physics problems (1)

1. The spring scale reads the horizontal force pulling the mass, which is 20N. The weight, being in a vertical direction, has no effect on the scale reading.
The force exerted by the mass on the spring is equal in magnitude and opposite in direction to the force exerted by the spring on the mass (Newton's third law)
Thus the force = 20N
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2. From Newton's law of gravitation, the attractive force F acting on an object on earth surface by the earth's graviational pull is:
F = GmM/R^2
where G is the Universal Gravitational Constant, m is the mass of an object, M is the mass of earth and R is the radius of the earth.

Thus F=[GM/R^2]m
the slope is [GM/R^2] when F is ploted against m

Since the earth's graviational pull on the object is just its weight mg,
thus mg = GmM/R^2
i.e. g = GM/R^2

the slope is thus equal to g, the acceleration due to gravity.
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3. I think this question asks for the order of magnitude.
A football player is generally of mass several tens of kilograms, e.g. 70 kg or 7x10^1 kg. The order of magnitude is thus 10^1 kg.