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12. On sister, 200 N and on Harry 400 N

13. Horizontal- 250 N, Vertical- 0 N.

14. The object is experiencing net force of 250 N to the left.

15. Horizontal- 0 N, Vertical- 0 N.

16. The object is experiencing no net force.

17.

First law:

When viewed in an inertial reference frame, an object either remains at rest or continues to move at a constant velocity, unless acted upon by an external force.^{[2]}^{[3]}

Second law:

The vector sum of the external forcesF on an object is equal to the massm of that object multiplied by the acceleration vectora of the object: F = ma.

Third law:

When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body.

18. 0 N.

19. 2 m/s^2

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