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9th grade · Science · Unit 28 of 28

Forces at a Distance & Heat Flow

How gravity and electric forces change with distance, and how heat evens out.

What your child practices

Practices reasoning with Newton's law of gravitation and Coulomb's law (how forces change when masses, charges or distances change), and how thermal energy flows from hotter to colder objects until they reach the same temperature, including simple mixing calculations.

New York State Learning Standards learning standard: HS-PS2-4; HS-PS3-4 · how gravitational and electric forces depend on mass, charge and distance, and how thermal energy flows until objects reach the same temperature

How we explain it

Children can open this short lesson before they practice.

  1. Gravity and electric force grow when masses or charges grow: double one, double the force.
  2. Both weaken quickly with distance: double the distance, a quarter of the force.
  3. Thermal energy flows from hotter to colder objects until they reach the same temperature.

Example: One of two charges triples. What happens to the electric force between them? Force is proportional to each charge. Triple the charge, triple the force. Answer: it triples

Forces at a Distance & Heat Flow in New York

In the New York State Learning Standards, this is unit 28 of 28 in 9th grade Science, after Nuclear Energy & Bond Energy.

Gradelings describes a child's practice on this unit as Beginning, Approaching, Meeting or Exceeding (Four practice steps). That reflects practice, not a report-card mark: the teacher decides the mark.

Source: New York State Education Department (NYSED)

Sample questions

A taste of this unit. In the app, questions change every time and get easier or harder to match your child.

  1. Question 1

    Two like charges…

    • attract each other
    • repel each other
    • have no force between them
    • join together

  2. Question 2

    If the distance between two masses doubles, the gravitational force becomes…

    • 1/2 as large
    • 2 times as large
    • 1/4 as large
    • 4 times as large

  3. Question 3

    If the distance between two charges doubles, the electric force becomes…

    • 4 times as large
    • 2 times as large
    • 1/2 as large
    • 1/4 as large

  4. Question 4

    If one of two masses doubles, the gravitational force between them…

    • becomes 4 times as large
    • doubles
    • halves
    • stays the same

  5. Question 5

    Specific heat is the energy needed to…

    • move an object 1 m
    • melt any object
    • raise the temperature of 1 g of a substance by 1°C
    • boil water instantly

  6. Question 6

    Why is gravity, not the electric force, what holds planets in orbit?

    • Gravity only works on charged objects
    • Electric forces do not exist in space
    • Planets have huge mass but almost no overall charge
    • Planets have no mass

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