# Homework Help: Need help

1. Mar 7, 2006

### rachael

8 A boy of mass 65 kg stands on the floor of an elevator.
a What is the weight of the boy?
b What is the reaction of the floor of the elevator on
the boy?
The elevator starts to move upwards with an
acceleration of 1.0 m s–2.
c What is now the reaction of the floor of the elevator
on the boy?
After accelerating upwards for 2.0 s the elevator
continues with a constant upwards velocity.
d What is now the reaction of the floor of the elevator
on the boy?
part c if the elevator had an acceleration of 1.0 m s–2
downwards?

a. i use w=mg
b.i don't understand y the answer remains the same shouldnt it be ma=mg-N, where mg=650 and ma=65?
c,d,e.i dont get these questions

2. Mar 7, 2006

### xman

for part (a) yeah m g=w and for part (b) since there is no acceleration in the problem then mg=N. for (c) now there is an acceleration so draw a free body diagram of the forces acting on the boy? what direction is the acceleration and how does this effect the boy? this will answer (c) and (d) and (e)

3. Mar 7, 2006

### mezarashi

From your own experience on an elevator, you notice that when the elevator is static, you feel... normal.

When it starts moving up, you feel heavier. After a while you feel normal again. When it's about to stop you feel lighter.

When it starts moving down, you feel lighter. After a while you feel normal again. When it's about to stop you feel heavier.

It is asking you about these concepts. If you feel heavier, indeed you must be "heavier". That is, if you were standing on a scale on the elevator, it would read a weight higher than your usual.

Weight is caused by the reactionary force from whatever it is you are standing on, in this case, the floor of the elevator. While standing, there are two forces acting on you to produce a net force: gravity (pulling downwards) and the floor (pushing upwards). If the elevator is still, acceleration is zero, so what does this say?

Fnet = ma (a = 0)
Fnet = 0

As we discussed, Fnet consists of two forces, gravity and the floor, so:

Fnet = 0
Fnet = Fgrav + Ffloor = 0
Fnet = mg + Ffloor = 0

Can you take it from here?

4. Mar 7, 2006

### rachael

thank you..........