Let us examine the given situations one at a time.
Case a. A 200-pound barbell is held over your head.
The barbell is in static equilibrium because it is not moving.
Answer: STATIC EQUILIBRIUM
Case b. A girder is being lifted at a constant speed by a crane.
The girder is moving, but not accelerating. It is in dynamic equilibrium.
Answer: DYNAMIC EQUILIBRIUM
Case c: A jet plane has reached its cruising speed at an altitude.
The plane is moving at cruising speed, but not accelerating. It is in dynamic equilibrium.
Answer: DYNAMIC EQUILIBRIUM
Case d: A box in the back of a truck doesn't slide as the truck stops.
The box does not slide because the frictional force between the box and the floor of the truck balances out the inertial force. The box is in static equilibrium.
Answer: STATIC EQUILIBRIUM
The acceleration due to gravity of the planet X is 1 m/s².
The given parameters;
- height above the ground, h = 100 m
- initial velocity of the rock, u = 15 m/s
- time of motion of the rock, t = 10 s
The acceleration due to gravity is calculated as follows;

Thus, the acceleration due to gravity of the planet X is 1 m/s²
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No work is done because the object needs to be moved. The formula for work is Work = Force x Distance.
Explanation:
In the given question, the two metal spheres were hanged with the nylon thread.
When these two spheres were brought close together, they attracted each other. The attraction between these spheres is the result of the opposite charges between them.
The possible ways by which these two metal spheres can be charged are by induction that is touching the metal or by rubbing them.
During induction, the same charges are transferred to each sphere. In this case, either both the spheres will be negatively charged or positively charged.
It is not possible that after the sphere touch each other they will cling together because the same charge repels each other and during touching, if one sphere is neutral, then the charged one will transfer the same charge. And as we know that same charge repel each other therefore they will repel each other.
Wavelength = (speed) / (frequency)
Wavelength = (300 thousand km per second) / (10.5 billion per second)
Wavelength = (300 / 10.5) (thousand km per second) / (billion per second)
Wavelength = (28.57) (million meters / second) / (thousand million / second)
Wavelength = (28.57) (meters / second) / (thousand / second)
Wavelength = (28.57) (meters / thousand)
<em>Wavelength = (28.57) (millimeters) </em>