Cycle 3 Grade 9 Advanced Physics Chapter 8 Work, Energy and Machines Work Definition: Work is the transfer of energy that occurs when a force (F) is applied through a displacement (d). Unit is the Joule (J) or the Newton-Meter (N.m). *Note Force x Displacement = Newton x Meter = N.m Work at an angle When dealing with forces exerted at an angle, work can then be calculated as follows: Work at an angle The work done may be positive, zero, or negative, depending on the angle between the force and the displacement. Total Work Done on a System The total work done on a system is the sum of the work done by each body that exerts a force on the system. Work done by a variable force Force (N) The area under a force-displacement graph is equal to the work. Area = workdone Displacement (m) Work – Problem Solving Strategies Practice Questions Q1. Ahmed uses a force of 25 Newtons to lift his grocery bag while doing 50 Joules of work. How far did he lift the grocery bags? Answer = 2 m Q2. A baseball player does 1234 Joules of work when hitting a baseball into left field. Assuming the baseball landed 100 meters away from home plate, how much force did the player use to hit the ball? Answer = 12.34 N Q3. A force of 70 N acts on a 12 kg object at 60ᵒ to the horizontal and as a result, the object is displaced horizontally through a displacement of 23 m. Determine the work done on the object. Answer = 805 J Energy Energy is quite difficult to define! Definition: the ability to do work. Unit is the Joule (J) There are essentially two types of energy in nature: 1. Potential 2. Kinetic Kinetic Energy Kinetic energy is the energy of a moving object. The term ‘kinetic’ means movement. When stored energy is being used up, it is making things move or happen. K.E. = ½ mv2 Units = (kg)(m/s)2 = 1 Joule Potential Energy Potential energy is the energy stored in an object. The term ‘potential’ means the energy has the ability to do something useful later on. Gravitational Potential Energy (PEGRAV) is the potential energy a physical object with mass has in relation to another massive object due to gravity. PEGRAV = -mgh Units = (kg)(m/s2)(m) = 1 Joule Work Done by Gravity Work = Force x Displacement (or height) W=Fxd W = -mg x h W = - mgh Work-Energy Theorem and Power Work done on a system is equal to the change in the system’s energy. W = ΔE Power is equal to the change in energy divided by the time required for the change. P = ΔE/t = W/t Unit is the Watt (W). W = J/s One Watt is equal to 1 J of energy transformed in 1 s. The Law of Conservation of Energy The total energy of an isolated system remains constant; it is said to be conserved over time. This law means that energy can neither be created nor destroyed; rather, it can only be transformed or transferred from one form to another. Practice Questions A rollercoaster is released from rest at point A and moves along the track to the right. The mass of an empty coaster is 500 kg. Assuming there is no friction, answer the questions below. a. At what location has: i) ii) iii) iv) Greatest potential energy? A Lowest potential energy? C Greatest kinetic energy? C Lowest kinetic energy? A Practice Questions A rollercoaster is released from rest at point A and moves along the track to the right. The mass of an empty coaster is 500 kg. Assuming there is no friction, answer the questions below. b. Calculate the: i) ii) iii) iv) Potential Energy at point A 98000 J Potential Energy at point E 49000 J Kinetic Energy at point C 98000 J Velocity at point C 19.79 m/s