PS1 Solutions

PS1 Solutions - Physics 217: Problem Set 1 Solutions by...

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Unformatted text preview: Physics 217: Problem Set 1 Solutions by Jolyon Bloomfield Due Friday, Jan 23rd, 2009 1 Purcell 1.1 The attractive gravitational force is FG = Gm2 p r2 and the repulsive electrostatic force is FE = The ratio of these two is e2 . 4π 0 r2 4π 0 Gm2 FG p = 7.4 × 10−37 . = FE e2 The electrostatic repulsion, using r = 10−13 cm, is 23 N. 2 Purcell 1.2 Balancing the Coulomb and gravitational forces, if e2 /4π 0 r2 = mg , then r= e2 . 4π 0 mg Substituting in numbers, we find r = 4.80 m. 3 Purcell 1.3 Look at a single volleyball. There are three forces acting on it - gravitational, electrostatic, and tension. From the Coulomb force law, we can rearrange to obtain q=r 4π 0 F where F is the force. To find out what the force should be, we need to calculate what the horizontal tension force is which opposes the electrostatic repulsion. Defining θ to be the angle between the horizontal and the string from a volleyball, we find the horizontal component of the force is T cos θ, and the vertical component is T sin θ. As the vertical component has to balance with gravity, we find M g = T sin θ. 1 This solves for T . The horizontal component of the force from the tension is then F = Mg cos θ Mg = . sin θ tan θ We have tan θ = 10 from the geometry. Substituting this in to find the charge, we have q=r 4π 0 Inserting numbers obtains q = 2.86 × 10−6 C. 2 × 0.1M g. ...
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This note was uploaded on 06/10/2011 for the course PHYS 2217 taught by Professor Leclair, a during the Spring '06 term at Cornell.

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PS1 Solutions - Physics 217: Problem Set 1 Solutions by...

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