## PS122 – A car is travelling at an average speed of 100 km/h. In how many minutes does the car travel 70 km

PS122 Physics for General Science I Semester 1DUBLIN CITY UNIVERSITYSEMESTER 1 EXAMINATIONS 2014/2015 (January 2015)Instructions:Attempt ALL FOUR questions. Approximate marks are indicated for eachquestion. Show your work and give explanations with all answers. Atable of physical constants and useful formulae is provided at the end ofthe paper.Requirements for this paperPlease mark (X) as appropriateX Log TablesGraph PaperDictionariesStatistical TablesThermodynamic TablesActuarial TablesMCQ – Do not publish on WebPLEASE DO NOT TURN OVER THIS PAGE UNTIL YOU ARE INSTRUCTED TO DO SOThe use of programmable or text storing calculators is expressly forbidden. Please note that where a candidate answers more than the required number of questions, the examiner will markall questions attempted and then select the highest scoring ones.PS122 Physics for General Science I Semester 1 Page 2 of 6Please remember to provide explanations with all of your answers. Typically the answerto a question carries one mark, the remainder of the marks goes towards theexplanation. Provide brief explanations with your calculations, e.g. why a particularequation is appropriate.Question 1 (30 marks)a. [3 marks] A car is travelling at an average speed of 100 km/h. In how many minutesdoes the car travel 70 km?b. [4 marks] A car speeds up to 25 m/s from rest and covers a distance of 83 m. What isits acceleration, assumed constant?c. [4 marks] As it grows, an orange increases in radius r by a factor of four. By whatfactor do the volume V and area A increase during the growth?d. [4 marks] A typical cow will produce enough meat to make about 2000 hamburgers, andenough skin to make 6 footballs. If you were to raise a cow large enough to producemeat for 3000 hamburgers, approximately how many footballs could be made from theskin of that cow?e. [3 marks] A cyclist is moving to the right but accelerating to the left. What is thedirection of the net force on the cyclist?f. [4 marks] A person pushes on a 20 kg box to the right with a force of 100 N. Thecoefficient of friction between the box and the floor is 0.3. What is the magnitude anddirection of the friction force?g. [4 marks] When 10,000 J of work is done on a car it accelerates from 0 to 5 m/s. Howmuch work is required to accelerate the car from 5 m/s to 15 m/s?h. [4 marks] A force F is applied to a2.5 kg model car in the direction of itsmotion. The force exerted on the carvaries with the position of the car asshown at right. If the car begins with aspeed of 2.0 m/s at x = 0.0 m, what is thespeed of the car at x = 6.0 m?PS122 Physics for General Science I Semester 1 Page 3 of 6Question 2 (20 marks)As shown in the first lecture, a basketball and a tennis ball move with identical acceleration g(the acceleration due to gravity) when both are released at the same time from a distance habove the ground.a. [4 marks] Draw separate free-body diagrams for the basketball and the tennis ball sometime after they have been released but before they hit the ground. Explain how you drewthe diagrams.b. [3 marks] Just before the balls hit the ground, is the momentum of the basketball greaterthan, less than, or equal to the momentum of the tennis ball?c. [3 marks] Just before the balls hit the ground, is the kinetic energy of the basketballgreater than, less than, or equal to the kinetic energy of the tennis ball?In a second experiment, the tennis ball was placed on top of the basketball. Both werereleased at the same time from a distance h above the ground, and the two balls traveledtogether with the same acceleration g until they hit the ground. The basketball then stayed onthe ground while the tennis ball shot up and reached a height much greater than h.d. [4 marks] A student says: âWhile the two balls were falling, they touched each other sothey exerted a normal force on each otherâ. Do you agree with this statement?e. [3 marks] Explain how the motion of the two balls is consistent with the law ofconservation of momentum. Consider the motion before, during, and after hitting theground.f. [3 marks] Explain how the motion of the two balls is consistent with the law ofconservation of energy. Consider the motion before, during, and after hitting the ground.PS122 Physics for General Science I Semester 1 Page 4 of 6Question 3 (25 marks)a. [5 marks] Tightrope walkers typically carry a long pole to help them balance. Explainbriefly the physics behind how the pole helps.b. [7 marks] If Mars orbits the sun with an angular velocity of 1.06 Ã 10-7 rad/s, how longis a year on Mars in earth days, assuming it travels in a circular orbit?c. [8 marks] You decide to rob a bank that is filled with gold bars, and bring a briefcasewith dimensions 60 cm Ã 30 cm Ã 10 cm. If the density of gold is 19,300 kg/m3 and itsvalue is â¬30,000 per kg, what value of gold can you fit in the case? Are there any reasons,given your calculations, why you think the robbery might not be successful?d. [5 marks] Will a cube of gold (density = 19,300 kg/m3) float if it is placed in a beakerfilled with mercury (density = 13,594 kg/m3)? If so, what fraction of the cube will beabove the liquid?Question 4 (25 marks)a. [10 marks] A 70 L steel fuel tank in a car is completely filled up with petrol at 20oC.The car is left in the sun and the tank heats up to 40oC. If the petrol cap was left off thetank, how much petrol would spill out? The coefficient of volumetric expansion of thesteel in the tank is 36 Ã 10-6 oC-1, and that of the petrol is 950 Ã 10-6 oC-1.b. [8 marks] 1 L of air is cooled at constant pressure until its volume is halved and then itexpands isothermally back to its original volume. Draw this entire process on a P-Vdiagram.c. [7marks] Calculate the volume of 1 mole of an ideal gas at standard temperature andpressure.PS122 Physics for General Science I Semester 1 Page 5 of 6USEFUL EQUATIONSPS122 Physics for General Science I Semester 1 Page 6 of 6Table of Physical Constants for use in Physics ExaminationsQuantity Symbol Value UnitsSpeed of light in vacuum c 2.998Ã108 ms-1Acceleration due to gravity g 9.81 ms-2Gravitational constant G 6.67Ã10-11 Nm2kg-2Avogadroâs number NA 6.02Ã1023 mol-1Boltzmannâs constant k 1.38Ã10-23 JK-1Stefan-Boltzmann constant ? 5.67Ã10-8 Wm-2K-4Molar gas constant R 8.31 Jmol-1K-1Rydberg constant RH 1.1Ã107 m-1Electron charge e 1.6Ã10-19 CElectron mass me 9.1Ã10-31 kgPermittivity of free space ?0 8.85Ã10-12 Fm-1Permeability of free space Âµ0 4?Ã10-7 Hm-1Planckâs constant h 6.63Ã10-34 JsBohr magneton ÂµB 9.27Ã10-24 JT-1Atomic mass unit u 1.66Ã10-27 kgConversions1 atm=1.01Ã105N/m21 kcal=4.18Ã103J

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