture, turn a nut with a wrench or a bull dozer pushes a load of soil, the applied force is called an effort. When a vehicle is driven forwards by an engine, or carriages are pulled by a locomotive, the force which causes motion is known as the tractive effort
This is the force exerted by gravity on an object. It depends on the mass of the object and varies slightly depending on where it is located on the planet and the distance from the center of the Earth. An object's weight is less on the Moon and this is why the Apollo astronauts seemed to bounce around a lot and could jump higher. However it could be greater on other planets. Weight is due to the gravitational force of attraction between two bodies. It is proportional to the mass of the bodies and inversely proportional to the square of the distance apart. In engineering, this force acting on a structure is known as a load.
Tensile or Compressive Reaction
When you stretch a spring or pull on a rope, the material exerts an equal reactive force pulling back in the opposite direction. This is known as tension. If you try to compress an object such as a spring, sponge, gas or simply place an object on a table, the object pushes back. Working out the magnitude of these forces is important in engineering so that structures can be built with members which will withstand the forces involved, i.e they won't stretch and snap, or buckle under load.
Friction is a reactive force which opposes motion. Friction can have beneficial or detrimental consequences. When you try to push a piece of furniture along the floor, the force of friction pushes back and makes it difficult to slide the furniture. This is an example of a type of friction known as dry friction, static friction or stiction.
Friction can be beneficial. Without it everything would slide and we wouldn't be able to walk along a sidewalk without slipping. Tools or utensils with handles would slide out of our hands, nails would pull out of timber and brakes on vehicles would slip and nott be of much use.
Viscous friction or drag
When a parachutist moves through the air or a vehicle moves on land, friction due to air resistance, slows them down. If you try to move your hand through water, the water exerts a resistance and the quicker you move your hand, the greater the resistance. These reactive forces are known as viscous friction or drag.
Newton's Laws of Motion
"An object will continue in its state of rest or motion in a straight line provided no external force acts on it"
Basically, this means that if for instance a ball is lying on the ground, it will stay there. If you kick it into the air, it will keep moving. If there was no gravity, it would go on for ever. However, the external force, in this case, is gravity which causes the ball to follow a curve, reach a max altitude and fall back to the ground.
Another example is if you put your foot down on the gas and your car accelerates and reaches top speed. When you take your foot off the gas, the car slows down, The reason for this is that friction at the wheels and friction from the air surrounding the vehicle (known as drag) causes it to slow down. If these forces were magically removed, the car would stay moving forever.
"The acceleration of a body is directional proportional to the force which caused it and inversely proportional to the mass and takes place in the direction which the force acts"
This means that if you have an object and you push it, the acceleration is greater for a greater force. So a 400 horse power engine in a sports car is going to create loads of thrust and accelerate the car to top speed rapidly. Imagine if that engine was placed into a heavy train locomotive and could drive the wheels. Because the mass is now so large, the force creates much lower acceleration and the locomotive takes much longer to reach top speed.
If F is the force
m is the mass
and a is the acceleration
F = ma
Acceleration is measured in metres per second squared or m/s2
Example 1: A force of 10 newtons is applied to a mass of 2 kilos. What is the acceleration?
F = ma
So a = F/m = 10 / 2 = 5 m/s2
The velocity increases by 5 m/s every second
Weight as a Force
In this case, the acceleration is g, and is known as the acceleration due to gravity.
g is approximately 9.81 m/s2 in the SI system of units.
Again F = ma
So if the force F is renamed as W, and substituting for F and a gives:
Weight W = ma = mg
Example 2: What is the weight of a 10 kg mass?
W = mg = 10 x 9.81 = 98.1 newtons
"For every action there is an equal and opposite reaction"
This means that when a force is exerted on an object, the object pushes back.
- When you push on a spring, the spring exerts a force back on your hand. If you push against a wall, the wall pushes back.
- When you stand on the ground, the ground supports you and pushes back up. If you try to stand on water, the water cannot exert enough force and you sink.
- Foundations of buildings must be able to support the weight of the construction. Columns, arches, trusses and suspension cables of bridges must exert enough reactive compressive or tensile force to support the weight of the bridge and what it carries.
- When you try to slide a heavy piece of furniture along the floor, friction opposes your effort and makes it difficult to slide the object
Dry Friction or "Stiction"
As we saw above, friction is an example of a force. When you attempt to slide a piece of furniture along a floor, friction opposes your effort and makes things more difficult. Friction is an example of a reactive force, and doesn't exist until you push the object (which is the active force). Initially, the reaction balances the applied force i.e. your effort pushing the furniture, and there is no movement. Eventually, as you push harder, the friction force reaches a maximum, known as the limiting force of friction. Once this value is exceeded by the applied force, the furniture will start to slide and accelerate. The friction force is still pushing back and this is what makes it so difficult to continue to slide the object. This is why wheels, bearings, and lubrication come in useful as they reduce friction between surfaces, and replace it by friction at an axle and leverage to overcome this friction. Friction is still necessary to stop a wheel sliding, but it doesn't oppose motion. Friction is detrimental as it can cause overheating and wear in machines resulting in premature wear. So engine oil is important in vehicles and other machines, and moving parts need to be lubricated.
Dry static friction, also known as stiction (see above diagram)
If F is the applied force on a body
The mass of the body is m
Weight of the body is W = mg
μs is the coefficient of friction (low μ means the surfaces are slippery)
and Rn is the normal reaction. (the reactive force at right angles to the surface due to the object being pushed against the surface)
Reaction Rn = Weight W
limiting friction force is Ff = μsRn = μsW = μsmg
Remember this is the limiting force of friction just before sliding takes place. Before that, the friction force equals the applied force F trying to slide the surfaces along each other, and can be anything from 0 up to μRn.
So the limiting friction is proportional to the weight of an object. This is intuitive since it is harder to get a heavy object sliding on a specific surface than a light object. The coefficient of friction μ depends on the surface. "Slippery" materials such as wet ice and Teflon have a low μ. Rough concrete and rubber have a high μ. Notice also that the limiting friction force is independent of the area of contact between surfaces (not always true in practice)
Once an object starts to move, the opposing friction force becomes less than the applied force. The friction coefficient in this case is μk.
Remember this is the limiting force of friction just before sliding takes place. Before this occurs, the opposing friction force equals the applied force F trying to slide the surfaces along each other, and can be anything from 0 up to μRn.
Newton's Equations of Motion
There are three basic equations which can be used to work out the distance traveled, time taken and final velocity of an accelerated object.
If u is the initial velocity
v is the final velocity
s is the distance covered
t is the time taken
and a is the acceleration
For uniform acceleration
v = u + at
s = ut + 1/2 at2
v2 = u2 + 2as
(1) A force of 100 newtons accelerates a mass of 5 kg for 10 seconds. If the mass is initially at rest, calculate the final velocity.
Firstly it is necessary to calculate the acceleration.
F = 100 newtons
m = 5 kg
t =10 seconds
u = initial velocity = 0 since the object is at rest
F = ma so a = F/m = 100/5 = 20 m/s2
v = u + at = 0 + 20 x 10 = 200 m/s
(2) A mass of 10 kg is dropped from the top of a building which is 100 metres tall. How long does it take to reach the ground?
In this example, it doesn't make any difference what the value for the mass is, the acceleration due to gravity is g irrespective of the mass. Galileo demonstrated this when he dropped two balls of equal sizes but differing masses from the leaning tower of Pisa.
We know s = 100
g = 9.81
u = 0
We can use the equation s = ut + 1/2 at2
u = 0 and a = g so s = 1/2gt2
or t = √(2s/g) = √(2 x 100) / 9.81 = 4.5 seconds approx
Commander David Scott's Hammmer and Feather Experiment from Apollo 15
Adding and Resolving Force Vectors
As mentioned briefly above, a force can be represented graphically by an arrow with a given direction known as a vector. If two or more forces are involved, problems in mechanics can be solved graphically by drawing the vectors, the head of one vector ending at the tail of the 2nd vector and so on. The vectors are drawn to scale, the length representing the magnitude of the force and the angle being the angle of action of the force. The "Triangle of Forces" or "Parallelogram of Forces" is then a method for visualizing or finding the resultant of forces.
Forces can also be resolved. In the diagram below, a mass rests on a slope. Using the parallelogram of forces in reverse, the weight force can be resolved into a force parallel to the slope and perpendicular to the slope. This is useful for these type of problems because it enables the normal reaction to be worked out (the force exerted by the slope on the mass as explained earlier) and also the friction forces involved.
Engineering Mathematics by K.A. Stroud
This is an excellent math textbook for both engineering students and anyone with an interest in the subject. The material has been written for part 1 of BSc. Engineering Degrees and Higher National Diploma courses.
A wide range of topics are covered including matrices, vectors, complex numbers, calculus, calculus applications, differential equations, series, probability theory, and statistics. The text is written in the style of a personal tutor, guiding the reader through the content, posing questions, and encouraging them to provide the answer.
This book basically makes learning mathematics fun!
Couples and Torque
When a force acts on an object, it produces what is known as a turning moment or just simply a moment. An example is when you push on the trunk of a small tree.This produces a turning moment about the base of the tree which is balanced by the tension in the trunk (called torsion). If you push too hard, you exceed a breaking limit and the trunk snaps. The moment of a force about a point is the magnitude of the force multiplied by the perpendicular distance between the force and the point. When 2 forces act in opposite directions, this is known as a couple and the magnitude of the twisting force or couple is called torque. If the forces are both of magnitude F, and the perpendicular distance between them is d, then:
Torque T = Fd
As you can see, if the force is increased or the distance is increased, the torque becomes greater. So this is why it is easier to turn something if it has a larger diameter handle or knob. A tool such as a socket wrench with a longer handle has more torque.
A gearbox is a device which converts high-speed low torque to lower speed and higher torque (or vice versa). Gearboxes are used in vehicles to provide the initial high torque required to get a vehicle moving and accelerate it. Without a gearbox, a much higher powered engine with a resulting higher torque would be needed. Once the vehicle has reached cruising speed, lower torque is required (just sufficient to create the force required to overcome the force of drag and rolling friction at the road surface).
Gearboxes are used in a variety of other applications including power drills, cement mixers (low speed and high torque to turn the drum), food processors and windmills (converting low blade speed to high rotational speed in the generator)
A common misconception is that torque is equivalent to power and more torque equals more power. Remember however torque is a turning force and a gearbox which produces higher torque also reduces speed proportionately. So the power output from a gearbox is equal to the power in (actually a little less because of friction losses, mechanical energy being wasted as heat)
© 2012 Eugene Brennan