#### Transcript Work, energy and momentum

Work, energy and momentum Revise the following: • Calculating work • Calculating kinetic energy • Momentum Work and energy When a force causes a body to move through a distance, energy is transferred and work is done. What is the link between work and energy? work done = energy transferred This means the units for work are the same as the units for energy – joules. For example, if a person does 500 J of work, then 500 J of energy is transferred. In the same way, if a person transfers 250 J of energy, then 250 J of work is done. How is work calculated? The work done on an object can be calculated using this equation: work done = force (Joules, J) x distance moved (Newtons, N) (metres, m) Force is measured in newtons (N). Distance moved is measured in metres (m). Work done is measured in joules (j). done against frictional forces is mainly transformed into heat How is kinetic energy calculated? (Higher tier) The kinetic energy (KE) of an object depends on 2 things: 1. mass and 2. speed The kinetic energy (KE) of an object can be calculated using this equation: KE = ½ x mass x velocity2 = ½mv2 Mass is measured in kilograms (kg). Velocity is measured in metres per second (m/s). KE is measured in joules (j). What is momentum? All moving objects have momentum. This is a measure of how difficult it is to stop a moving object. If these two cars have the same mass but one is quicker than the other, which has the most momentum? The faster car. If both cars travel at the same velocity, but one is full with luggage and the other is empty, which will have the most momentum? The heavier car. The bigger an object is and the faster it moves, the more momentum it will have and the more difficult it will be to stop. How is momentum calculated? The momentum of an object can be calculated using this equation: momentum = mass x velocity Mass is measured in kilograms (kg). Velocity is measured in metres per second (m/s). Momentum is measured in kilogram metres per second (kg m/s). When two objects collide, and no other forces act, then conservation of momentum applies. What does the term conservation of momentum mean? When the total momentum after the collision equals the total momentum before the collision Force and change in momentum (higher tier) When a force is applied to an object, the object’s velocity changes. This means that its momentum will also change. The change in momentum depends on the size of the force and the time for which it is applied. The relationship between this values is shown by this equation: force = change in momentum time Momentum is measured in kilogram meters per second (kg m/s). Time is measured in seconds (s). Force is measured in newtons (N). Gravitational potential energy depends on mass and height above ground Books on a shelf have gravitational potential energy. Book A has more than book C as it's higher. Book B has more than book A because it has a greater mass 9 of 28 © Boardworks Ltd 2007 Gravitational field strength on earth The gravitational field at the Earth's surface produces a force of approximately 10N (Newtons) on every mass of 1kg. Gravitational field strength is symbolised by the letter 'g'. On larger planets, like Jupiter where the gravitational field strength is greater, the gravitational potential energy would also be greater 10 N/Kg 10 of 28 © Boardworks Ltd 2007 Gravitational potential energy is the energy that an object has by virtue of its position in a gravitational field. Ep = m x g x h 11 of 28 © Boardworks Ltd 2007 Gravitational potential energy is the energy that an object has by virtue of its position in a gravitational field. Ep = m x g x h Worked example Abid the IT technician lifts a 15kg monitor from the floor onto a table 1.2m high. Calculate its gain in GPE. GPE = mgh GPE = 15 × 10 × 1.2 GPE = 180 J 12 of 28 © Boardworks Ltd 2007 A force acting on an object may cause a change in shape of the object. Forces don’t always make an object move, sometimes they make an object change shape Work is done whenever a force is applied to an object. This means that if a force changes the shape of an object, work must be done 13 of 28 © Boardworks Ltd 2007 A force applied to an elastic object such as a spring will result in the object stretching and storing elastic potential energy Elastic objects such as elastic bands and squash balls can change their shape. They can be stretched or squashed, but energy is needed to change their shape. This energy is stored in the stretched or squashed object as elastic potential energy. 14 of 28 © Boardworks Ltd 2007 For an object that is able to recover its original shape, elastic potential energy is stored in the object when work is done on the object to change its shape. 1. A falling ball transfers GPE into KE. 2. When the ball hits the ground, its shape changes and the KE is transferred into EPE. 3. As its shape is restored, the EPE changes back into KE. 4. As the ball gets higher in the air, KE is transferred back into GPE. 15 of 28 © Boardworks Ltd 2007 The extension of an elastic object is directly proportional to the force applied, provided that the limit of proportionality is not exceeded: F= k x e F is the force in newtons, N k is the spring constant in newtons per metre, N/m e is the extension in metres, m 16 of 28 © Boardworks Ltd 2007