Remember that you will be given the value for the gravitational field strength in the question (unless you are asked to calculate it!).
__Question: __Calculate the velocity of a 5kg object which was dropped from a height of 10m. g = 10N/kg
Working: __The first step is to see that we need to find out we are missing. To calculate velocity, the only equation we know is __Ek= ½ x m x v². We need to find v and have been given m; so we are missing Ek.
The question also gives us the height and a value for g. The only equation that suits this is Ep=mgh. We should remember that the kinetic energy at the bottom is the same value as the gravitational energy at the top.
Therefore: Ep= 5 x 10 x 10 = 500J
So: 500 = ½ x m x v²
v² = (500 x 2) / 10 = 100
Remember to square root both sides so:
v = 10m/s
Often you will be asked to fill in one of the missing values when given the other two. Remember that the total in must equal the total out.
The line width of the arrow represents how much energy is being transferred. You may be asked to take a look at the scale (i.e. one box = 10J).
- What is the kinetic energy of a 3kg ball that is rolling at 2m/s?
- What is the gravitational energy of a ball that is on the ground?
Explanation: The ball cannot fall, h=0
- What is the spring constant of a spring that took 0.7J of energy to stretch it 5cm?