Question

3. A 40.0 kg child is in a swing that is attached to ropes 2.00 m long. Find the gravitational potential energy associated with the child relative to the chid s lowest position under the following conditions: a. when the ropes are horizontal b. when the ropes make a 30.0° angle with the vertical c. at the bottom of the circular arc SECTION 2 FORMATIVE ASSESSMENT O Reviewing Main Ideas 1. A pinball bangs against a bumper, giving the ball a speed of 42 cm/s. If the 2. A student slides a 0.75 kg textbook across a table, and it comes to rest after ball has a mass of 50.0 g, what is the ball's kinetic energy in joules? traveling 1.2 m. Given that the coefficient of kinetic friction between the book and the table is 0.34, use the work-kinetic energy theorem to find the book's initial speed. 3. A spoon is raised 21.0 cm above a table. If the spoon and its contents have a mass of 30.0 g, what is the gravitational potential energy associated with the spoon at that height relative to the surface of the table? Critical Thinking 4. What forms of energy are involved in the following situations? a. a bicycle coasting along a level road b. heating water c. throwing a football d. winding the mainspring of a clock

Answer 1

mass = m = 40 kg

length = l = 2 m

A) when ropes are horizontal

potential energy = u = m* g * h = 40 * 9.8 * 2 = 784 J

b) if theta = 30 degrees

             U = m g l ( 1- cos theta )

              U = 40 * 9.8 * 2 ( 1- cos 30 ) = 105 J

C) at the bottom height h= 0

potential energy = U = 0