Applied Physical Science

Force and Motion

Newton's 1st Law of Motion

Physical Science can be called the study of “why things work”. In this course, you will learn specific laws and principles of physical science that will help you understand why things work in the process technology industry. Included in this course are topics in mechanics, electricity, fluid dynamics, thermodynamics, and material balancing. In this topic, you will learn the basic laws of motion and how various types of force effect movement.

Isaac Newton’s first law of motion states that a body at rest remains at rest and a body in motion remains in motion unless acted upon by an external force.

This car is at rest and is in fact resistant to change.

But, what happens when an external force exceeds the cars resistance to change?

The car changes position. The rate at which the car changes position over time is the car’s velocity. Common measurements of velocity are meters per second (m/s), miles per hour (mph), and feet per second (ft/s).

If after 2.57 seconds the car has moved 68 inches, what was the cars average velocity?

Our car changes from 0 in/s to a top velocity of 26.46 in/s in 2.57 seconds. When the rate of velocity changes over time, we call this acceleration. For example, our car went from 0 to 40 in/s in 5 seconds. It accelerated 8 in/s².

When the rate of velocity changes over time, we call this acceleration.For example, if the cars velocity changes from 0 in/s to 67.5 in/s in 1.2 seconds. The acceleration of the car would be 56.25 in/s².


Newton's 2nd Law of Motion

Newton’s second law of motion states that a force acting on a body causes its velocity to change, or in other words, accelerate.

To cause a body to accelerate requires a force that pushes or pulls on it. Force is measured in pounds.

Force is also measured in pound force. If an actual car’s mass is 3000 lbs, how much force is required to accelerate the car at 2 ft/s²?

But if the first law of physics says that a body in motion remains in motion unless acted upon by a force, why does the car stop?  The answer is that there are other forces acting on the car.In order to understand why machines and equipment behave the way that they do, it is important to understand the many other forces that can act upon an object.  Lets explore those other forces further to see how they impact our world.


Gravity, Friction, Normal Forces

Gravity is the force that pulls a body toward the center of the earth. Gravity pulls on the car with a force of 32.17 ft-lbm/lbf-s².

Friction is the force between two surfaces that is dependent upon the roughness or characteristics of the surfaces and the weight, but not the surface area. In the case of the car, friction is due to roughness of the surface it is on.  Some surface to surface interfaces cause more friction than others.  For example, the runners on a sled encounter more friction on concrete than they do on ice.

Liquids also experience friction as the fluid comes in contact with surfaces such as the walls of a pipe. Friction caused when a fluid encounters bends, joints, valves and other obstructions causes turbulence in fluid flow.

This vase has force because of its weight. But when the vase rests on the table, the table exerts a support force that is equal to the force of the vase. This is called the normal force. It is different than frictional force because it does not involve two surfaces sliding across each other.


Drag, Tension, Spring Forces

Other types of forces include drag, the force of tension on a cable or wire, spring force, torque, buoyancy, and electromagnetic force.

An object moving through a fluid experiences a drag force in the direction opposite to its motion. Aircraft and boats experience drag as they move through the atmosphere or water. Both are designed so that drag is minimized.

Tension is the magnitude of the pulling force exerted by a string, wire, cable, or chain that is pulled tight.Tension force is equal at all points on the rope.

Most springs obey Hooke’s Law which states that the force developed is proportional to the distance it is stretched or compressed. The spring constant is a function of the type of material the spring is made of, the diameter of the wire it is made from, the radius of the coils, and the number of coils it contains. The further the spring is stretched or compressed, the greater the force.


Torque, Bouyancy, Electromagnetic Forces

Torque is a twisting or turning force and is often measured in pound-feet or newton-meters. Torque is the force that causes something to rotate. The force that drives this drill is an example.

Buoyancy is the force that keeps ships afloat. An object in a fluid will be pushed up by a force equal to the weight of the fluid that is displaced by the object.

Electromagnetic force is caused by the interaction between positively charged protons in atoms and negatively charged electrons. When electrons move from one atom to another, the result is a positively charged ion and a negatively charged ion. If enough of these ions build up on a surface, we have a surface that is positively or negatively charged. Two surfaces that have the same charge will repel each other. Two surfaces that have opposite charges will pull toward each other.

We can use vectors to show force. A vector is a line with an arrow indicating distance and direction. The force exerted to move the car forward is represented by this vector. The pull of gravity can be represented this way.  Air resistance is shown here. The combined forces of gravity, friction, and to a small extent air resistance, cause the car to slow, and then stop.


Balanced and Unbalanced Forces

In this example, there are multiple forces at work, some of them pushing or pulling down and some pushing or pulling up. The net result is that the weight is suspended and does not move up or down. We say that these forces are in equilibrium, or, are balanced.

The car resting on a surface is another example of balanced forces.  Gravity pulls the car toward the surface while the surface pushes back with equal force.  The net result is the car staying in place.

When you push the accelerator control of the car sitting at rest, you add an additional force or net force causing the car to accelerate.

Mechanical vibrations occur when unbalanced force causes an object to move. A vibration is a movement that is side to side, up and down, or back and forth. Vibration can result from uneven weight distribution such as a badly balanced tire, fan, or rotor. As the blade turns, the net force of the uneven weight distribution causes the fan to move back and forth. The rate of vibration is called frequency. The distance the object travels each time it vibrates is the amplitude. Some vibration is inherent in motors and other machines, but the amplitude of frequency for a vibrating piece of equipment can be reduced by proper design, balance, mounting, and maintenance.

Centripetal force is the net force that makes a body follow a curve, or, the force that makes a body move toward a central axis point. One example is spinning an object on a string.  Another example is a car turning a corner. Inertia causes the car to want to move in a straight line.  Friction on the tires makes the car want to turn in a curved direction. A car or any object moving in a circular motion is pulled toward the center of the circle by centripetal force, which is relative to the mass and velocity of the object and the radius of the circle.


Newton's 3rd Law of Motion

Newton’s Third Law states that for every action there is an equal and opposite reaction. For example, friction is the force exerted on the car tires that keeps the car on the road as it goes around a corner pulling the car towards the center of the curve. At the same time the circular motion of the car creates a centrifugal force on the car that is trying to force the car off the road towards the outside of the curve. These two forces are equal and opposite so the car can go around the corner and still be under control. When they are not equal the car slides off into the outer ditch like it might when it hits an icy patch in a curve.

The concept of opposing forces helps to explain how a centrifugal pump works. A rotating impeller forces the fluid in a circular motion, creating a centrifugal force that accelerates the fluid away from the center.  With an almost equal force, the centripetal force from the outer housing pushes the fluid toward the center as well as pulling fresh fluid into the pump. The centrifugal force and the momentum of the fluid spinning around in the pump then forces it out the discharge nozzle of the pump.

You may not realize it, but when an external force acts on an object, like this car, the car pushes back with the exact same force. To illustrate this, lets place a 2nd car in the path of the 1st car. The 1st car accelerates and crashes into the 2nd car, causing the 2nd car to accelerate. But what happens to the 1st car?  The force of the 2nd car pushes back on the 1st car causing it to slow down and stop.

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