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Mass and Force

  1. Jun 17, 2009 #1
    "The ball weights about one sixth as much on the moon, and lifting the ball on the moon requires one sixth of the force. However, because the mass of the ball is the same on the moon as on the earth, throwing the ball horizontally at a specified speed requires the same force on the moon as on Earth."

    On the moon, what if we were to throw the ball downwards, would that require more force?

    I'm confused when they say it requires the same amount of force to throw ball horizontally on moon. The weight of the object is LESS on the moon. Therefore, the object would seem lighter. So, isn't it true that you can probably throw a lighter object a distance with less force than throwing a heavy object same distance?
  2. jcsd
  3. Jun 17, 2009 #2
    I think

    the ball droped in moon will have acceleration due to gravity less than earth,so you must apply extra force to make accelerate it as of earth

    I think you will feel the weight of the ball as you were in earth because your weight has been decreased by same ratio, But the truth is weight has been decreased

    And i think you need the same force because the horizantal velocity is not effected by gravity
    Last edited: Jun 17, 2009
  4. Jun 17, 2009 #3
    A^2 =Ax^2+Ay^2 ( A = acceleration , x+ horizontal and y= vertical)
    Ay cannot affect Ax , so forse you apply in horizontal direction is not affected by gravity
    You might have confused because we have never tried projectile than in earth
    And your sense of lightier object would travel longer distance by same force came from daily experience of throwing different objest ypward, but its not fort projectile
  5. Jun 17, 2009 #4
    Coverme has it spot on.

    There are vertical and horizntal components to a trajectory when an obect is in flight. Gravity (so by extension weight) has no effect what so ever of the acceleration on the horizontal component.

    From Newtons second law:(it thrown from 9.81m above ground)

    Horizontal F = MA
    Vertical F = M(A+/-G) (plus or minus depending on the coordinate system)

    Its shows clearly that the force required for a certain acceleration depends only on the mass, therefore it takes a set force to accelerate said mass to a given velocity in the horizontal.

    The reason why you can throw things further on the moon is that the vertical component pulling the object down is less.

    For example if an object was thrown from a height that it took 1 second to hit the ground on earth it would take 6 seconds to hit the ground on the moon. Which means (ignoring any losses) it would travel 6x further for a given horzontal velocity.
  6. Jun 17, 2009 #5
    You say that it's possible to throw things further on the moon with same amount of force, due to less gravity. Doesn't this mean that the vertical acceleration is influencing the range of the object? Even though the acceleration components for x and y are independent?

    So, when they say it requires the "same force" for a specified speed, that means that it is NOT possible to throw things farther on the moon than on Earth, horizontally, when xxChrisxx stated that the lesser pull of the moon made it possible to throw things farther with specified force.
  7. Jun 17, 2009 #6
    You can treat vertical and horizontal components totally seperately.

    If I threw a ball in the air 10 meters vertically upwards, or at an angle so it peaked at 10m the ball would have the same flight time becuase gravity pulls the ball to the ground with the same force in either case (acceleration downwards in the same).

    So obviously the faster it is moving in a given direction the further it will go in that set flight time.

    On the moon the pull is less to the acceleration downwards is less (thus for a given vertical force the flight time will be 6 times longer).

    Also it is wrong for them to say that the same force will give the same speed. The same force will give the same acceleration in the horizontal.
  8. Jun 17, 2009 #7
    "Also it is wrong for them to say that the same force will give the same speed. The same force will give the same acceleration in the horizontal. "

    Ah okay, I think that clarifies things.

    The horizontal componant of acceleration, being independent of vertical component, stays the same, since gravity changes on the moon, not horizontal comp of acc. Thanks Chris and coverme :)
  9. Jun 18, 2009 #8
    There is more thing to tell and my previous posting will be in contradiction
    Actually aX=o , because there is uniform Ux, so resultant acceleration
    a^2= ax^2 +ay^2= o + ay^2= ay^2 = g^2
    therefore resultant acceleration is always a=g for the projectile in the whole path And furthermore , R=u^2 sin(2x)/ g ( General formulae for the range of the projectile)

    In moon acceleration due to gravity happens to be g/6
    Since u , x(angle ) remains constants the new R is 6 times the previous Range
    So xx Chriss xx is total correct
    Sorrry for some middle errors, because I always do immediate reaction that does mistakes
    Thank you all.
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