Introduction to Energy Types of Energy and Its Conservation

Cheetah Breaks Speed Record - Beats Usain Bolt by Seconds Types of Energy Kinetic Energy (EK) The energy a body has because it is moving Units: Joules (J) http://news.nationalgeographic.com/news/2012/08/120802-cheetah-sarah-cincinnati-zoo-fastest-record-science-usain-bolt-olympics/# Cheetah Breaks Speed Record - Beats Usain Bolt by Seconds

Potential Energy Types of Energy The amount of energy that is stored in a body There are different types of potential energy: Gravitational energy Elastic potential energy (energy in an stretched spring) Others (magnetic, electric, chemical, …). Won’t deal with those here.

Gravitational Potential Energy (EP) Types of Energy Gravitational Potential Energy (EP) The amount of energy that an object has as a result of its position above a reference point (in Earth’s gravitational field) Where m = mass (kg) g = acceleration due to gravity (9.80 m s 2 ) and h = height (m) above reference level Units: Joules (J)

Forms of Energy: Types of Energy Thermal Energy Chemical energy the kinetic energy of atoms and molecules (remember, “heat” is the transfer of this energy between systems) Chemical energy energy associated with electronic structure of atoms and the electromagnetic force Nuclear energy energy associated with nuclear structure of atoms and the strong nuclear force

Forms of Energy: Types of Energy Electrical energy associated with an electric current (kinetic energy of electrons in a conductor) Radiant (light) energy energy associated with photons of light Mechanical energy associated with the movement or position of physical bodies (kinetic and potential energy)

Conservation of Energy The Universe contains a static amount of energy. Energy cannot be created nor destroyed, it transforms from one type to another (or transferred from one object to another)

Let’s Look At This Useful Example! Conservation of Energy Let’s Look At This Useful Example!

How is energy conserved? Conservation of Energy How is energy conserved?

Transformation of Energy Potential Energy Consider a roller coaster… At the top, it has potential energy. The energy is stored just as the energy in a rubber band Potential energy is energy of position Many times, it is energy waiting to be used for motion

Transformation of Energy Kinetic Energy Falling objects convert potential to kinetic (total energy is a constant) As the roller coaster moves toward the bottom, the kinetic energy increases along with the roller coaster’s speed

Transformation of Energy Conservation of energy means it can change forms between kinetic and potential, but it never disappears. The total amount of energy is always the same Energy is often times changed into heat due to friction and is not lost Example: Sliding a book across the floor will heat up the book and floor

The Principle of Energy Conservation Conservation of Energy The Principle of Energy Conservation The total amount of energy a body possesses will remain constant, although the type of energy may be transformed from one form to another Note: many times the energy transforms into a “useless” form, so it appears that energy has been lost…when it really hasn’t! Conservation of Mechanical Energy:

Example 1 The land-speed record for a wheel-driven car is nearly 738 km/h (approximately 205 m/s - recorded at the Bonneville Salt Flats on 10/18/01) If the mass of this car was approximately 1.0x103 kg, what was its kinetic energy? 𝐸 𝐾 = 1 2 𝑚 𝑣 2 𝐸 𝐾 = 1 2 (1000 kg) (205 m s ) 2 𝑬 𝑲 =𝟐.𝟏× 𝟏𝟎 𝟕 𝐉 459 mi/hr http://www.loqu.com/top-10-most-significant-land-speed-records-of-all-time/blogs/hotday-5390

Example 2 The land-speed record for a mammal was set by Sarah, and 11 year old cheetah for 98 km/h (approximately 27 m/s - recorded at the Cincinnati Zoo on 06/20/12) If the mass of Sarah was approximately 45 kg, what was her kinetic energy? 𝐸 𝐾 = 1 2 𝑚 𝑣 2 𝐸 𝐾 = 1 2 (45 kg) (27 m s ) 2 𝑬 𝑲 =𝟏.𝟔× 𝟏𝟎 𝟒 𝐉 http://www.loqu.com/top-10-most-significant-land-speed-records-of-all-time/blogs/hotday-5390

Example 3 You are 1.80 m tall. A 0.100 kg apple is hanging1.00 meter above your head. What is its potential energy? 𝐸 𝑃 =𝑚𝑔ℎ 𝐸 𝑃 =(0.100 kg)(9.80 m s 2 )(2.80 m) 𝑬 𝑷 =𝟐.𝟕𝟒 𝐉 1 m

Example 4 Three balls are thrown from the top of a building, all with the same initial speed. The first is thrown horizontally, the second with some angle above the horizontal and the third with some angle below the horizontal. They have more potential energy to convert to kinetic energy. Describe the motion of the balls. Rank the speed of the balls as they hit the ground. They all follow a parabolic path v2 = v1 = v3

Example 5 How much energy does a 875 g flower pot have if it is on a shelf 95.0 cm above the ground? 𝐸 𝑃 =𝑚𝑔ℎ 𝐸 𝑃 =(0.875 kg)(9.80 m s 2 )(0.950 m) 𝐸 𝑃 =8.15 J How fast is the flower pot traveling when it is 35.0 cm above the ground? 𝐸 𝐾(1) +𝐸 𝑃(1) = 𝐸 𝐾(2) +𝐸 𝑃(2) 1 2 𝑚 𝑣1 2 +𝑚𝑔 ℎ 1 = 1 2 𝑚 𝑣2 2 +𝑚𝑔 ℎ 2 0+𝑚(9.80 m s 2 )(0.950 m)= 1 2 𝑚 𝑣2 2 +𝑚(9.80 m s 2 )(0.350 m) 0+(9.31 m 2 s 2 )= 1 2 𝑣2 2 +(3.43 m 2 s 2 ) 𝒗 𝟐 =𝟑.𝟒𝟑 𝐦 𝐬

Example 6 A car slides down an icy hill that has a height from top to bottom of 14.0 m. What is its speed when it hits the snow bank at the bottom of the hill? 𝐸 𝐾(1) +𝐸 𝑃(1) = 𝐸 𝐾(2) +𝐸 𝑃(2) 1 2 𝑚 𝑣1 2 +𝑚𝑔 ℎ 1 = 1 2 𝑚 𝑣2 2 +𝑚𝑔 ℎ 2 0+𝑚 9.80 m s 2 14.0 m = 1 2 𝑚 𝑣2 2 +0 9.80 m s 2 14.0 m = 1 2 𝑣2 2 𝒗 𝟐 =𝟏𝟔.𝟔 𝐦 𝐬