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Physics Calculation Revision - answers 1. A man lifts a brick of mass 5kg from the floor to a shelf 2 metres high. 6. A 2kW fire, a 200W TV and three 100W lamps are all switched on from 6pm until a] What is the weight of the brick? (g = 10N/kg) 10pm. What is the total cost at 8p per kW-h? m = 5kg W = m x g = 5kg x 10N/kg = 50N fire P = 2kW h = 2m TV P = 200W = 0.2kW W=? lamp P = 100W = 0.1kW g = 10N/kg total P = 2kW + 0.2kW + (3 x 0.1kW) = 2.5kW t = 4hr b] How much work is done? cost = ? Wd = ? Wd = F x d = 50N x 2m = 100J unit cost = 8p cost = P x t x unit cost = 2.5kW x 4hr x 8p = 80p 2. A weight lifter raises a mass of 200kg up to a height of 2m. How much gravitational 7. An electric kettle is rated at 2kW. How many joules of energy are transferred in potential energy does the mass gain? (g = 10N/kg) 10 seconds? m = 200kg GPE = m x g x h = 200kg x 10N/kg x 2m = 4000J P = 2kW = 2000W P = Wd/t h = 2m Wd = ? GPE = ? t = 10s Wd = P x t = 2000W x 10s = 20000J g = 10N/kg 8. An athlete exerts a force of 80N for a distance of 100m, how much work does she 3. A step down transformer is required to transform 240V a.c. to 12V a.c. for a model do? She uses up 40000J of food energy, what is her efficiency? railway. If the primary coil has 1000 turns, how many turns should the secondary coil F = 80N Wd = F x d = 80N x 100m = 8000J have? d = 100m V1 = 240V N2/N1 = V2/V1 Wd = ? V2 = 12V N1 = 1000turns N2 = (V2/V1) x N1 = (12V/240V) x 1000 = 50turns efficiency = (useful E/total E) x 100% = (8000J/40000J) x 100% = 20% N2 = ? 9. What voltage is needed to send a 2A current through a 5Ω resistor? 4. A crane lifts a load weighing 3000N through a height of 5m in 10s. What is the power V=? V = I x R = 2A x 5Ω = 10V of the crane? I = 2A W = 3000N P = Wd/t = (F x d)/t = (3000N x 5m)/10s = 1500W R = 5Ω h = 5m t = 10s 10. A car travels 100m in 5s. What is its average speed? P=? d = 100m s = d/t = 100m/5s = 20m/s t = 5s 5. A car is travelling at 20m/s. The driver has a reaction time of 0.7s. How far does she s=? travel before she starts to brake? s = 20m/s s = d/t 11. An electric lamp uses 100J of electrical energy each second but only transfers 2J t = 0.7s to light energy. How efficient is the lamp? d=? d = s x t = 20m/s x 0.7s = 14.0m efficiency = (useful E/total E) x 100% = (2J/100J) x 100% = 2% KRa9a5e91a-c400-4944-a0a7-55095686f03b.doc Physics Calculation Revision - answers 12. A car accelerates from 5m/s to 25m/s in 10 seconds. What is its acceleration? 17. A car-driver of mass 100kg is in a crash. He decelerates from 30m/s to rest in 2 u = 5m/s a = (v-u)/t = (25m/s - 5m/s)/10s = 2m/s 2 seconds. Calculate: a] his deceleration, v = 25m/s m = 100kg a = (v-u)/t = (0m/s - 30m/s)/2s = -15m/s2 t = 10s u = 30m/s a=? v = 0m/s t = 2s 13. A sealed syringe contains 60cm3 of air at a pressure of 1.0atm. The piston is pushed a=? in slowly until the volume is 40cm3. What is the pressure now? V1 = 60cm3 P2/P1 = V2/V1 b] the force exerted by his seat-belt. P1 = 1.0atm P2 = (V2/V1) x P1 = (60cm3/40cm3) x 1.0atm = 1.5atm a = -15m/s2 F = m x a = 100kg x -15m/s2 = -1500N V2 = 40cm3 F=? P2 = ? 18. A ship sends out a sound wave and receives an echo after 1s. If the speed of sound in 14. A wave has a frequency of 2Hz and a wavelength of 10cm. What is the speed of the water is 1500m/s, how deep is the water? wave? t = 1s s = d/t f = 2Hz v = f x = 2Hz x 0.1m = 0.2m/s s = 1500m/s d = s x t = 1500m/s x 1s = 1500m = 10cm = 0.1m d=? depth = 1500m/2 = 750m v=? 19. A current of 1A flows through a lamp for 1 minute. How much charge passes through 15. A 2000W electric fire is used for 10 hrs. What is the cost at 8p per kW-h? the lamp? P = 2000W = 2kW cost = P x t x unit cost = 2kW x 10hr x 8p = 160p I = 1A Q = I x t = 1A x 60s = 60C t = 10hr t = 1min = 60s cost = ? Q=? unit cost = 8p 20. If a potential difference of 10V causes a current of 2A to flow for 1 minute. 16. An man has a mass of 60kg. a] What is his mass on the Moon? 60kg How much energy is transferred? b] What is his weight on Earth? (g = 10N/kg) V = 10V E = I x V x t = 2A x 10V x 60s = 1200J m = 60kg W = m x g = 60kg x 10N/kg = 600N I = 2A g = 10N/kg t = 1min = 60s W=? E=? KRa9a5e91a-c400-4944-a0a7-55095686f03b.doc Physics Calculation Revision - answers 21. An electric kettle connected to the 230V mains supply draws a current of 10A. 25. Galileo drops a stone from the leaning tower of Pisa. The tower is 45m high. At what Calculate: a] the power of the kettle, speed does the stone hit the ground? (hints: g = 10N/kg; GPE lost = KE gained) V = 230V P = I x V = 10A x 230V = 2300W h = 45m I = 10A v=? P=? GPE lost = KE gained b] the energy transferred in 1 minute. m x g x h = ½ x m x v2 E=? E = I x V x t = 10A x 230V x 60s = 138000J g x h = ½ x v2 t = 1min = 60s v = √(2 x g x h) = √(2 x 10 x 45) = 30m/s 22. An elephant of mass 2000kg is travelling at 5m/s. How much kinetic energy does it 26. A car of mass 800kg is travelling at 10m/s. When the brakes are applied, it comes to have? rest in 8m. What is the average force exerted by the brakes? (hint: kinetic energy m = 2000kg KE = ½ x m x v2 = ½ x 2000 x 52 = 25000J lost = work done by the brakes) v = 5m/s m = 800kg KE = ? v = 10m/s 23. A potential difference of 4V is applied to two resistors (6Ω and 2Ω ) connected in d = 8m series. Calculate: a] the combined resistance and the current flowing, F=? V = 4V V=IxR R = 6Ω + 2Ω = 8Ω I = V/R = 4V/8Ω = 0.5A KE lost = Wd by brakes I=? ½ x m x v2 = F x d F = (½ x m x v2)/d = (½ x 800 x 102)/8 = 5000N b] the potential difference across the 6Ω resistor. V = (6Ω /8Ω) x 4V = 3V 24. When a force of 6N is applied to a block of mass 2kg, it moves along a table at constant velocity. a] What is the force of friction? 6N b] When the force applied is increased to 10N, what is the acceleration? m = 2kg F=mxa F = 10N - 6N = 4N a=? a = F/m = 4N/2kg = = 2m/s2 KRa9a5e91a-c400-4944-a0a7-55095686f03b.doc

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posted: | 2/24/2010 |

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