Physics - Work and Energy - Power - Please help!!!!!!!!!!!!!!!?

Question

1. A driver notices that her 1000kg car slows down 90 km/h to 70 km/h in about 6.0s on the level when it is in neutral. Approximately what power (watts and hp) is needed to keep the car traveling at a constant 80 km/h?

2. A shot putter accelerates a 7.3kg shot from rest to 14m/s. If this motion takes 2.0s, what average power was developed?


The answers from the book of the book for #1 is 2.1 x 10^4 W = 28 h, and #2 is 3.6 x 10^2 W.

I'm not sure how to get these answers so please show work and an explanation. Thanks so much!!

Answer 1

First some conversions
90 km/h = 25.0 m/s
80 km/h = 22.2 m/s
70 km/h = 19.4 m/s

acceleration = change in velocity / time
a = (19.4 - 25.0) / 6 = -0.933 m/s

so magnitude of F = ma = 1000 * 0.933 = 933 N. This is from kinetic friction of the tires. So the engine must apply this same magnitude to keep at constant speed.

Power = Force * average speed
= 933 * 22.2
= 2.1 * 10^4 Watts

you can convert to hp by dividing by 0.7457
================
same again
a = (14 - 0) / 2 = 7

F = 7.3 * 7 = 51.1

P = 51.1 * 7 = 3.6 * 10^2