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Page 1 Edurev123 4. Work & Energy 4.1 A body is describing an ellipse of eccentricity e under the action of a central force directed towards a focus and when at the nearer apse the centre of the force is transferred to the other focus. Find the eccentricity of the new orbit in terms of the eccentricity of the original orbit. (2009: 12 Marks) Solution: Approach : The motion in an ellipse in under inverse square force and the velocity determines the eccentricity of the ellipse. So balance the velocity between the two orbits. Let ?? and ?? ' be the foci of the original orbits and the force initially be directed towards ?? . The nearer apse is at ?? . Since the motion is in an ellipse the velocity at any point is given by ?? 2 =?? ( 2 ?? - 1 ?? ) At ?? ,?? =???? =???? -???? =?? -???? =?? (1-?? ) ? ?? 2 =?? ( 2 ?? (1-?? ) - 1 ?? )= ?? (1+?? ) ?? (1-?? ) When the centre of force is changed to ?? ' the velocity remains same. Let ?? ' be the semi major axis of the new orbit. Then: ?? 2 = ?? (1+?? ) ?? (1-?? ) =?? ( 2 ?? - 1 ?? ' ) ?? =?? ?? ' =???? +???? =?? +???? =?? (1+?? ) Page 2 Edurev123 4. Work & Energy 4.1 A body is describing an ellipse of eccentricity e under the action of a central force directed towards a focus and when at the nearer apse the centre of the force is transferred to the other focus. Find the eccentricity of the new orbit in terms of the eccentricity of the original orbit. (2009: 12 Marks) Solution: Approach : The motion in an ellipse in under inverse square force and the velocity determines the eccentricity of the ellipse. So balance the velocity between the two orbits. Let ?? and ?? ' be the foci of the original orbits and the force initially be directed towards ?? . The nearer apse is at ?? . Since the motion is in an ellipse the velocity at any point is given by ?? 2 =?? ( 2 ?? - 1 ?? ) At ?? ,?? =???? =???? -???? =?? -???? =?? (1-?? ) ? ?? 2 =?? ( 2 ?? (1-?? ) - 1 ?? )= ?? (1+?? ) ?? (1-?? ) When the centre of force is changed to ?? ' the velocity remains same. Let ?? ' be the semi major axis of the new orbit. Then: ?? 2 = ?? (1+?? ) ?? (1-?? ) =?? ( 2 ?? - 1 ?? ' ) ?? =?? ?? ' =???? +???? =?? +???? =?? (1+?? ) ? ?? (1+?? ) ?? (1-?? ) =?? ( 2 ?? (1+?? ) - 1 ?? ' ) ? 1+?? 1-?? = 2 1+?? - ?? ?? ' Again the direction of velocity is not changed, the velocity still being perpendicular to the direction of motion the point ?? is an apse for the new orbit as well. ? ?? ' ?? =?? ' (1-?? ' )=?? (1+?? ) ? ?? ?? ' = 1-?? ' 1+?? ? 1+?? 1-?? = 2 1+?? · 1-?? ' 1+?? = 1+?? ' 1+?? ? ?? ' = (1+?? ) 2 1-?? -1= ?? 2 +3?? 1-?? = ?? (?? +3) 1-?? 4.2 A particle moves with a central acceleration ?? (?? ?? -?? ?? ) , being projected from an apse at a distance v?? with velocity ?? v?? ?? . Show that its path is the curve ?? ?? + ?? ?? =?? . (2010 : 20 Marks) Solution: The equation of motion is h 2 (?? + ?? 2 ?? ?? ?? 2 )= ?? ?? 2 where ?? = 1 ?? ?? = central accel =?? ( 1 ?? 5 - 9 ?? ) ? Equation becomes h 2 (?? + ?? 2 ?? ?? ?? 2 ) = ?? ?? 2 ( 1 ?? 5 - 9 ?? ) ? h 2 (?? + ?? 2 ?? ?? ?? 2 ) =?? ( 1 4 7 - 9 4 3 ) Multiplying both sides by 2 ???? ???? and integrating, we get Page 3 Edurev123 4. Work & Energy 4.1 A body is describing an ellipse of eccentricity e under the action of a central force directed towards a focus and when at the nearer apse the centre of the force is transferred to the other focus. Find the eccentricity of the new orbit in terms of the eccentricity of the original orbit. (2009: 12 Marks) Solution: Approach : The motion in an ellipse in under inverse square force and the velocity determines the eccentricity of the ellipse. So balance the velocity between the two orbits. Let ?? and ?? ' be the foci of the original orbits and the force initially be directed towards ?? . The nearer apse is at ?? . Since the motion is in an ellipse the velocity at any point is given by ?? 2 =?? ( 2 ?? - 1 ?? ) At ?? ,?? =???? =???? -???? =?? -???? =?? (1-?? ) ? ?? 2 =?? ( 2 ?? (1-?? ) - 1 ?? )= ?? (1+?? ) ?? (1-?? ) When the centre of force is changed to ?? ' the velocity remains same. Let ?? ' be the semi major axis of the new orbit. Then: ?? 2 = ?? (1+?? ) ?? (1-?? ) =?? ( 2 ?? - 1 ?? ' ) ?? =?? ?? ' =???? +???? =?? +???? =?? (1+?? ) ? ?? (1+?? ) ?? (1-?? ) =?? ( 2 ?? (1+?? ) - 1 ?? ' ) ? 1+?? 1-?? = 2 1+?? - ?? ?? ' Again the direction of velocity is not changed, the velocity still being perpendicular to the direction of motion the point ?? is an apse for the new orbit as well. ? ?? ' ?? =?? ' (1-?? ' )=?? (1+?? ) ? ?? ?? ' = 1-?? ' 1+?? ? 1+?? 1-?? = 2 1+?? · 1-?? ' 1+?? = 1+?? ' 1+?? ? ?? ' = (1+?? ) 2 1-?? -1= ?? 2 +3?? 1-?? = ?? (?? +3) 1-?? 4.2 A particle moves with a central acceleration ?? (?? ?? -?? ?? ) , being projected from an apse at a distance v?? with velocity ?? v?? ?? . Show that its path is the curve ?? ?? + ?? ?? =?? . (2010 : 20 Marks) Solution: The equation of motion is h 2 (?? + ?? 2 ?? ?? ?? 2 )= ?? ?? 2 where ?? = 1 ?? ?? = central accel =?? ( 1 ?? 5 - 9 ?? ) ? Equation becomes h 2 (?? + ?? 2 ?? ?? ?? 2 ) = ?? ?? 2 ( 1 ?? 5 - 9 ?? ) ? h 2 (?? + ?? 2 ?? ?? ?? 2 ) =?? ( 1 4 7 - 9 4 3 ) Multiplying both sides by 2 ???? ???? and integrating, we get h 2 (?? 2 +( ???? ???? ) 2 )=?? ( -2 6?? 6 + 9×2 2?? 2 )+?? ? h 2 (?? 2 +( ???? ???? ) 2 )=?? ( 9 ?? 2 - 1 3?? 6 )+?? =?? 2 ? ?? (9×3- 1 3 ×27)+?? =18?? ? ?? +18?? =18?? ??? =0 ? h 2 (?? 2 +( ???? ???? ) 2 )=?? ( ?? ?? 2 - 1 3?? 6 ) ???????? ,???? ???????? , ( ???? ???? )=0 h 2 ( 1 3 )=?? (9×3- 1 3 ×27)=18?? h=54?? ? Equation becomes 54(?? 2 +( ???? ???? ) 2 )=?? ( 9 4 2 - 1 3?? 6 ) ? ?? 2 +( ???? ???? ) 2 = 1 5?? ( 9 ?? 2 - 1 3?? 6 ) ? ( ???? ???? ) 2 = 27?? 4 -1-162?? 8 162?? 6 ? ???? ???? = v 27 162 ?? 4 - 1 162 -?? 8 ? = ?? 3 ???? v ?? 4 6 - 1 162 -?? 8 ???? integrating both sides, we get ? 36?? 3 ???? v1-(36?? 4 -3) 2 =? ???? ? sin -1 (36?? 4 -3) 4 =?? +?? Now, at ?? = 1 ?? = 1 v3 ,?? =0 (apse) Page 4 Edurev123 4. Work & Energy 4.1 A body is describing an ellipse of eccentricity e under the action of a central force directed towards a focus and when at the nearer apse the centre of the force is transferred to the other focus. Find the eccentricity of the new orbit in terms of the eccentricity of the original orbit. (2009: 12 Marks) Solution: Approach : The motion in an ellipse in under inverse square force and the velocity determines the eccentricity of the ellipse. So balance the velocity between the two orbits. Let ?? and ?? ' be the foci of the original orbits and the force initially be directed towards ?? . The nearer apse is at ?? . Since the motion is in an ellipse the velocity at any point is given by ?? 2 =?? ( 2 ?? - 1 ?? ) At ?? ,?? =???? =???? -???? =?? -???? =?? (1-?? ) ? ?? 2 =?? ( 2 ?? (1-?? ) - 1 ?? )= ?? (1+?? ) ?? (1-?? ) When the centre of force is changed to ?? ' the velocity remains same. Let ?? ' be the semi major axis of the new orbit. Then: ?? 2 = ?? (1+?? ) ?? (1-?? ) =?? ( 2 ?? - 1 ?? ' ) ?? =?? ?? ' =???? +???? =?? +???? =?? (1+?? ) ? ?? (1+?? ) ?? (1-?? ) =?? ( 2 ?? (1+?? ) - 1 ?? ' ) ? 1+?? 1-?? = 2 1+?? - ?? ?? ' Again the direction of velocity is not changed, the velocity still being perpendicular to the direction of motion the point ?? is an apse for the new orbit as well. ? ?? ' ?? =?? ' (1-?? ' )=?? (1+?? ) ? ?? ?? ' = 1-?? ' 1+?? ? 1+?? 1-?? = 2 1+?? · 1-?? ' 1+?? = 1+?? ' 1+?? ? ?? ' = (1+?? ) 2 1-?? -1= ?? 2 +3?? 1-?? = ?? (?? +3) 1-?? 4.2 A particle moves with a central acceleration ?? (?? ?? -?? ?? ) , being projected from an apse at a distance v?? with velocity ?? v?? ?? . Show that its path is the curve ?? ?? + ?? ?? =?? . (2010 : 20 Marks) Solution: The equation of motion is h 2 (?? + ?? 2 ?? ?? ?? 2 )= ?? ?? 2 where ?? = 1 ?? ?? = central accel =?? ( 1 ?? 5 - 9 ?? ) ? Equation becomes h 2 (?? + ?? 2 ?? ?? ?? 2 ) = ?? ?? 2 ( 1 ?? 5 - 9 ?? ) ? h 2 (?? + ?? 2 ?? ?? ?? 2 ) =?? ( 1 4 7 - 9 4 3 ) Multiplying both sides by 2 ???? ???? and integrating, we get h 2 (?? 2 +( ???? ???? ) 2 )=?? ( -2 6?? 6 + 9×2 2?? 2 )+?? ? h 2 (?? 2 +( ???? ???? ) 2 )=?? ( 9 ?? 2 - 1 3?? 6 )+?? =?? 2 ? ?? (9×3- 1 3 ×27)+?? =18?? ? ?? +18?? =18?? ??? =0 ? h 2 (?? 2 +( ???? ???? ) 2 )=?? ( ?? ?? 2 - 1 3?? 6 ) ???????? ,???? ???????? , ( ???? ???? )=0 h 2 ( 1 3 )=?? (9×3- 1 3 ×27)=18?? h=54?? ? Equation becomes 54(?? 2 +( ???? ???? ) 2 )=?? ( 9 4 2 - 1 3?? 6 ) ? ?? 2 +( ???? ???? ) 2 = 1 5?? ( 9 ?? 2 - 1 3?? 6 ) ? ( ???? ???? ) 2 = 27?? 4 -1-162?? 8 162?? 6 ? ???? ???? = v 27 162 ?? 4 - 1 162 -?? 8 ? = ?? 3 ???? v ?? 4 6 - 1 162 -?? 8 ???? integrating both sides, we get ? 36?? 3 ???? v1-(36?? 4 -3) 2 =? ???? ? sin -1 (36?? 4 -3) 4 =?? +?? Now, at ?? = 1 ?? = 1 v3 ,?? =0 (apse) ? sin -1 ( 36 9 -3) 4 =0+?? ? sin -1 (1) 4 =?? ??? = ?? 8 ? sin -1 (36?? 4 -3) 4 =?? + ?? 8 ? sin -1 (36?? 4 -3)=4?? + ?? 2 ? 36?? 4 -3=sin ( ?? 2 +4?? )=cos 4?? ? (cos 2 2?? -sin 2 2?? )=36?? 4 -3 ? 2cos 2 2?? =36?? 4 -2 ? cos 2 2?? =18?? 4 -1 ? (cos 2 ?? -sin 2 ?? ) 2 +(cos 2 ?? +sin 2 ?? ) 2 =18?? 4 ? cos 4 ?? +sin 4 ?? -2cos 2 ?? sin 2 ?? +cos 4 ?? +sin 4 ?? +2cos 2 ?? sin 2 ?? =18?? 4 ? 2cos 4 ?? +2sin 4 ?? =18?? 4 ? cos 4 ?? +sin 4 ?? =9?? 4 = 9 ?? 4 ? ?? 4 cos 4 ?? +?? 4 sin 4 ?? =9 ? ?? 4 +?? 4 =9 ? Path is the curve ?? 4 +?? 4 =9. 4.3 A particle moves in a plane under a force, towards a fixed centre, proportional to the distance. If the path of the particle has two apsidal distance ?? ,?? (?? >?? ) , then find the tyuation of the path. (2015 : 15 Marks) Solution: Page 5 Edurev123 4. Work & Energy 4.1 A body is describing an ellipse of eccentricity e under the action of a central force directed towards a focus and when at the nearer apse the centre of the force is transferred to the other focus. Find the eccentricity of the new orbit in terms of the eccentricity of the original orbit. (2009: 12 Marks) Solution: Approach : The motion in an ellipse in under inverse square force and the velocity determines the eccentricity of the ellipse. So balance the velocity between the two orbits. Let ?? and ?? ' be the foci of the original orbits and the force initially be directed towards ?? . The nearer apse is at ?? . Since the motion is in an ellipse the velocity at any point is given by ?? 2 =?? ( 2 ?? - 1 ?? ) At ?? ,?? =???? =???? -???? =?? -???? =?? (1-?? ) ? ?? 2 =?? ( 2 ?? (1-?? ) - 1 ?? )= ?? (1+?? ) ?? (1-?? ) When the centre of force is changed to ?? ' the velocity remains same. Let ?? ' be the semi major axis of the new orbit. Then: ?? 2 = ?? (1+?? ) ?? (1-?? ) =?? ( 2 ?? - 1 ?? ' ) ?? =?? ?? ' =???? +???? =?? +???? =?? (1+?? ) ? ?? (1+?? ) ?? (1-?? ) =?? ( 2 ?? (1+?? ) - 1 ?? ' ) ? 1+?? 1-?? = 2 1+?? - ?? ?? ' Again the direction of velocity is not changed, the velocity still being perpendicular to the direction of motion the point ?? is an apse for the new orbit as well. ? ?? ' ?? =?? ' (1-?? ' )=?? (1+?? ) ? ?? ?? ' = 1-?? ' 1+?? ? 1+?? 1-?? = 2 1+?? · 1-?? ' 1+?? = 1+?? ' 1+?? ? ?? ' = (1+?? ) 2 1-?? -1= ?? 2 +3?? 1-?? = ?? (?? +3) 1-?? 4.2 A particle moves with a central acceleration ?? (?? ?? -?? ?? ) , being projected from an apse at a distance v?? with velocity ?? v?? ?? . Show that its path is the curve ?? ?? + ?? ?? =?? . (2010 : 20 Marks) Solution: The equation of motion is h 2 (?? + ?? 2 ?? ?? ?? 2 )= ?? ?? 2 where ?? = 1 ?? ?? = central accel =?? ( 1 ?? 5 - 9 ?? ) ? Equation becomes h 2 (?? + ?? 2 ?? ?? ?? 2 ) = ?? ?? 2 ( 1 ?? 5 - 9 ?? ) ? h 2 (?? + ?? 2 ?? ?? ?? 2 ) =?? ( 1 4 7 - 9 4 3 ) Multiplying both sides by 2 ???? ???? and integrating, we get h 2 (?? 2 +( ???? ???? ) 2 )=?? ( -2 6?? 6 + 9×2 2?? 2 )+?? ? h 2 (?? 2 +( ???? ???? ) 2 )=?? ( 9 ?? 2 - 1 3?? 6 )+?? =?? 2 ? ?? (9×3- 1 3 ×27)+?? =18?? ? ?? +18?? =18?? ??? =0 ? h 2 (?? 2 +( ???? ???? ) 2 )=?? ( ?? ?? 2 - 1 3?? 6 ) ???????? ,???? ???????? , ( ???? ???? )=0 h 2 ( 1 3 )=?? (9×3- 1 3 ×27)=18?? h=54?? ? Equation becomes 54(?? 2 +( ???? ???? ) 2 )=?? ( 9 4 2 - 1 3?? 6 ) ? ?? 2 +( ???? ???? ) 2 = 1 5?? ( 9 ?? 2 - 1 3?? 6 ) ? ( ???? ???? ) 2 = 27?? 4 -1-162?? 8 162?? 6 ? ???? ???? = v 27 162 ?? 4 - 1 162 -?? 8 ? = ?? 3 ???? v ?? 4 6 - 1 162 -?? 8 ???? integrating both sides, we get ? 36?? 3 ???? v1-(36?? 4 -3) 2 =? ???? ? sin -1 (36?? 4 -3) 4 =?? +?? Now, at ?? = 1 ?? = 1 v3 ,?? =0 (apse) ? sin -1 ( 36 9 -3) 4 =0+?? ? sin -1 (1) 4 =?? ??? = ?? 8 ? sin -1 (36?? 4 -3) 4 =?? + ?? 8 ? sin -1 (36?? 4 -3)=4?? + ?? 2 ? 36?? 4 -3=sin ( ?? 2 +4?? )=cos 4?? ? (cos 2 2?? -sin 2 2?? )=36?? 4 -3 ? 2cos 2 2?? =36?? 4 -2 ? cos 2 2?? =18?? 4 -1 ? (cos 2 ?? -sin 2 ?? ) 2 +(cos 2 ?? +sin 2 ?? ) 2 =18?? 4 ? cos 4 ?? +sin 4 ?? -2cos 2 ?? sin 2 ?? +cos 4 ?? +sin 4 ?? +2cos 2 ?? sin 2 ?? =18?? 4 ? 2cos 4 ?? +2sin 4 ?? =18?? 4 ? cos 4 ?? +sin 4 ?? =9?? 4 = 9 ?? 4 ? ?? 4 cos 4 ?? +?? 4 sin 4 ?? =9 ? ?? 4 +?? 4 =9 ? Path is the curve ?? 4 +?? 4 =9. 4.3 A particle moves in a plane under a force, towards a fixed centre, proportional to the distance. If the path of the particle has two apsidal distance ?? ,?? (?? >?? ) , then find the tyuation of the path. (2015 : 15 Marks) Solution: ?? =?? 2 h 2 [?? + ?? 2 ?? ?? ?? 2 ] ? - ?? 1 =?? 2 h 2 (?? + ?? 2 ?? ?? ?? 2 )?- ?? ?? 3 =h 2 (?? + ?? 2 ?? ?? ?? 2 ) ? - ?? ?? 3 · 2?? ???? =h 2 [?? ·2 ???? ???? + ?? 2 ?? ?? ?? 2 · 2???? ???? ] ? ?? + ?? ?? 2 =h 2 [?? 2 +( ???? ???? ) 2 ] or ?? 2 =h 2 [?? 2 +( ???? ???? ) 2 ]= ?? ?? 2 +?? At ?? = 1 ?? , ???? ?? 6 =0??? +?? ?? 2 =h[ 1 ?? 2 ] ? ?? +??ˆ ?? 2 = h 2 ?? 2 Similarly, ?? +?? ?? 2 = h 2 ?? 2 ? ?? (?? 2 -?? 2 )=h 2 ( 1 ?? 2 - 1 ?? 2 )? ?? h 2 = -1 ?? 2 ?? 2 ? ?? +?? ?? 2 = -?? ?? 2 ?? 2 ?? 2 ??? =-?? (?? 2 +?? 2 ) Thus, -?? ?? 2 ?? 2 [?? 2 +( ???? ???? ) 2 ]=-?? (?? 2 +?? 2 )+ ?? ?? 2 ? ?? 2 ( ???? ???? ) 2 = (?? 2 +?? 2 - 1 ?? 2 ) ?? 2 ?? 2 ? ( ???? ???? ) 2 = ?? 2 +?? 2 ?? 2 ?? 2 - 1 ?? 2 ?? 2 ?? 2 -?? 2 = 1 ?? 2 [ ?? 2 +?? 2 ?? 2 ?? 2 ?? 2 - 1 ?? 2 ?? 2 -?? 4 ] = 1 ?? 2 [-( 1 2 ?? 2 +?? 2 ?? 2 ?? 2 -?? 2 ) 2 + 1 4 ( ?? 2 +?? 2 ?? 2 ?? 2 ) 2 - 1 ?? 2 ?? 2 ] ?????? ?? ?? 2 = 1 4 ( ?? 2 +?? 2 ?? 2 ?? 2 ) 2 - 1 ?? 2 ?? 2 = 1 4(?? 2 ?? 2 ) 2 [(?? 2 +?? 2 ) 2 -4?? 2 ?? 2 ]Read More
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