Physics 100-Chapter 7-Plug and Chug

Show that 50 W of power is required to give a brick 100 J of PE in a time of 2 s. Power = work/time: P=W/t

-P = 100 J/2 s = 50 Watts OR

Calculate the work done when a force of 5 N moves a book 1.2 m. (1 N m = 1 J) Work = force x distance: W=Fd

-W = (5N)(1.2m) W = 6.0 J

Show that 24 J of work is done when a 3.0 kg block of ice is moved from rest to a speed of 4.0 m/s. Work - energy theorem: Work = DeltaKE

1/2 (3 kg)(4 m/s)2 = 24 J (3)(16) = 48 1/2 (48) = 24 J

When you are driving 90 km/h, how much more distance do you need to stop compared with driving at 30 km/h?

Nine times more. The car has 9 times as much kinetic energy when it travels 3 times as fast: 1/2m(3v)squared = 1/2 m9vsquared = 9(1/2 mvsquared). The friction force will ordinarily be the same in either case; therefore, 9 times as much work required 9 times as much distance.

Show that the kinetic energy of a 1.0 kg book tossed across the room at a speed of 3.0 m/s is 4.5 J. (1 J is equivalent to 1 kg*m/s2) Kinetic energy = 1/2 mass x speed squared: KE = 1/2 mv2

m = 1 kg v = 3 m/s 1/2 (1 kg)(3 m/s)2 = 4.5 J 1/2 (9) = 4.5 J

Show that when a 3.0 kg book is lifted 2.0 m its increase in gravitational potential energy is 60 J. Gravitational potential energy = weight x height: PE=mgh

m = 3 kg h = 2 m (3 kg)(10m/s2)(2m) = 60 J