This question is incomplete, the complete question is;
A weightlifter holds a 1,300 N barbell 1 meter above the ground. One end of a 2-meter-long chain hangs from the center of the barbell. The chain has a total weight of 400 N. How much work (in J) is required to lift the barbell to a height of 2 m?
What is the average force (average with respect to height of the barbell from the ground) exerted by the weightlifter in the process?
Answer: Average force exerted by the weightlifter in the process = 1600N
Explanation:
To find Work done to lift a barbell and half of the hanging chain we say;
W₁ = ( 1300N + (1/2 × 400N)) × 1m
W₁ = (1300 + 200) Nm
W₁ = 1500J
now work done to lift the upper half of the chain we say:
W₂ = (1/2 × 400N) × (1/2 × 1m)
W₂ = 200N × 0.5m
W₂ = 100J
So total work done will be
W = W₁ + W₂
W = 1500J + 100J
W = 1600J
To find the average force exerted by the weight lifter, we say;
F = W/D
F = (1600 / 1m) N
F = 1600N
∴Average force = 1600N
Answer:

Explanation:
In this case we will use the Bohr Atomic model.
We have that: 
We can calculate the centripetal force using the coulomb formula that states:

Where K=
and r is the distance.
Now we can say:

The mass of the electron is =
Kg
The charge magnitud of an electron and proton are= 
Substituting what we have:
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so:

The question is incomplete. The complete question is :
Two loudspeakers are placed 1.8 m apart. They play tones of equal frequency. If you stand 3.0 m in front of the speakers, and exactly between them, you hear a minimum of intensity. As you walk parallel to the plane of the speakers, staying 3.0 m away, the sound intensity increases until reaching a maximum when you are directly in front of one of the speakers. The speed of sound in the room is 340 m/s.
What is the frequency of the sound?
Solution :
Given :
The distance between the two loud speakers, 
The speaker are in phase and so the path difference is zero constructive interference occurs.
At the point
, the speakers are out of phase and so the path difference is 
Therefore,




Thus the frequency is :


Hz
Answer:
x(t) = 0.077cos(6.455t)
Explanation:
If the spring can be stretched 0.2 m by a force of 50 N, then the spring constant is:
k = 50 / 0.2 = 250 N/m
The equation of simple harmonic motion is as the following:

where 
We also know that the initial velocity is 0.5 m/s, which is also the maximum speed at the equilibrium:


is the initial phase
Therefore, the position of the mass after t seconds is
x(t) = 0.077cos(6.455t)
Because the earth revolves around the sun and the whole earth isn’t always facing the sun it changes that’s why we have night and day and summer and winter etc