questions about force?

Question

1) A race car has a mass of 718 kg. It starts from rest and travels 44.0 m in 3.0 s. The car is uniformly accelerated during the entire time. What net force is exerted on it?
N=?

2)Your mass is 80 kg. You stand on a bathroom scale in an elevator on Earth.

(a) What force would the scale exert when the elevator moves up at a constant speed?
N=?
(b) What force would the scale exert when it slows at 2.2 m/s2 while moving upward?
N=?
(c) What force would the scale exert when it speeds up at 2.2 m/s2 wile moving downward?
N=?
(d) What force would the scale exert when it moves downward at a constant speed?
N=?
(e) What force would the scale exert when it slows to a stop at a constant magnitude of acceleration?



3) A boy exerts a 38 N horizontal force as he pulls a 54 N sled across a cement sidewalk at constant speed. What is the coefficient of kinetic friction between the sidewalk and the metal sled runners? Ignore air resistance.

Answer 1

1. Let's use famous F=ma

Average a = V/t
so
F=m v/t= 718 x 44.0/3.0=
F=10530 N

2.
(a ) did you say constant speed? then W=mg

(b) 2.2 m/s2 mowing upward then
F=W+Fe= mf +ma=m(g+a)

(c)2.2 m/s2 mowing downward then
F=W-Fe= mf +ma=m(g-a)

(d) same as (a)

(e) constant negative acceleration then it is the ame as (c)


3. Force of pull equal to force of friction
F=f
F=uN
u=F/N=38/54=0.704

Have fun!