• Question: A heavy rock and a light rock in free fall (zero air resistance) have the same acceleration. The reason the heavy rock doesn’t have a greater acceleration is that the?

    Asked by to Aimee, Chris, Dave, Greig, Laurence on 19 Jun 2014. This question was also asked by .
    • Photo: Greig Cowan

      Greig Cowan answered on 19 Jun 2014:

      Gravity is acting equally on both rocks.

    • Photo: Dave Jones

      Dave Jones answered on 19 Jun 2014:

      Force of gravity which causes the acceleration is proportional to the rock’s mass. Meaning a rock twice as heavy experiences twice the force. But, a rock twice as massive needs twice the force to give it the same acceleration (that’s what Newton’s second law tells us). So, even though the force of gravity is greater on the heavier rock, its acceleration will be exactly the same.

    • Photo: Laurence Perreault Levasseur

      Laurence Perreault Levasseur answered on 19 Jun 2014:

      inertial mass and gravitational mass for a given object are the same. So the acceleration of both objects is the same, even if the forces are different!

    • Photo: Aimee Hopper

      Aimee Hopper answered on 20 Jun 2014:

      gravity is an acceleration towards a mass, so near the surface of the Earth it is 9.8 meters per second per second. This is independent of the mass of the rock.
      The FORCE on the second rock is more, because Force = mass x acceleration, so higher mass = higher force.

      you may have seen the clip from the Apollo 15 mission to the moon of the astronaut dropping a hammer and a feather…

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