A.) <span>If no unbalanced force acts upon the house of cards, then it will remain standing forever.

[ Other Statements doesn't make any sense in Physics, they can be true in some situation & can't be false, option A is the only answer ]

Hope this helps!</span>

**Answer:**

618.03 J

**Explanation:**

Potential energy can be calculate using the following formula.

PE=m*g*h

where m is the mass, g is the acceleration due to gravity and h is the height.

In this situation, we have a 3 kilogram rock on a 21 meter high cliff. Therefore, the mass is 3 kg and the height is 21 m.

On Earth, the acceleration due to gravity is 9.81 m/s^2.

m=3 kg

h=21 m

g=9.81 m/s^2

Substitute these values into the formula.

PE=3*9.81*21

Multiply all the numbers together

PE=63*9.81

PE=618.03

The potential energy is **618.03 kg m^2/s^2, or 618.03 Joules**

place an flashlight in an plastic tube and then color it

**Answer:**

Convection is the transfer of thermal energy from one place to another by the movement of gas or liquid particles. How does this happen? As a gas or liquid is heated, the substance expands. This is because the particles in liquids and gases gain kinetic energy when they are heated and start to move faster.