Gravitational Potential and Newton's Law

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Questions and Answers

Explain why gravitational potential is always a negative value.

Gravitational potential is zero at infinity, and work is done by the gravitational force when moving masses closer together or work is done on the mass to move it to infinity.

With reference to gravitational field lines, explain why the gravitational field near the Earth's surface is approximately constant for small changes in height.

The change in height is negligible compared to the radius of the Earth, so the field lines are effectively parallel.

State one similarity and one difference between gravitational potential due to a point mass and electric potential due to a point charge.

Similarity: Inversely proportional to distance from point, points of equal potential lie on concentric spheres, zero at infinite distance. Difference: Gravitational potential is always negative, electric potential can be positive or negative.

Explain how the force(s) on a satellite can result in the satellite being in a circular orbit around a planet.

Gravitational force provides the centripetal force or causes centripetal acceleration. Signup and view all the answers

State two other conditions that must be met for the orbit to be geostationary.

Orbit must be above the Equator and the direction must be from west to east. Signup and view all the answers

Explain why there is a point X on the line between the centers of the Earth and the Moon where the resultant gravitational field strength due to the Earth and the Moon is zero.

Fields due to Earth and the Moon have equal magnitudes and the fields due to earth and the moon are in opposite directions. Signup and view all the answers

Explain how the electric force between the electron and the positron causes the path of the moving particles to be circular.

Electric force is perpendicular to the velocity of particles or force perpendicular to velocity causes centripetal acceleration or does not change the speed of the particles or force has constant magnitude. Signup and view all the answers

Explain why the gravitational potential energy of the rock is negative.

Potential energy is zero at infinity, gravitational forces are attractive, and work must be done on the rock to move it to infinity. Signup and view all the answers

Explain, if the car remains in contact with the track, why the centripetal acceleration of the car at point Y must be greater than 9.8 $ms^{-2}$.

9.8 $ms^{-2}$ is caused by weight of car or centripetal force must be greater than weight of car, requires contact foce from track. Signup and view all the answers

Explain why $\phi$ is negative throughout the entire range x = 2R to x = D – R.

Potential is zero at infinity, and gravitational forces are attractive or test mass getting closer from infinity loses potential energy. Signup and view all the answers

Explain how it is possible for the gravitational field strength to be zero despite the presence of two large masses nearby.

Gravitational fields from the Earth and Moon are in opposite directions and the resultant is zero where gravitational fields are equal in magnitude. Signup and view all the answers

Rockets on the satellite are fired so that the satellite enters a different circular orbit that has a period of 150 minutes. The change in the mass of the satellite may be assumed to be negligible. State, with a reason, whether the gravitational potential energy of the satellite increases or decreases.

Separation increases so (potential energy) increases or movement is against gravitational force so (potential energy) increases. Signup and view all the answers

The satellite is travelling from west to east or from east to west. Explain why this affects the launch energy.

Smaller gain in energy required if orbit is west to east or smaller change in velocity if orbit is west to east or satellite already moving west to east at launch or earth's rotation is from west to east. Signup and view all the answers

A second identical solid metal sphere B is now placed near sphere A. Sphere B has -Q charge. Explain why.

Fields due to each sphere have equal magnitude and point in opposite direction. Signup and view all the answers

State the reason why two objects that are at the same temperature are described as being in thermal equilibrium.

No net thermal energy is transferred between them. Signup and view all the answers

State two of the basic assumptions of the kinetic theory of gases.

Particles are in (continuous) random motion, particles have negligible volume (compared with the gas) and negligible forces between particles (except during collisions), collisions are perfectly elastic, time of collision negligible (in comparison with time between collisions). Signup and view all the answers

By referring to both kinetic energy and potential energy, explain what is meant by the internal energy of an ideal gas.

Total kinetic energy associated with random motion of molecules plus total potential energy of molecules, but potential energy is zero. Signup and view all the answers

Why is the model of an ideal gas unlikely to be used for a star?

Molecules are very close together, and forces are now non-negligible. Signup and view all the answers

Use the first law of thermodynamics to explain what happens to the internal energy of a spring when it is stretched at constant temperature within its elastic limit

No thermal energy transferred, work is done on the spring by increasing the potential energy of the particles, so internal energy increases Signup and view all the answers

Explain what happens to the internal energy of a sample of water when it evaporates from a rain puddle on a hot day using the first law of thermodynamics.

Thermal energy transferred to water, work is done by water expanding against atmosphere as it vaporizes, more thermal energy transferred than work done, so internal energy increases Signup and view all the answers

Use the first law of thermodynamics to describe and explain the energy transfers associated with one complete cycle ABCDA.

More work done by gas in CD than is done on gas in AB or no work done on gas in BC and DA so gas does work. The overall input of thermal energy is equal to the overall change in internal energy that is zero. Signup and view all the answers

Use the first law of thermodynamics to explain why, if work is done on a gas, the temperature of the helium gas increases.

Work is done on the gas and internal energy increases, so the temperature also increases. Signup and view all the answers

The first law of thermodynamics may be expressed as $\Delta U = q + W. $ What is meant by q?

Thermal energy supplied to the system. Signup and view all the answers

Using a simple kinetic model of matter, describe the structure of a solid.

Particles, atoms/molecules/ions very close together or touching, vibrating in a lattice, held together in a regular pattern. Signup and view all the answers

The specific latent heat of vaporization is much greater than the specific latent heat of fusion for the same substance. Explain this, in terms of the spacing of molecules.

Greater increase in spacing of molecules for vaporization compared with fusion. Signup and view all the answers

Explain, with reference to the first law of thermodynamics, why the increase in internal energy of the gas is Q.

Internal energy increases the gas + work done, constant volume so no work done. Signup and view all the answers

By reference to intermolecular forces, explain why the change in internal energy of an ideal gas is equal to the change in total kinetic energy of its molecules.

In an ideal gas no intermolecular forces, no potential energy (so change in kinetic energy is change in internal energy). Signup and view all the answers

State and explain the effect, if any, in the internal energy of a solid metal ball as it falls under gravity in a vacuum.

Potential and kinetic energies don't actually change (cancel out). There is no work or resistive force, so little change in internal energy. Signup and view all the answers

Use kinetic theory to explain why, when the piston is moved so that the gas expands, this causes a decrease in the temperature of the gas.

Speed of molecule decreases on impact with moving piston. Mean square speed (directly) proportional to temperature or kinetic energy. Signup and view all the answers

State what is meant by resonance.

Oscillation of object at maximum amplitude when driving frequency = natural frequency of system. Signup and view all the answers

State what is meant by damping.

Loss of (total) energy of system due to resistive forces. Signup and view all the answers

Explain why, when the sphere remains in the horizontal plane, the path of the moving particles are circular.

Magnetic force on the sphere is perpendicular to its velocity, which is the centripetal force, therefore does not change the speed of the molecules. Signup and view all the answers

The block in is now released so that it oscillates vertically. Explain why the oscillations of the block are simple harmonic.

F= -Ag$\rho$g, A, g$,\rho$ are constant. F is propotional to x, negative sign. Signup and view all the answers

At any given frequency, the amplitude of oscillation of the trolley is constant. Explain how this indicates that there are resistive forces opposing the motion of the trolley.

Oscillator supplies energy continuously. The energy of trolley constant also the energy needs dissipation. Signup and view all the answers

Explain how this indicates that there are resistive forces opposing the motion of the trolley

Either current causes thermal energy dissipated in resistor, or thermal energy or comes from energy of magnet with constant magnitude. Signup and view all the answers

Explain why there is an induced e.m.f. across wire Q during the discharge of the capacitor.

Give rise to magnetic field from which cutting magnetic flux causes emfs. Signup and view all the answers

State two conditions that must be satisfied for a copper wire, placed in a magnetic field, to experience a magnetic force.

Here must be a current and also non-zero energy. Signup and view all the answers

Explain why two long parallel current-carrying wires exert a magnetic force on each other.

Wire creates field around others, sits within field, is perpendicular Signup and view all the answers

A Hall probe is placed in What does it mean by “magnetic”, and explain why they are parallel in the lines

Either plane is magnetic, in the field or on the plane. Signup and view all the answers

State an explain on why a a coil undergoes dissipation.

Current or magnetic. Leads to magnetic field between coil. Signup and view all the answers

Flashcards

Gravitational Potential

Work done per unit mass to move a mass from infinity to that point.

Radian

The angle subtended at the center of a circle when the arc length equals the radius.