What are my units when finding % diff. In kinetic energy?

• astru025
In summary, the problem asks for the kinetic energy before and after a collision between Phrank, Phrancine, and a skateboard. The kinetic energy before the collision is 88.1 J and after the collision is 51.88 J. The percent difference is 51.75, which is a dimensionless quantity.

astru025

What are my units when finding % diff. In kinetic energy?

Homework Statement

Phrank (36.4kg) runs at 2.2 m/s and jumps onto a large stationary skateboard (2.3kg) on which Phrancine (22.7kg) is sitting. After the collision, Phrank, Phrancine, and the skateboard move at 1.3 m/s. Find the kinetic energy before and after the collision along with the percent difference.

Homework Equations

Kinetic energy before equals= 88.1 J
Kinetic energy after= 51.88 J
Percent difference= 88.1-51.88 / (88.1 + 51.88/2 ) x 100 = 51.75 WHAT ARE MY UNITS HERE?

The Attempt at a Solution

Need help with my units for percent difference! J is no correct!

Percent difference is a unit-less number.

More precisely, it is a dimensionless number.

And not percent? I can be horribly wrong, but I was always under impression that if it goes like

$$\frac{3J}{2J}100\% = 150\%$$

ratio is dimensionless but the answer is in %.

Not that % is a physical unit. Actually now that I think about it, I have no idea what % is.

Borek said:
ratio is dimensionless but the answer is in %.

Not that % is a physical unit. Actually now that I think about it, I have no idea what % is.
You could certainly argue that dimensionless quantities nevertheless have units - whole numbers, percentages, millions... But if I ask how many km/h a certain speed limit is the answer could be 50 - it would not, strictly speaking, be 50km/h. In the same way, if asked what percentage 3 is of 4 then the answer is 75, not 75% (which could legitimately be interpreted as 75/100 or 0.75). Does that make sense?

haruspex said:
Does that make sense?

More or less... not your fault, I am slow today :zzz:

1. What exactly is kinetic energy?

Kinetic energy is the energy an object possesses due to its motion. It is dependent on the mass and velocity of the object.

2. How do you calculate kinetic energy?

Kinetic energy is calculated using the formula KE = 1/2 * m * v^2, where m is the mass of the object and v is its velocity.

3. What units are used to measure kinetic energy?

Kinetic energy is measured in joules (J) in the SI system of units. In some cases, it may also be measured in foot-pounds (ft-lb) or kilogram-meters squared per second squared (kg * m^2 / s^2).

4. What is the formula for calculating percent difference in kinetic energy?

The formula for calculating percent difference in kinetic energy is % diff. = (KE2 - KE1) / ((KE1 + KE2) / 2) * 100%, where KE1 and KE2 are the initial and final kinetic energies, respectively.

5. How do you interpret the units when finding percent difference in kinetic energy?

The units for percent difference in kinetic energy will be in percentage (%), as it is a ratio of the difference between two kinetic energies to the average of those two energies.