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A-level Physics Unit G484: The Newtonian World Newton’s law of gravitationCircular motion
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Gravitation – lesson 1 recall LOs To do/answer 1. Complete the following: ‘a field is a region of space in which a particular type of object experiences a … ‘ 2. Name the type of object needed to detect the following: a. a magnetic field b. an electric field c. a gravitational field 3. Sketch a diagram to show the gravitational field in this lab.. Be ready to explain what you are attempting to show. 4. Sketch a diagram to show the Earth’s gravitational field. How do we describe this field? Where do the field lines i) come from, ii) go to? What have you assumed about the Earth? 5. Define ‘gravitational field strength’.Circular motion
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Lesson focus • Newton’s law of universal gravitation Learning objectives At the end of the lesson you will be able to: • state Newton’s law of gravitation; • select and use the equation for centripetal force F = -GMm/r2 for the force between two point or spherical objects.Circular motion
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Learning outcomes All of you should be able to • state Newton’s law of gravitation in words; • state this law as an equation and define the symbols used; • use this equation to solve simple problems. Most of you will be able to • use Newton’s law of gravitation to solve more complex problems.Circular motion
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The gravitational force between objects LOs Imagine two masses m1 m2 We know that masses give rise to gravitational fields. Each mass will, therefore, be in the gravitational field of the other. 1. Sketch the situation and show the forces on each mass. 2. List the factors that determine the size of the forces on the masses. Isaac Newton and gravity: videoCircular motion
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Newton’s law of universal gravitation LOs This law states that the ‘gravitational force between two point masses is proportional to the product of the two masses and inversely proportional to the square of the distance between them’. G m1 m 2 F = - r2 G - universal gravitational Notes: constant (‘big G’) • a point mass has no spatial extent • the minus sign in the equation shows that the force is attractiveCircular motion
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Newton’s law of universal gravitation LOs G m1 m2 where, F = - G - universal gravitational r2 constant (‘big G’)2/3 Newton conceived of this force extending (instantaneously) throughout the entire Universe (hence universal). He did not attempt to explain the origin of the force.Circular motion
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Newton’s law of universal gravitation LOs To do Using the data given find a value for G. Data: G m 1 m2 • your mass F = - • the mass of the Earth (5.97 x 1024 kg) r2 • radius of the Earth (6.37 x 103 km)3/3 G is one of the least well defined constants of nature (hw: why is this?). The accepted value is 6.674×10−11 N m2 kg−2 . To do Calculate the percentage difference between this and your value. Circular motion
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