Question

Blackie, a cat whose mass is 6-kg, is napping on top of the refrigerator when he rolls over and fall. Blackie has a KE of 90-J just before he lands on his feet on the floor.
How tall is the refrigerator?

Answer 1

To answer this question, you need to understand the law of conservation of energy. Essentially, the law states that energy cannot be created or destroyed; it is always conserved.

Knowing this law, how can we answer this question? Well, let's look at what the question tells us. We know that Blackie was initially at the top of a refrigerator before rolling over and falling to the ground. At ground level, he only has Kinetic energy when he lands. Since we know that he started at the top of the fridge, we know that Blackie started off with potential gravitational energy.

That would make sense, right? Written out as an equation, it'd be:

$U_{g}$ = $K_{E}$

This follows the law of conservation of energy, as all the potential gravitational energy is converted into Kinetic energy.

Now, we know what energies are converted. What can we do with it? Recall the equation of potential gravitational energy:

$U_{g}$ = $F_{g}$∆H

$U_{g}$ = mg∆H

Do you see it now? ∆H is the distance that Blackie falls, and since Blackie jumps from the fridge to the ground, ∆H must be the height of the fridge!

Let's start solving for ∆H:

$U_{g}$ = $K_{E}$

Substitute potential gravitational energy with our equation:

mg∆H = $K_{E}$

We were given the value of Kinetic Energy:

mg∆H = 90

Isolate ∆H by dividing both sides by mg:

∆H = $\frac{90}{mg}$

Input values for 'm' and 'g' (m is the mass of Blackie and g is Earth's acceleration)

∆H = $\frac{90}{6*9.80}$

∆H = $\frac{90}{58.8}$

∆H = 1.53

The refrigerator is 1.53 meters tall.

And that's it! Let me know if you need me to explain anything I did here.

- breezyツ