Question about Newton's laws of motion

Question

what does a 138N rock weigh if it is accelerating a)upward at 12m/s^2 (b) downward at 3.5m/s^2 (c) what would be the answers to part a and b if the rock were moving with a constant upward velocity of 23m/s?

Answer 1

Mass of a body is constant. But weight changes, it is the force acting on the body

W= mg derived from F= ma where W, F, m, a, g are weight, mass, acceleration of the body, acceleration due to gravity

Now your question is the weight of the body 138 N means its mass is 14.06 kg (138/9.8)

1. The apparent weight when the left has an upward uniform acceleration is given by

W= m (g+ a) where a is the acceleration of the left or other words the body

W = 14.06 (9.8 + 12) = 14.06(20.8)= 292.45 N

2. The apparent weight when the left has a downwards uniform acceleration is given   by W= m (g- a) where a is the acceleration of the left or other words the body

W = 14.06 (9.8 – 3.5) =14.06(5.3)=  74.52 N

3. When the elevator is moving with constant velocity its acceleration is “Zero”. So there is no change in the weight of the body.

As a simple rule you can remember that

When the left is moving up or down with a constant velocity there is no change in the bodies’ weight

When the left is moving up with acceleration, add it to g value

When the left is moving down with acceleration, substrate it from g value

Best of luck

Answer 2

use w=mg (weight is equal to mass times grav. acc (9.8m/s^2)  

also f=ma (force is equal to mass times acc)

good luck..need more help email me: [e-mail]

Answer 3

the weight is the same irrespective of whether it is moving up or down or accelerating (ignoring the very small change due to its distance from earth changing).  

Answer 4

Weight of an object doesn't depend on it's velocity or acceleration. Thus a rock weighing 138N will weigh 138N only. It doesn't matter  whether it is accelerating upward or downward or moving with a constant velocity.

Weight is a measure of magnitude of the force with which the earth atttracts a body towards itself by gravitational force.

Answer 5

(a) the force you just gave is dependent on the acceleration being 9.81 m/s/s. Remove that first and then figure out the mass of the object. After that, multiply the mass by 12 m/s/s. The difference between the two forces in (a) and (b) would be a negative sign indicating your chosen coordinate system.

(c) there is no acceleration with a constant velocity. So, using Newton's Second Law, Fnet=m*a; a = 0, therefore Fnet = 0.

Answer 6

Its weight W = mg = 138 kg-m/sec^2 (aka Newton)

a) However, the net weight w = W - F = mg + ma = m(g + a); where g = 9.81 m/sec^2, a = 12 m/sec^2, and m = W/g = 138/9.81.  You can plug the numbers.  The sign is plus because we have - m(-a) = + ma as -a means acceleration in a direction opposite to graivty's g.

b) Similarly w = W - F = mg - ma = m(g - a); where a = 3.5 m/sec^2.

c) Again w = W - F = mg - ma = mg = W = 138 kg-m/sec^2 (N).  Note we don't care what velocity it is, when it's constant a = 0.