Global exam of the 3 rd term of Physics and Chemistry for 3 rd ESO F

Transcription

1 Global exam of the 3 rd term of Physics and Chemistry for 3 rd ESO F Name: Date: 1. In atoms, electrons, which have negative charge, orbit around the nucleus, which has positive charge, thanks to an electrostatic force of attraction between the positive nucleus and the negative electrons. Look at the following atom of hydrogen, which is made by one proton and one electron: As you can see in the picture, the distance between the nucleus and the electron is m. The charge of the proton is C and the charge of the electron is C (as you can see they have the same charge but with different sign). a) Calculate the electrostatic force between the proton of the nucleus and the electron that is orbiting around it. (0.75 points) b) As protons and electrons have mass, there is also a gravitational force between them. Knowing that the mass of a proton is kg and the mass of an electron is kg, calculate the gravitational force between them in the atom of the problem. Compare this force with the electrostatic force you have calculated in a). What conclusion do you have about the importance in the atom of these two forces? (0.75 points) 2. Calculate the acceleration of gravity on Mars at a height of 100 km, knowing that the mass of Mars is kg and its radius is 3397 km. In Mars, at that height, what is the weight of an object that has a mass of 300 Hg? If you need to change units in this problem, it is obligatory to use conversion factors. 3. A spring has an elastic constant of 200 N/m. a) You are on the Earth. Calculate the elongation of this spring when you hang on the spring an object that has a mass of 225 Dag. b) Imagine that you go to the Moon (g Moon =1.6 m/s 2 ). Calculate the elongation of the spring if you hang from it an object that has a mass of 9750 dg. If you need to change any units in this problem, use conversion factors. 4. An object has a speed of 10 Hm/min. It begins to slow down with a constant acceleration of a=-1.5 m/s 2. Calculate the time it takes for this object to completely stop. 5. The distance between Madrid an Alicante is Dam. Calculate the time it takes an AVE train to reach Alicante from Madrid if its average speed in the journey is 200 km/h.

2 6. In the following graph of speed vs time of an object, calculate: The acceleration in each section. The space travelled in each section. 7. We study the motion of a student of IES Nicolás Copérnico that is in Diego s cafeteria and starts moving towards the Head of Studies (Jefatura de Estudios). We choose as our reference frame: The position x=0 is Diego s cafeteria. The positive axis of the frame reference is the path that goes from Diego s cafeteria to the Head of Studies (Jefatura de Estudios). We represent the position of the student vs the time and obtain this graph: a) What is the position of the student in time 10 s? What is the position of the student in time 30 s? And in time 50 s? (0.5 points) b) Calculate: The average speed in the whole trip.

3 Global exam of the 3 rd term of Physics and Chemistry for 3 rd ESO F (answers) Name: Date: 1. In atoms, electrons, which have negative charge, orbit around the nucleus, which has positive charge, thanks to an electrostatic force of attraction between the positive nucleus and the negative electrons. Look at the following atom of hydrogen, which is made by one proton and one electron: As you can see in the picture, the distance between the nucleus and the electron is m. The charge of the proton is C and the charge of the electron is C (as you can see they have the same charge but with different sign). a) Calculate the electrostatic force between the proton of the nucleus and the electron that is orbiting around it. (0.75 points) b) As protons and electrons have mass, there is also a gravitational force between them. Knowing that the mass of a proton is kg and the mass of an electron is kg, calculate the gravitational force between them in the atom of the problem. Compare this force with the electrostatic force you have calculated in a). What conclusion do you have about the importance in the atom of these two forces? (0.75 points) The electrostatic force is !!! times bigger than the gravitational force. This means that the force that is important in an atom is the electrostatic force, the gravitational force is so small that it is not important in the atom.

4 2. Calculate the acceleration of gravity on Mars at a height of 100 km, knowing that the mass of Mars is kg and its radius is 3397 km. In Mars, at that height, what is the weight of an object that has a mass of 300 Hg? If you need to change units in this problem, it is obligatory to use conversion factors. 3. A spring has an elastic constant of 200 N/m. a) You are on the Earth. Calculate the elongation of this spring when you hang on the spring an object that has a mass of 225 Dag. b) Imagine that you go to the Moon (g Moon =1.6 m/s 2 ). Calculate the elongation of the spring if you hang from it an object that has a mass of 9750 dg. If you need to change any units in this problem, use conversion factors. 4. An object has a speed of 10 Hm/min. It begins to slow down with a constant acceleration of a=-1.5 m/s 2. Calculate the time it takes for this object to completely stop.

5 5. The distance between Madrid an Alicante is Dam. Calculate the time it takes an AVE train to reach Alicante from Madrid if its average speed in the journey is 200 km/h. Answer: 2 hours, 6 minutes and 18 seconds. 6. In the following graph of speed vs time of an object, calculate: The acceleration in each section. The space travelled in each section. Section 1: a 1 =0 because the speed remains constant (v=10 m/s). In the graphs of speed vs time the space is the area of the surface between the graph and the X-axis: e 1 =10 5=50 m The average speed is equal to 10 m/s because v remains constant in the whole section.

6 Section 2: The acceleration is constant because the graph is a straight line. As the acceleration is constant, we could also have calculated the average speed like this: Section 3: a 3 =0 because the speed remains constant (v=20 m/s). In the graphs of speed vs time the space is the area of the surface between the graph and the X-axis: e 3 =20 5=100 m The average speed is equal to 20 m/s because it remains constant in the whole section. Section 4: The acceleration is constant because the graph is a straight line. As the acceleration is constant, we could also have calculated the average speed like this:

7 7. We study the motion of a student of IES Nicolás Copérnico that is in Diego s cafeteria and starts moving towards the Head of Studies (Jefatura de Estudios). We choose as our reference frame: The position x=0 is Diego s cafeteria. The positive axis of the frame reference is the path that goes from Diego s cafeteria to the Head of Studies (Jefatura de Estudios). We represent the position of the student vs the time and obtain this graph: a) What is the position of the student in time 10 s? What is the position of the student in time 30 s? And in time 50 s? (0.5 points) x(t=10)=10 m x(t=30)=20 m x(t=50)=30 m b) Calculate: The average speed in the whole trip. In section 1 the speed is constant (v m =v): In section 2 the speed is 0 because the student is not moving. In section 3 the speed is constant (v m =v):