B2.2 Potential Energy

Mass Weight vs The mass of an object is a measure of the amount of matter that makes it up. The mass of an object has nothing to do with the amount of gravity. An object will have the same mass on Earth as it does on the moon, or even in a region where there is no gravity. Mass is a scalar quantity, and is measured in kilograms (kg).

Only PUNKS symbolize weight with a W. So, don’t do it!! The weight of an object is the gravitational force exerted on it by a large body (usually Earth). Weight is a force, and is therefore a vector quantity and measured in newtons (N). = weight (N) m = mass (kg) = acceleration due to gravity (m/s2) On Earth, g = 9.81 m/s2 Kelso sez: Only PUNKS symbolize weight with a W. So, don’t do it!! For example, your mass stays the same on the moon, but your weight will change due to a difference in the amount of gravity acting on you.

Potential Energy Potential energy is energy stored in an object because of its state or position. chemical potential energy Examples include: elastic potential energy Chemical potential energy is the energy stored in the bonds of chemical compounds In this course, we will focus on gravitational potential energy. The reference point is usually whatever the object will hit if it is dropped. Oprah sez: = gravitational potential energy (J) m = mass (kg) g = acceleration due to gravity (m/s2) h = height above reference (m)

Only CHANGES in potential energy can be measured!! When held above the table top, we see that the apple does not have a lot of gravitational potential energy. h1 Move the apple horizontally so that the ground is now the reference point and the apple has a lot more gravitational potential energy. We are NOT creating energy when we do this. h2 Instead, we are just changing the proportion of Ep versus other forms of energy. Only CHANGES in potential energy can be measured!! The point is: the gravitational potential energy can be set to zero at any point you choose.

examples: Practice Problems p. 174 1) A child with a mass of 25.0 kg is at the top of a slide in an amusement park. If the vertical height of the slide is 4.00 m, calculate the gravitational potential energy of the child relative to the ground. 981 J 2) An 800-g bird has 47.0 J of gravitational potential energy when it is perched high up in a tree. Calculate the bird’s vertical height from the ground. 5.99 m 3) A hanging sign is 3.00 m above the ground and has 1.47 x 103 J of gravitational potential energy. Calculate the mass of the sign. 49.9 kg

Homework: read pages 173 – 178 B2.2 Check and Reflect REMEMBER: Show all work for mathematical calculations Review how to convert units