FORENSIC SCIENCE--
1 answer:
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a) The work done is 920 J
b) The work done is 5200 J
Explanation:
a)
In this first part of the problem, the crate is moved horizontally at constant speed.
The work required in this case is given by
where
F is the magnitude of the force applied
d is the displacement of the crate
is the angle between the direction of the force and of the displacement
Here the crate is moved at constant speed: this means that the acceleration of the crate is zero, and so according to Newton's second law, the net force on the crate is zero: this means that the force applied, F, must be equal to the force of friction (but in opposite direction), so
F = 230 N
The displacement is
d = 4.0 m
And the angle is , since the force is applied horizontally. Therefore, the work done is
b)
In this case, the crate is moved vertically. The force that must be applied to lift the crate must be equal to the weight of the crate (in order to move it a constant speed), therefore
F = W = 1300 N
The displacement this time is again
d = 4.0 m
And the angle is , since the force is applied vertically, and the crate is moved also vertically. Therefore, the work done on the crate this time is
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The gravitational force acting on the man is 800 N towards the Earth's centre
Explanation:
The weight of an object on the Earth is exactly the gravitational force exerted by the Earth on the object.
The gravitational force exerted by the Earth on an object located at the Earth's surface is given by:
where
G is the gravitational constant
M is the Earth's mass
m is the mass of the object
R is the radius of the Earth
And the direction of the force is towards the Earth's centre.
Since G, M and R are constant, they are grouped into a single constant called g, acceleration of gravity:
therefore the gravitational force can be rewritten as
And this is the usual equation that we use to calculate the weight of an object.
Therefore, weight and gravitational force acting on an object on Earth are the same thing: so, the gravitational force acting on the man is equal to his weight, 800 N, and it acts towards the Earth's center.
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