Download this poster at www.hoddereducation.co.uk/physicsreviewextras PhysicsReviewExtras 17 www.hoddereducation.co.uk/physicsreview 16 Physics Review February 2017 at a glance Ideas about the motion of falling objects date back long before modern theories of gravitation. A common idea of the time was that a projectile rises in a straight line until it runs out of ‘impetus’, at which point it falls to the ground (1); the heavier an object, the more rapidly it would fall. The Italian scientist Galileo Galilei (1564–1642) designed an experiment in which a ball rolling down a slope hit a series of bells. By adjusting their positions, he made the bells ring at regular intervals (2), showing that the rolling ball had a constant acceleration (sometimes called ‘diluted gravity’). Galileo showed that all free-falling objects have the same acceleration and deduced that a projectile moves with constant horizontal velocity combined with a constant vertical acceleration (3). Isaac Newton (4) proposed that the forces responsible for an apple falling from a tree, and for keeping a planet in orbit, had the same origin. He argued that any two objects attract one another with a force that is proportional to each of their masses and gets weaker as their separation increases. By studying the Moon’s orbit, he derived the inverse-square law for universal gravitation (5). Gravit a tion massive star 7 Gravitational redshift; photon energy E = hf = hc/λ 9 The rate of universal expansion changes over time time present deceleration acceleration Big Bang Big Rip Big Crunch constant dark energy future scale of the universe 8 Artist’s impression of a black hole surrounded by a nebula of hot gas 1 3 5 7 2 Markers correspond to equal time intervals for a ball rolling down the slope m 1 F 1 F 1 = F 2 = G F 2 r m 2 m 1 × m 2 r 2 5 Newton’s law of universal gravitation 1 A missile trajectory drawn in the fifteenth century 3 A freely moving projectile follows a parabolic path 4 A portrait of Isaac Newton (1642–1727) superimposed on a diagram of orbits calculated using his laws of motion and theory of gravitation 6 A ‘rubber sheet’ model illustrates how masses distort space-time The general theory of relativity (GR), published in 1915 by Albert Einstein (1879–1955), presented a radical new way of thinking about gravity. Any mass distorts space-time so that other masses fall towards it. In a two-dimensional model, it is as if the masses are resting on a stretchy rubber sheet (6). GR describes how gravity affects electromagnetic radiation as well as matter. Radiation travelling away from a massive object undergoes a gravitational redshift (7). The radiation’s wavelength is ‘stretched’ and its photons lose energy, rather like a material object losing kinetic energy. Close to a very massive, compact object, such as a collapsed star, the gravitational field can be so large that no matter or radiation has enough energy to escape; the object is a black hole. Black holes can be detected by their gravitational influence on other objects, and material swirling towards a black hole heats up, emitting radiation (8). In 1998, observations of very distant galaxies showed that, rather than slowing down due to the gravitational attraction between all matter, the expansion of the universe is accelerating (9). The idea of ‘dark energy’ has been proposed to explain the observations. Einstein’s GR equations include a ‘cosmological constant’ that might account for the acceleration, but its value has yet to be determined, and there are difficulties reconciling GR with modern theories of particle physics and quantum mechanics. Maybe we need a new theory of gravitation?
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Download this poster at www.hoddereducation.co.uk/physicsreviewextras
PhysicsReviewExtras
17www.hoddereducation.co.uk/physicsreview16 Physics Review February 2017
at a glance
Ideas about the motion of falling objects date back long before modern theories of gravitation. A common idea of the time was that a projectile rises in a straight line until it runs out of ‘impetus’, at which point it falls to the ground (1); the heavier an object, the more rapidly it would fall.
The Italian scientist Galileo Galilei (1564–1642) designed an experiment in which a ball rolling down a slope hit a series of bells. By adjusting their positions, he made the bells ring at regular intervals (2), showing that the rolling ball had a constant acceleration (sometimes called ‘diluted gravity’). Galileo showed that all free-falling objects have the same acceleration and deduced that a projectile moves with constant horizontal velocity combined with a constant vertical acceleration (3).
Isaac Newton (4) proposed that the forces responsible for an apple falling from a tree, and for keeping a planet in orbit, had the same origin. He argued that any two objects attract one another with a force that is proportional to each of their masses and gets weaker as their separation increases. By studying the Moon’s orbit, he derived the inverse-square law for universal gravitation (5).
Gravitationmassive star
7 Gravitational redshift; photon energy E = hf = hc/λ
9 The rate of universal expansion changesover time
timepresent
deceleration
acceleration
BigBang
Big Rip
Big Crunch
constantdark energy
future
scal
e o
f th
e u
niv
erse
8 Artist’s impression of a black hole surroundedby a nebula of hot gas
13
5
7
2 Markers correspond to equal time intervals for a ball rolling down the slope
m1 F1
F1 = F2 = G
F2
r
m2
m1 × m2
r2
5 Newton’s law of universal gravitation
1 A missile trajectory drawn in the fifteenth century
3 A freely moving projectile follows a parabolic path
4 A portrait of Isaac Newton (1642–1727)superimposed on a diagram of orbits calculated using his laws of motion and theory of gravitation
6 A ‘rubber sheet’ model illustrates how masses distort space-time
The general theory of relativity (GR), published in 1915 by Albert Einstein (1879–1955), presented a radical new way of thinking about gravity. Any mass distorts space-time so that other masses fall towards it. In a two-dimensional model, it is as if the masses are resting on a stretchy rubber sheet (6).
GR describes how gravity affects electromagnetic radiation as well as matter. Radiation travelling away from a massive object undergoes a gravitational redshift (7). The radiation’s wavelength is ‘stretched’ and its photons lose energy, rather like a material object losing kinetic energy. Close to a very massive, compact object, such as a collapsed star, the gravitational field can be so large that no matter or radiation has enough energy to escape; the object is a black hole. Black holes can be detected by their gravitational influence on other objects, and material swirling towards a black hole heats up, emitting radiation (8).
In 1998, observations of very distant galaxies showed that, rather than slowing down due to the gravitational attraction between all matter, the expansion of the universe is accelerating (9). The idea of ‘dark energy’ has been proposed to explain the observations. Einstein’s GR equations include a ‘cosmological constant’ that might account for the acceleration, but its value has yet to be determined, and there are difficulties reconciling GR with modern theories of particle physics and quantum mechanics. Maybe we need a new theory of gravitation?
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