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Intro to Potential Energy

Just because an object is not moving does not mean it does not have any energy. It could have “stored” or potential energy.

Read pages 169 -172 (in the old book us 177-178) in your textbook and answer the following.

  1. What is the symbol for potential energy? PEg
  1. What “type” of energy is potential energy? Potential energy is stored energy.
  1. What is the equation for gravitational potential energy? Peg = mgh
  1. What three quantities does gravitational potential energy depend on? Height from zero level, free-fall acceleration, and the mass of the object.
  1. Fill in the title and graph:

P.E. vs. M Accel. vs. P.E.

  1. Why to we need a zero level to calculate gravitational potential energy?

The zero level is the vertical coordinate at which gravitational potential energy is defined by zero.

  1. Which of the following has more gravitational potential energy and why?

a)A 5 kg bowling ball or a 10 kg boulder both 2 m above the ground?

The 10 kg boulder because it has more mass.

b)A 5 kg bowling ball 3 m above the ground or an identical bowling ball 7 m above the ground?

The bowling ball that is 7 m above the ground because it has a higher distance from the ground than the other.

c)A heavy object a short distance above the ground or a light object a large distance above the ground?

They would have the same potential energy because the distance and mass cancel.

  1. What are the units of potential energy?

Joules (J)

  1. What is another type of potential energy? Describe a situation in which an object could have this type of potential energy.

Elastic potential energy is another type of potential energy which can be found in a spring being stretched/\.

Complete the Section Review #3 on 172 and #1,2,3 in Practice D on the same page. (Answers - #3SR 6.18x10-2 Joules, #1 3.3 Joules, #2 3.1x10-2 Joules, #3a 785 Joules, #3b 105 Joules, #3c 0 Joules)

Potential Energy

Section Review

3. A pinball bangs against a bumper, giving the ball a speed of 42 cm/s. If the ball has a mass of 50g, what is the ball’s kinetic energy in joules?

PEg= mgh

PEg= (50)(9.8)(42)

Peg= 6.18x10-2 J

Practice D

1. A spring with a force constant of 5.2 N/m has a relaxed length of 2.45 m. When a mass is attached to the end of the spring and allowed to come to rest, the vertical length of the spring is 3.57m. Calculate the elastic potential energy stored in the spring.

PEelastic = (1/2)kx2

PEelastic = (1/2)(5.2)(1.12)

PEelastic = 3.3 J

2. The staples inside a stapler are kept in a place by a spring with a relaxed length of .115m. If the spring constant is 51 N/m, how much elastic potential energy is stored in the spring when its length is .15m?

PEelastic = (1/2)kx2

PEelastic = (1/2)(51)(0.035)

PEelastic = 3.1x10-2 J

3. A 40.0 kg child is in a swing that is attached to ropes 2.00 m long. Find the graviatational potential energy associated with the child relative to the child’s lowest position under the following conditions:

a) when the ropes are horizontal

PEg= mgh

PEg= (2)(9.8)(40)

PEg= 784 J

b) when the ropes make a 30 degree angle with the vertical

PEg= mgh

PEg= (2)(9.8)(30)

PEg= 105 J

c) at the bottom of the circular arc

PEg= mgh

PEg= (2)(9.8)(0)

PEg= 0 J