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DULWICH COLLEGE SHANGHAI IB MOTION IN FIELDS DEFINITIONS AND CONCEPTS COMPONENTS OF PROJECTILE MOTION: The horizontal component has constant velocity. The vertical component has a constant acceleration. They are independent and together create a parabolic trajectory. AIR RESISTANCE: A force which opposes motion and is proportional to the velocity. This leads to terminal velocity. GRAVITATIONAL POTENTIAL ENERGY: The energy a body has because of its position in a gravitational field. Zero GPE is defined as the energy an infinite distance away; GPE is a negative quantity. CONSERVATIVE FIELD: The energy needed to bring an object to a point in a field is the same whichever path the object follows. This follows from the principle of conservation of energy. Gravitational and electric fields are conservative. GRAVITATIONAL POTENTIAL: The energy needed per kilogram to bring a test mass from infinity to a point in a gravitational field. Since zero potential is defined at an infinite distance, all gravitational potential is a negative quantity. Measured in Jkg-1. DEFINING EQUATION OF GRAVITATIONAL POTENTIAL: m is a test mass. EQUATION FOR GRAVITATIONAL POTENTIAL DUE TO A MASS, m (eg a planet) POTENTIAL GRADIENT: The change in potential per metre. Gravitational field strength is the negative of potential gradient. r and g are in opposite directions, hence the minus sign. Measured in Vm-1. EQUATION FOR RELATION BETWEEN g AND GRAVITATIONAL POTENTIAL GRADIENT EQUIPOTENTIAL: Lines or surfaces which join points at the same potential. They are always perpendicular to the gravitational field lines. ESCAPE SPEED: The minimum speed of a small mass to escape from the gravitational field of a planet to infinity. DULWICH COLLEGE SHANGHAI ELECTRIC POTENTIAL ENERGY: The energy a charge has due to its position in an electric field. This can be positive or negative; potential around a positive charge is positive since work is done to bring a positive test charge nearer. Unit: joule. ELECTRIC POTENTIAL: The energy per coulomb needed to bring a positive test charge from infinity to a point in an electric field. Zero potential is at infinity . Unit: JC-1 or volt (V). DEFINING EQUATION OF ELECTRIC POTENTIAL: q is a test charge. Quic kTime™ and a decom pres sor are needed to s ee this pic ture. EQUATION FOR POTENTIAL DUE TO A CHARGE q. QuickTime™ and a decompressor are needed to see this picture. QuickTime™ and a decompressor are needed to see this picture. ELECTRIC POTENTIAL GRADIENT: The change in potential per metre. Electric field strength is the negative of potential gradient. r and E are in opposite directions, hence the minus sign. Measured in Vm-1. ELECTRIC EQUIPOTENTIAL: Lines or surfaces joining points in an electric field which are at the same potential. They are perpendicular to the electric field lines. EQUATION FOR RELATION BETWEEN g AND GRAVITATIONAL POTENTIAL GRADIENT CENTRIPETAL FORCE: Orbital motion requires an accelerating force. This is provided by the gravitational attraction of the masses. KEPLER’S THIRD LAW: The square of the time of revolution of a planet about the Sun is directly proportional to the cube of the mean radius of the planet’s orbit. WEIGHTLESSNESS IN SPACE: True weightlessness would only occur an infinite distance from the Sun (and other stars). A body in freefall (eg in orbit) experiences no reaction force from another object (such as a spacecraft) which is also falling and so the body feels weightless.
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