Atomic Structure

By Ivan Newman

All 1) elem_ntary forms of 2) m_tter are composed of very small unit 3) quant_ties called atoms. The atoms of a given element all have the same size and 4) wei_ht. The atoms of different elements have different sizes and weights. Atoms of the same or different elements 5) un_te with each other to form very small unit quantities of compound 6) sub­st_nces called 7) mol_cules.

Atoms are composed of a 1) ______ charged central core or nucle­us, surrounded by a 2) ______ of negatively charged electrons. The 3) ______ are equal, so that the atom is electrically 4) ______. Atoms form 5) ______ elements, as hydrogen, iron, or lead. Two or more atoms sticking together form a 6) ______. Molecules form chemical 7) ______.

Nucleus is the central part of the atom, which makes up most of the weight of the atom and 1) (is charging, is charged) with positive electricity. Proton is a 2) (small, smallest) particle with a positive electric charge equal nu­merically to the negative electric charge of the electron. Neutron is a particle with no electric charge, but with a mass 3) (approximate, approximately) the same as that of the proton. Electron is the smallest known particle having a negative electric charge. The part of an atom outside the nucleus is made of electrons, the number of which, being equal to the protons in the nucleus, 4) (is, are) the same as the atomic number of the atoms. For an atom in 5) (it’s, its) normal state the number of protons in the nucleus is equal to the number of electrons re­volving around it, and the atom as a whole 6) (displaies, displays) no electric charge.

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From COSMOS

10.03.2009

Three Laws of Motion

By Richard Horton

Newton's Laws of Motion help to describe the 1) inter_ction of forces in physical 2) s_stems — from the motions of objects, animals, and humans on Earth to the motions of rockets in 3) spa_e. Isaac Newton was the first to state the laws in a published form that future 4) s_ientists could use, although other scientists had made similar discoveries before Newton. In order to understand the Laws of Motion it is 5) ne_essary to understand speed, velocity, and acceleration. Velocity adds to speed the 6) d_mension of direction — a change in direction or a change in speed is a change in velocity. Acceleration refers to any change in velocity—therefore it can mean slowing, stopping, or 7) t_rning as well as speeding up.

Gravity and friction are two 1) ______ important in the workings of Newton's Laws. Friction occurs 2) ______ touching surfaces and always acts opposite to the 3) ______ of motion. Friction depends on the 4) ______ of the touching surfaces and the amount of 5) ______ pressing the surfaces 6) ______. Gravity is a force that all objects exert on all other objects, although the pull of Earth is by far the greatest gravitational force experienced on 7) ______.

Newton's First Law states that 1) (a, an) object at rest tends to stay at rest, and an object in motion tends to stay in motion in a straight line and at a constant speed unless 2) (acted, acting) upon by a net force. The Second Law of Motion states that the acceleration of an object 3) (depends, has depended) on the net force acting on that object and on the mass of 4) (that, those) object. Newton's Third Law of Motion states that whenever an object exerts a force on a 5) (secondth, second) object, the second object exerts an equal and opposite force on 6) (the, a) first object.

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From The Future of Physics: Scientific American Magazine

11.11.2008