Answer:
The static frictional force is greater than the kinetic frictional force, so the static frictional force is greater than 1200 N
Explanation:
Frictional force is a force that acts against the motion of any object sliding on a surface, and it is due to the contact between the molecules on the surface of the object and the molecules of the surface on which the object is sliding.
There are two types of frictional forces:
- Static friction: this occurs when the object is at rest. When the students push the piano and the piano is still at rest, this force acts in the opposite direction to the students' push. In order to be able to move the piano, the students must apply a force which is greater than the maximum value of the static force, which is given by:
where is the coefficient of static friction and (mg) is the weight of the piano.
- Kinetic friction: this occurs when the object is already moving. Its direction is opposite to the direction of motion of the piano, and its magnitude is given by
where is the coefficient of kinetic friction and (mg) the weight of the piano.
Generally, , which means that the static friction is always greater than the kinetic friction (this is why it is usually harder to make an object start moving rather than keep it in motion along a surface).
If we apply all of this to this problem, we see that:
- When the piano is sliding, it is pulled with a force of 1200 N and it is in equilibrium. This means that the net force is zero, so the force of kinetic friction is equal in magnitude to 1200 N.
- Since the static friction is always greater than the kinetic friction, this also means that the static friction is greater than 1200 N, and this is why the students need a force greater than 1200 N to put the piano in motion.
Answer:
41.053 A
Explanation:
given,
three phase kVA = 25-kVA
voltage = 440 Volt
current = ?
To determine three phase kVA when volts and amperes are know
three phase kVA = 1.73 x V x I
I = 32.84 A
the maximum overcurrent protection value is equal to
= 125 % of I
= 1.25 x 32.84
= 41.053 A
From the momentum conservation we know that the initial momentum is equal to the final momentum. The momentum in a singular way can be defined as the product between the mass and the velocity of an object. In the presented system, however, there are two objects, therefore the mass of both and the speed of both, before and after the collision must be taken into account. Mathematically we could describe this as
Here,
= Mass of each object
= Initial velocity of each object
= Final velocity of each object
From here we can realize that it is necessary to use the system on both cars to be able to predict what will happen either with their masses, or their speeds.
The correct answer is C.