The only other factor you need is the height of the object in real life (otherwise you could be photographing a model which is much closer to the camera).
The maths isn't actually that complex, the ratio of the size of the object on the sensor and the size of the object in real life is the same as the ratio between the focal length and distance to the object.
To work out the size of the object on the sensor, work out it's height in pixels, divide by the image height in pixels and multiply by the physical height of the sensor.
So the whole sum is:
Distance to object equation
Answer:
The girl is doing work on the ball because the energy in her muscles changed, even though the ball is not displaced.
Explanation:
The complete question is...
Which of these correctly describes whether a girl holding a ball in the same position is doing work on the ball?
-The girl is doing work on the ball because the energy of the ball changed, even though it is not displaced.
-The girl is doing work on the ball because the energy in her muscles changed, even though the ball is not displaced.
-The girl is doing no work on the ball because the ball is not displaced.
-The girl is doing no work on the ball because she is exerting a net force on the ball.
Holding up a ball costs energy, which is used to counter the work that would have otherwise be done on the ball by gravity. Although no physical distance is moved, we should consider the fact that by holding the ball, the girls hand exerts physical force to hold the ball in place. Also, there is a potential gravitational work on the ball due to gravity, but the force exerted by the girls hand does an equivalent of this gravito-potential work in order to counter it and hold the ball in place. All these activities eventually lead to a change in energy in her hand muscle to show that energy is expended.
Hopefully this will help you.
Answer:
potential energy decreases
kinetic energy increases
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