"An object in motion tends to remain in motion along a straight line unless acted on by an outside force." -Isaac Newton
When one begins to learn more about physics they expect to study the related entities of energy and force. The laws of gravity, elasticity, level and gears, describing motion and the pressure in fluids are all concepts that are covered by the scientific discipline of physics in the topic of forces.
For those who are interested in a scientific career that involves acquiring the fundamentals of physics, the GCSE Physics Syllabus is a great choice. It can be studied during the last two years of secondary school and is offered by exam boards in the United Kingdom.
Superprof is here to analyze the fifth topic of the GCSE Physics Syllabus, also known as forces, that is offered by the AQA exam board. Now, who's ready for a purely scientific article?
Scalar and Vector Quantities
Scientists are well-known for making measurements. The physical quantities that they measure can be divided into two groups: scalar and vector.
Scalar quantities generally only have a magnitude or size and here some examples:
- Temperature: either measured in Celsius or Fahrenheit.
- Mass: in kilograms or pounds it is usually measured.
- Energy: always calculated by using joules.
- Distance: often gauged in metres, kilometres or miles.
- Speed: determined by using kilometres per hour (km/h) or metres per second (m/s).
- Density: most often than not it is calculated in kilograms per metre cubed but can also be done using grams per centimetre cubed.
The entire sum of scalar quantities can be determined by adding the sums together.
Vector quantities are a bit more advanced than scalar quantities due to the fact that they have both magnitude and an associated direction. Some of the most common examples of vector quantities are as follow:
- Force: determined using newtons to the left.
- Displacement: calculated using measurements of distance like kilometres or miles east.
- Velocity: gauged using metres upwards.
- Acceleration: metres per second squared downwards can be used as an example.
- Momentum: kilogram metres per second can be utilized as an example.
The direction of a vector is always given as a written description or drawn as an arrow.
Resultant forces or two forces acting together can easily be calculated when they are in a straight line. When two forces are going in the opposite direction you subtract the magnitude of the smaller force from the magnitude of the larger force.
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Contact and Non-Contact Forces
Forces are responsible for all contact between particles and objects. They can easily be divided into contact and non-contact forces.
These are the forces that act between two objects that are physically touching each other. There are four different examples of contact forces:
- Reaction force: this can be described as an object at rest on a surface.
- Tension: this occurs when an object is being stretched.
- Friction: when two objects slide past each other and create friction forces.
- Air resistance: this happens when an object is moving around through the air.
These are forces that act between two objects but are not physically touching each other. Examples of non-contact forces include:
- Magnetic force: experience by any magnetic object in a magnetic field.
- Electrostatic force: this can happen to any charged particle that is in an electric field.
- Gravitational force: this can be experienced by any object that is in the gravitational field.
One of the most important forces in the entire universe. All objects with mass will produce a gravitational field and the more mass an object has, the greater the gravitational field will be.
Gravitational field strength (g) is measured in newtons per kilogram (N/kg).
The Earth's gravitational field strength is 9.8 N/kg. Therefore, this means that for every kilogram of mass, an object will experience 9.8N of force. Where there is a weaker gravitational field, the weight of an object is smaller than it would be if the gravitational field was stronger.
The feeling of weightlessness can be felt by astronauts on the ISS even though they are within Earth's gravitational field. This is due to the fact that both they and the space station is falling towards the earth.
Weight can be calculated using an equation where weight (W) is measured in newtons, mass (m) is calculated by using kilograms (kg) and the gravitational field strength (g) is gauged in newtons per kilogram (N/kg). The equation is as follows:
weight = mass x gravitational field strength or W = m g
Work is done when energy is transferred from one store to another and when a force causes an object to move. Work can be calculated using a simple equation:
work done = force x distance or W = F s
The work done (W) is measured in joules (J), the force (F) is measured in joules and the distance moved along the line of action of the force (s) is measured in metres (m).
Free body diagrams
Free body diagrams model the force acting on an object. The object on the diagram is shown by a box or a dot and the forces are shown as arrows pointing away from the box.
These diagrams can be used for many examples such as the weight and reaction force for a resting object, an object moving full speed downhill and for an accelerating speedboat.
These are just some examples that can be used to determine weight, upthrust, thrust, air resistance, friction and reaction force.
Two forces can be added together to find a resultant force. A single force can be broken down into two component forces at right angles to each other.
Vector diagrams are taught in this section and students learn how to draw them, often with the help of their maths and physics tutor.
Forces and Elasticity
Forces have the responsibility of changing objects. If there are various forces used at the same time, the entire shape of an object can also be changed.
The object may change shape by bending, stretching or compressing or even a combination of all three is possible for some shape changes. A change in shape is known as deformation and there are two types:
- When an object undergoes elastic deformation it is reversed to original shape after the force is removed.
- After the force is removed in an inelastic deformation the object does not reverse to its original shape, permanent damage can be observed.
Extension is when an object increases in length and compression occurs when it decreases in length. The extension of an elastic object can be described by using the equation of Hooke's Law:
force = spring constant x extension or F = K e
In this equation force (F) is measured in newtons (N), spring constant (K) is calculated in newtons per metre (N/m) and the extension (e) is gauged in metres (m).
It is important to note that there is a point where Hooke's Law is no longer true and this is referred to as the limit of proportionality. The elastic limit of the material is at its furthest point it can be stretched without returning to its previous shape.
Force-extension graphs are great for visual students because they are able to see the force (N), extension (m) and limit of proportionality.
Before ending their studies of this section of the forces topic, students learn more about potential energy stored in a spring and complete a required practical that has the purpose of investigating between force and extension for a spring
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Moments, Levers and Gears
Turning forces can be found in everyday situations and are essential for machines to work properly.
A moment is the turning effect of a force. They act in a clockwise or counterclockwise way and the magnitude of a moment can be calculated using a simple equation:
moment of a force = force x distance or M = F d
Levers consist of three basic objects such as a pivot, an effort and a load. Depending on the arrangement of levers there can be many examples such as a see-saw, crowbar, scissors, wheelbarrow and cooking tongs.
A simple lever can be as basic as a solid piece of wood laid across a pivot. Levers make use of moments to act as force multipliers. They make it a lot easier to move large objects. The longer the lever the greater the force on the load will be.
Gears are wheels with toothed edges that rotate on an axle or shaft. The teeth of the small gear and the larger one have to fit perfectly into each other.
As one gear turns, so does the other and the teeth must both move in the same direction.
Students are not left in the dark since there are many useful examples that can be studied and examined during class time or for homework.
Pressure in Fluids
Pressure is the force per unit area. Liquids and gases are considered fluids and the pressure can be calculated using a very simple formula:
The pressure (p) is measured in pascals (Pa), force (F) is measured in newtons (N) and the area is calculated in metres squared. The pressure that is in fluids causes a force that is normal to a surface.
Pressure in a Liquid - Higher
It is important to note that the pressure of a liquid varies at different depths. The pressure increases the deeper you go down. Students in this section learn how to calculate the pressure of a liquid depending on the depth:
pressure = height of column x density of the liquid x gravitational field strength
Upthrust force is also explained in this section to deepen the pupil's knowledge.
The atmosphere is the layer of air around the Earth and it becomes lense dense as the altitude increases.
Air molecules colliding with a surface cause atmospheric pressure and it decreases as the height above ground level increases. This is the reason why the cabins of aeroplanes that fly at high altitudes need to be pressurized.
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The movement of different objects can be described using motion graphs and numerical values.
When discussing motion in a straight line students discuss the distance, which is how far an object moves, and the speed, which is the rate of change of distance that can be influenced depending on the factors of age, terrain, fitness and distance travelled.
The calculation that can be used to determine the distance travelled by an object moving at a constant speed is the following:
distance travelled = speed x time
Velocity and Acceleration
Velocity is the speed of a certain object in a particular direction. Displacement is used in calculations rather than distance to measure velocity.
Now acceleration of an object can be calculated using the following equation:
(Images are courtesy of BBC.com/bitesize/guides)
Acceleration can be defined as the amount that velocity changes per unit of time.
Distance-time graphs and velocity-time graphs are constantly used by students in this section to help design faster moving vehicles and determine acceleration.
Forces, Acceleration and Newton's Laws
In this section of the forces topic of the GCSE Physics Syllabus, students learn more about terminal velocity and the fact falling objects eventually reach the point where their resultant force is zero. The three stages of falling are analyzed to better understand terminal velocity.
Newton's Three Laws of motion are considered in this section and are the following:
- An object remains in the same state of motion unless a resultant force acts on it,
- The second law of motion can be described using a simple equation: resultant force = mass x acceleration,
- Whenever two objects interact they exert equal and opposite forces on each other.
Pupils are also expected to complete two required practicals. The first one has the aim of investigating the effect of varying the force on the acceleration of an object and the aim of the second experiment is to investigate the effect of varying the mass of an object on the acceleration produced by a constant force.
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Momentum can be described as a combination of mass and velocity. It is also a vector quantity which means it has both a magnitude and an associated direction.
Momentum can be calculated using the following equation:
momentum = mass x velocity or p = m v
Conservation of Momentum
A "closed system" is something that is not affected by external forces. It is also called the principle of conservation of momentum. This is conserved using collisions and explosions.
In this section, students analyze examples and equations that help them understand the principles of conservation of momentum.
Force and momentum is an important concept to grasp and when a force acts on an object that is moving there is a change in momentum that can be noticed.
Sample Exam Questions
While studying a GCSE subject, examinations are to be expected. The following are the types of questions that can be found on your exam with studying the GCSE Physics Syllabus:
- Multiple choice questions,
- One and two mark questions,
- Three and four mark questions,
- Maths questions,
- Practical questions,
- Six mark questions,
Studying all the topics of the GCSE Physics Syllabus such as energy, electricity, particle model of matter, atomic structure, waves, magnetism and electromagnetism and space physics can develop your knowledge of the most basic concepts in physics.
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