Chapters 7 & 9

Question 1

If you push a crate horizontally with 100 N across a 10-m factory floor and the friction between the crate and the floor is a steady 70 N, how much kinetic energy does the crate gain?

Answer 1

Fxd so (100N x 10m) = 1000 Nm (J)
KE = (100-70N)x(10m) = 300 J

Question 2

What will be the kinetic energy of a pile driver ram that

starts from rest and undergoes a 10-kJ decrease in potential energy?

Answer 2

Its gain in KE will equal its decrease in PE, 10 kJ.

Question 3

If an input of 100 J in a pulley system increases the potential energy of a load by 60 J, what is the efficiency of the system?

Answer 3

Efficiency = useful output/total energy input
60J = useful energy output / 100J energy input = 60%

Question 4

Calculate the kinetic energy of an 88-kg scooter moving at 14 m/s .

Answer 4

KE = 1/2mv2
KE=1/2(88)x(14x14) = 8600J

Question 5

In raising a 6000-N piano with a pulley system, the workers note that for every 2.3 m of rope pulled downward, the piano rises 0.16 m .

Answer 5

Work-energy theorem:
W = PE and W=fd, therefore, fd=mgh

f=(6000)(.16)/2.3 = 420N

When there is force causing an item to be lifted in height, the height = gravitational pull (h=g) so you can omit g from equation