Physics Moments and Energy Study Guide PDF

Moments 5.1 When Does a Force Cause Something to Turn? A turning effect of a force is called a moment. Moment is proportional to: -The perpendicular distance from the pivot where the force is applied -The magnitude of the force applied Definition of moment The moment of a force M, or torque, ab...

Moments 5.1 When Does a Force Cause Something to Turn? A turning effect of a force is called a moment. Moment is proportional to: -The perpendicular distance from the pivot where the force is applied -The magnitude of the force applied Definition of moment The moment of a force M, or torque, about a pivot is the product of the force F and the perpendicular distance d from the pivot to the line of action of the force. Formula of moment M=Fxd where F is the force applied (N) ​ d is the perpendicular distance between the force and the pivot (m) ​ M is moments (N m) Clockwise & anticlockwise moments When there is a clockwise rotation about the pivot, it results in a clockwise moment. When there is an anticlockwise rotation about the pivot, it results in an anticlockwise moment. *Note that in order to create a moment, the line of action of the force must not pass through the pivot* Example question Explain why it is better to apply the force at 10 cm rather than at 5 cm for the diagram on the left. perpendicular distance from the pivot to line of action of force at 10 cm is greater than that of the perpendicular distance from the pivot to line of action of force at 5 cm. Based on the formula M = Fx d, for the same moment, less force is required at 10cm than at 5 cm Principle of moments The principle of moments states that when a body is in equilibrium, the sum of clockwise moments about a pivot is equal to the sum of anticlockwise moments about the same pivot. Sum of clockwise moments​ =​ Sum of anticlockwise moments about any pivot ​ ​ ​ ​ about the same pivot Example Question Conditions of equilibrium -The resultant moment on the body is zero -The resultant force on the body is also zero Not needed: not have any rotational or translational motion. there should be no resultant moments and no resultant force acting on the body. 5.2 How can we prevent objects from toppling Centre of gravity Definition *The centre of gravity of an object is an imaginary point where the entire weight of the object seems to act.* (Remember this) The centre of gravity for an object that has a regular shape with a uniform density is at the centre of the object. *Note: centre of gravity can be outside the object* As mentioned, when the line of action of weight passes through the pivot, there is no perpendicular distance from the pivot to the line of action of weight. No resultant moment Finding the centre of gravity of an irregular object The centre of gravity of the lamina is the point of intersection of the three dashed lines. (For some irregularly shaped lamina, the point of intersection could be lying outside the shape itself.) Stability 2 situations to start the sub chapter (A)The block will return to its ​ ​ ​ (B) The block topples after it is pushed original position after it is pushed slightly​​ more strongly definition The stability of an object is a measure of its ability to return to its original position. Explanation to situations that can happen a) The line of action of the weight falls within its base area causes an anticlockwise moment about pivot b) Line of action of weight passes through the pivot no perpendicular distance from the pivot to the line of action of the weight no moment c) The line of action of the weight falls outside its base area causes an clockwise moment about pivot Why is an object with lower centre of gravity more stable? Line of action of the weight of the object continues to be within the base of the object even when object is tilted by & large angle Why is an object with a larger base area more stable? line of action of the weight of the object continues to be within the base of the object even when the object tilted by a large angle *When the centre of gravity is below the pivot Centre of gravity is vertically below the pivot No perpendicular distance from line of action of weight to pivot No moment* (important concept) 7.1 What are energy stores and transfers Energy is required for things to work. The SI unit for energy is joule (J) Energy stores and some examples: -kinetic store (any object in motion) -elastic potential store (compressed spring) -internal (thermal) store (hot objects) -gravitational potential store (any object above the ground) -chemical potential store (fossil fuels) -nuclear store (nuclei of atoms) Energy transfer (not too important): Total amount of energy remains constant. (energy cannot be created or destroyed) An analogy to understand this is to think of energy as money and energy stores as different bank accounts where money can be stored. We can transfer money (energy) from one bank account (energy store) to another bank account (energy store) but the total amount of money is a constant. Money (energy) can be transferred between the bank accounts (energy stores) in different ways. For example, we can transfer money using Internet banking or at the Automated Teller Machines (ATMs). Similarly, energy can be transferred to different energy stores through different pathways. Types of energy transfer: -mechanically -by heating -propagation of waves (electromagnetic / mechanical sound waves) -electrically Examples: Mechanically some of the energy is transferred mechanically from the gravitational potential store to the internal stores of the pendulum bob and surrounding due to air resistance opposing the movement of the pendulum bob. By heating Propagation of electromagnetic waves Propagation of sound waves Electricity The energy in the chemical potential store is transferred electrically to the kinetic stores of the robot and food tray, and the internal store of the lights in the robot. Calculating the size of energy store Ek = ½mv2 ​ where Ek = energy in kinetic store (J) ​ ​ m = mass of the body(kg) ​ ​ v = speed of the body (m/s) Eg = mgh ​ where Eg = energy in gravitational potential store (J) ​ ​ m = mass of the body (kg) ​ ​ g = gravitational field strength (N/kg) *usually 10N* ​ ​ h = height (m) Principle of conservation of energy The principle of conservation of energy states that energy cannot be created or destroyed. Energy can be transferred from one store to another. The total energy of an isolated system is constant *Note: for a detailed example, refer to pg 120 of TB* 7.2 What are work done and power Definition of work done work done by a constant force on an object is the product of the force and the distance moved by the object in the direction of the force. *State what is meant by work (common question)* try to rmb this Is the product of the force and the distance moved by the object to the direction of the force. W = Fs ​ where W = work done by a constant force F (J) ​ ​ F = constant force (N) ​ ​ s = distance moved in the direction of the force (m) Power Definition of power Power is defined as the work done or energy transferred per unit time SI unit of power is watt (W) 1W = 1J/s P = W/t = E/t ​ Where P = power (W) ​ ​ W = word done (J) ​ ​ E = energy transfer (J) ​ ​ t time taken (s) Concept of efficiency Applying the principle of conservation of energy: Total input energy = useful output energy + non useful energy Efficiency = ( useful energy output / total energy input ) x 100%