CSEC SECTION A: MECHANICS Introduction-Physics, Base quantities What is Physics? It is the study of forces and their interactions In Physics we need to measure quantities to help us understand relevant calculations. The base S.I. quantities are shown below: Derived quantities, standard form Useful prefixes in Physics Density Questions Making measurements Instruments used to measure the following: Length Time Temperature Mass Current Common Instruments in the Lab Common instruments in the lab Question Quantity Mass Length Time Temperature Electric current Unit Instrument used to measure quantity Errors Accuracy and Precision The error associated with an instrument whether analogue or digital is usually considered to be half of the smallest reading available on the instrument Useful Definitions Accuracy- how close something is the true value Precision- how easily replicated a value is Value of error- half the value of the smallest unit of measurement on the instrument for example a ruler, the smallest reading is 1 mm, therefore the error is 0.5 mm Plotting Graphs Graphs show the relationship between two variables There are 2 axes, the y axis(vertical) and x axis(horizontal) Graphs are always plotted y versus x or responding/dependent variable versus the manipulated/independent variable Controlled variables are those that are considered so that they do not affect the results When plotting a graph, it is easier to use multiples of 2,5 and 10 on the axes making it easier to identify values Straight line graphs usually involve finding the gradient of the line, equation of the line and the y intercept of the line. Points are usually marked using either of the following Plotting Graphs If we need to plot a graph of temperature vs time, this means that temperature goes on the y-axis whereas time will go on the x-axis Let us plot a simple graph as shown below: Plot a graph of velocity vs time for the following values: Plotting Graphs Plotting Graphs Simple Pendulum Simple Pendulum Vectors and Scalars There are two classes of quantity in physics: scalars and vectors. A scalar quantity has magnitude (size) but no direction such as mass, time, temperature A vector quantity has magnitude and direction such as velocity, displacement, acceleration, weight Scalars can be resolved easily using arithmetic Resolving Vectors Forces Forces are pushes and pulls acting on an object can change its size/ shape , direction or motion. The unit of force is the newton (N) As forces are vector quantities (they have size and direction) they are represented in diagrams as arrows. The direction of the arrow gives the direction of the force and the length represents the size of the force When objects need to be touching for the force to exist, these forces are described as contact forces. When two surfaces move past each other, forces attempt to prevent this movement. These types of forces are known as friction, drag, or air resistance depending on where they originate from. This gives objects grip against each other. Floating objects experience upthrust from the fluid in which they float. In some situations the objects are not in direct contact but forces still exist between them. Planets are held in orbit around the Sun by the forces of gravity. Electrons are bound to atoms by electromagnetic forces. The same forces cause attraction and repulsion in magnets. The nucleus of an atom is held together by strong nuclear forces. Forces- force diagram of an airplane Moments When a force acts on an object it may cause a turning effect, known as the moment of the force. This turning effect depends on the size of the force applied and the distance from the pivot or point of rotation. The moment of a force is the product of the force and the perpendicular distance to the pivot: M (N m) = F (N) × d (m) When describing the action of moments, the terms clockwise and anticlockwise are used to describe the direction of action When an object is in equilibrium it is not accelerating or rotating. The two equilibrium conditions are: There is no resultant force acting on the object or the forces acting in one direction is equal to the forces acting in the opposite direction The total clockwise moment about a point is equal to the total anticlockwise moment about the same point, also known as the principle of moments Equilibrium means balanced, that is the sum of all forces is zero Moments-examples Moments questions Stability The centre of gravity of a body is the point on the body from which the entire weight appears to act. As the weight of an object acts from the centre of gravity an object will always be in equilibrium when it is suspended from a point directly above the centre of gravity. This idea can be used to find the centre of gravity for a lamina. A lamina is a thin sheet of material in any shape. Stability Hooke’s Law Hooke’s Law Hooke’s Law and Elastic bands Hooke’s Law-Questions A spring of length 15 cm stretches 8 cm if a mass of 1.2 kg is added. Find the length of the spring if a mass of 3.5 kg is added to the spring. Assume the elastic limit has not been exceeded. Motion, SUVAT Displacement vs time graphs Displacement vs time graphs Describe the graph below: Acceleration, velocity vs time graphs Motion-Question A body at rest begins to accelerate until it reaches a velocity of 30 m/s in 5s. It then keeps this velocity for 15 seconds before it decelerates to rest in 10 seconds. Show the motion of the body on a velocity time graph Calculate the acceleration and deceleration of the body Find the total distance travelled by the body Motion-Question continued If the body begins to reverse reaching a velocity of 10 m/s in 5 s and then decelerates to rest in 2 s. Show motion of body on same graph and calculate how far it moved while reversing Question-Explain and do all relevant calculations Velocity-time graphs A body at rest begins to accelerate so that it reaches a velocity of 40 m/s in 15 seconds, it keeps this velocity for 10 seconds and begins to accelerate again reaching 80 m/s in 10 seconds. It suddenly has to stop and goes to rest in 6 seconds. Sketch a velocity time graph Calculate the two accelerations and deceleration Calculate total distance travelled SUVAT equations v= u + at 𝑣 2 = 𝑢2 + 2𝑎𝑠 𝑠 = 𝑢𝑡 + 0.5𝑎𝑡 2 𝑠= Find the final velocity of a body that is at rest and begins to accelerate at a rate of 10 m/s^2 for a period of 8 seconds Find the distance travelled by a body if it is initially travelling at a velocity of 15 m/s if it accelerates uniformly for 20 s at a rate of 5 𝑚𝑠 −2 . What is the velocity of the body at the end of the acceleration Find the distance travelled by a body if it is initially travelling at 10 m/s and accelerates at a rate of 5 𝑚𝑠 −2 until it reaches a velocity of 100 m/s 𝑢+𝑣 ∗𝑡 2 Newtons Laws of Motion Newton’s second law Rate of change of momentum is directly proportional to the force applied and it occurs in the direction of the force Newtons Laws of Motion Newton’s Laws of Motion-questions Momentum and Impulse Momentum-questions Energy Energy is the capacity or ability to do work Energy is measured in Joules (J) Energy has many forms and changes from one form to another are referred to as transformations Energy Discuss some energy transformations In any energy transfer or transformation some of the energy is transformed into thermal energy which cannot be used to do any more useful work. This energy has not disappeared just spread out to the surroundings or dissipated Renewable and Non-renewable energy Electricity is an important form of energy and there is a constantly increasing demand for it which is met by using a wide range of energy sources. Electricity can be generated using renewable sources of energy or nonrenewable sources of energy Non-renewable sources cannot be regenerated or replenished easily, examples include: Fossil fuels such as coal, oil and natural gas, formed from the remains of plants and animals that died millions of years ago. These fuels are burnt in a furnace producing waste gases and large amounts of thermal energy. The heat is used to produce high pressure steam to spin turbines which drive electricity generators. Renewable and Non-renewable energy Nuclear power is another non-renewable source of energy. Nuclear power station produces heat using nuclear fuels such as uranium. These fuels are not burnt but release thermal energy inside a reactor core when the nuclei split through a process called nuclear fission. Nuclear power plants are very expensive to build and dismantle but are fairly cheap to operate. They can produce very large quantities of electricity. However, they also produce radioactive waste Renewable sources can be regenerated easily over time Sources include: Renewable and Non-renewable energy Renewable and Non-renewable energy Potential Energy Potential energy of a body is due to its position or condition Elastic potential energy is if the body is stretched or compressed Gravitational potential energy is when the body is at a height above the surface Kinetic energy Kinetic energy of a body is the energy due to its motion. It is given by the equation: Principle of Conservation of Energy The principle of conservation of energy states that energy cannot be created nor destroyed, rather changed from one from to another For the questions below, state the energy conversions that occur and use a diagram to illustrate Power and Efficiency Power of a body is the rate at which the body does work Efficiency of a body tells us how effectively the body does work Power and efficiency-question Pressure Pressure Pressure in liquids is given by the equation: Pressure Pressure-Questions Floating, sinking, density Archimedes is credited to making advancements on this topic Floating, sinking , density Floating, sinking, density Floating, sinking, density Floating, sinking, density Floating, sinking, density Remember a body floats if its own weight is equal to the upthrust on it or the weight of the fluid displaced How to solve floating and rising questions FOR A SHIP TO FLOAT IS MUST DISPLACE THE SAME WEIGHT OF WATER AS IT WEIGHS, for example if a ship is 5000 kg, it has a weight of 50000 N, then the weight of the water displaced is also 50000 N. It the weight of the ship is greater than the upthrust then the ship sinks FOR A FLOATING/RISING BODY, THE VOLUME OF THE BODY IS THE SAME AS THE VOLUME OF THE FLUID DISPLACED. THAT IS, if a 40 cubic metre object is moving through a fluid, then 40 cubic metres of the fluid is displaced to accommodate it as it moves up Floating and sinking can be considered in two ways: mass/weight or in volume Floating, sinking, density Measuring Pressure Measuring Pressure
