Topic 9 - Motion in fields

Question 1

Are the vertical and the horizontal components of velocity for a projectile in a uniform field independent?

Answer 1

Yes

Question 2

Describe the trajectory of projectile motion in the absence of air resistance

Answer 2

It’s parabolic.

Question 3

Sketch the trajectory of projectile motion in the absence of air resistance

Answer 3

Question 4

Describe the effect of air resistance on the trajectory of a projectile

Answer 4

• Air resistance provides a force which opposes motion
• Acceleration will be reduced for falling objects and increased for objects moving upwards
• The trajectory will no longer be parabolic or symmetric

Question 5

Define the instantaneous velocity of a projectile

Answer 5

Question 6

Define gravitational potential at a point

Answer 6

The work done per unit mass in bringing a small test mass from infinity to that position

Question 7

Define gravitational potential energy of a system

Answer 7

The work done when bringing all the masses of the system to their present positions, assuming that they were originally at infinity.

Gravitational potential energy stored between to masses m₁ and m₂, separated by a distance r:

Question 8

When is gravitational potential energy equal to zero?

Answer 8

When an object is at infinity.

Question 9

Define gravitational potential due to a point mass

Answer 9

Question 10

Define the gravitational potential difference between two points as the work done when moving a unit mass between those points

Answer 10

Question 11

How do you get the potential due to one or more point masses?

Answer 11

By adding up the potentials

Question 12

Is gravitational potential a vector or a scalar quantity?

Answer 12

Scalar.

Question 13

Sketch and describe the pattern of equipotential surfaces due to one point mass

Answer 13

• Circular lines around the point mass
• The lines get further apart as the distance increases

Question 14

Sketch and describe the pattern of equipotential surfaces due to two point masses

Answer 14

• Just like contour lines

Question 15

What is the relationship between equipotential surfaces and gravitational field lines?

Answer 15

Field lines are always perpendicular to equipotential lines, pointing from a higher potential to lower potential.

Question 16

Define escape speed, v_esc

Answer 16

A theoretical speed that a projectile of mass m₂ needs to escape from a planet’s (m₁) force of attraction

Question 17

Define electric potential (at a point)

Answer 17

The work done per unit charge in bringing a small positive test charge from infinity to that position:

Question 18

Define electric potential energy

Answer 18

The work done when bringing all the charges of the system to their present positions, assuming that they were originally at infinity.

Question 19

Define the electric potential due to a point charge

Answer 19

Question 20

How is the overall electric potential at a point determined?

Answer 20

By adding up individual potentials.

Question 21

What is the relationship between equipotential surfaces and electric field lines?

Answer 21

Field lines are always perpendicular to equipotential lines and point from higher to lower potential

Question 22

Sketch the pattern of equipotential surfaces due to one point charge

Answer 22

Question 23

Sketch the pattern of equipotential surfaces due to two equal point charges

Answer 23

Question 24

Sketch the pattern of equipotential surfaces due to two opposite point charges

Answer 24

Question 25

What is a *dipole*?

Answer 25

A pair of equal and opposite charges separated by a small distance

Question 26

What provides the centripetal force for circular orbital motion?

Answer 26

Gravitation

Question 27

State and derive Kepler's third law

Answer 27

For all the planets orbiting the sun, the average radius cubed is proportional to the period squared

Question 28

Derive and state the expressions for kinetic energy, potential energy, and total energy of an orbiting satellite

Answer 28

Question 29

Sketch a graph showing the variation with orbital radius of the kinetic, gravitational potential, and total energy of a satellite

Answer 29

Question 30

When is weightlessness possible?

Answer 30

- When a body is in deep space where there is no significant graviational field - When a body is at a point where the gravitational forces of two or more fields cancel each other out - During free fall