Topic 9: Motion in Fields Flashcards
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 work done in moving a test mass from infinity to a point in a gravitational field. Zero GPE is defined as the energy an infinite distance away; GPE is a negative quantity since the force is always attractive.
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 work done 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 and the force is attractive, all gravitational potential is a negative quantity. Measured in Jkg-1.
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.
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.
Electric potential energy
The work done in moving a small positive test charge from infinity to a point 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 work done 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).
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.
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.
