Concepts of Physics: Class Test I

Transcription

1 Phys 100 Fall 2011 Concepts of Physics: Class Test I 26 September 2011 Name: Total: /50 Instructions There are 10 questions on 5 pages. Show your reasoning and calculations and always justify your answers. Physical constants and useful formulae speed = acceleration = distance traveled time elapsed change in velocity time elapsed distance = 1 acceleration time2 d 2 acceleration = net force mass net force = mass acceleration s = d t a = v t = 1 2 a t2 a = F m F = ma earth s gravitational force = mass 9.8 F = m 9.8 gravitational force = mass 1 mass 2 distance 2 F grav = m 1 m 2 d 2 Question 1 A chlorine atom (Cl) has a mass exactly 35 times that of a hydrogen atom (H). Hydrogen atoms and chlorine atoms can combine to form hydrochloric acid, HCl (one chlorine atom and one hydrogen atom). Suppose that this is decomposed giving exactly 5 kg of hydrogen. Which of the following (choose one) is the mass of the chlorine produced by this decomposition? a) 5kg. b) 7kg. c) 35kg. d) 40kg. e) 165kg.

2 Question 2 The planet Mars can be observed from the Earth and its brightness can be monitored as it completes an entire orbit. Describe what each of the following predicts for the variation of the brightness of Mars as observed from Earth: 1. a geocentric model in which all planets orbit in prefect circles and 2. a heliocentric model. Use your descriptions to predict how one could decide which of these two models could be correct. Question 3 Which of the following (choose one) possibly constitutes experimentally observable evidence for the fact that oxygen atoms exist? a) We need oxygen to breathe so oxygen must consist of atoms. b) Individual oxygen atoms can be observed with an ordinary optical microscope. c) Water consists of oxygen atoms and by observing water we can directly observe the oxygen atoms. d) Brownian motion (random jiggling) of a visible dust particle in a container of oxygen gas. /6 2

3 Question 4 A ball moves horizontally from left to right. A data recorder records the speed and acceleration of the ball. It reports that from 0s to 2s, the ball accelerates and then from 2s to 4s it moves with constant speed. Three students reconstruct the position of the ball every second. Their depictions are illustrated. 0s 1s 2s 3s 4s 5s Avogadro 0s 1s 2s 3s 4s Maxwell 0s 1s 2s 3s 4s 5s Zemansky Whose depiction is correct? Explain your answer. Question 5 A hockey puck slides across an ice surface, which is frictionless and horizontal. Air resistance is negligible and the puck slides with constant high speed in a straight line. A spectator states that At all times while the puck is sliding at high speed, there must be a large force acting on the puck to keep the puck moving with high speed. Is this correct or not? Explain your answer. /7 3

4 Question 6 Three cars each move in a straight line. At one instant their speeds and accelerations are measured. This information, together with their masses is provided in the table. Car Mass (kg) Speed (m/s) Acceleration Ford Honda Audi Which of the following best describes the net forces on the cars? a) Force on Ford is largest, force on Honda is smaller, force on Audi is smallest. b) Force on Honda is largest, force on Ford is smaller, force on Audi is smallest. c) Force on Audi is largest, force on Honda is smaller, force on Ford is smallest. d) Force on Ford is largest, force on Audi is smaller, force on Honda is smallest. Question 7 A car travels around a corner at a constant speed. Is the net force acting on the car zero or non-zero? Explain your answer. Question 8 A ball drops to the surface of the Earth, hits and bounces off. During the time in which the ball is in contact with the Earth, the Earth exerts a force of 100N on the ball. Which of the following (choose one) is true during the time that the ball is in contact with the Earth? a) The ball does not exert a force on the Earth. b) The ball exerts a force on the Earth and this is less than 100N. c) The ball exerts a force on the Earth and this is exactly 100N. d) The ball exerts a force on the Earth and this is more than 100N. /5 4

5 Question 9 A skydiver of mass 100kg falls directly down toward the surface of the Earth. The skydiver s weight (gravitational force exerted by Earth) is 980N. The air offers a resistance which amounts to a force of 680N opposite to the direction in which the skydiver moves. Determine the acceleration of the skydiver. Question 10 A satellite orbits the Earth in a circle at constant speed. At one particular instant, it is at the illustrated position (it orbits clockwise according to this view). Consider the net force exerted on the satellite at this instant and assume that the Sun, Moon and other objects exert forces that are so small that they can be ignored. Which of the following (choose one) best represents the net force exerted on the satellite at this instant? Satellite Earth /8 a) Net force is zero. b) Net force points c) Net force points d) Net force points e) Net force points 5