# Check my simple physics task for calculating weight

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#### qratman

##### Newbie level 2 Hi, I wanted to calculate the weight(in Newtons) the object has at the moment when car crashes (driving 100kmh against wall for example). So I wonder if my calculations are right?

Using the following formulas.
Q = gm ± am [N]; Weight = gravity x mass + force x mass [Newtons]
F = a x m ; force = acceleration x mass [Newtons]
a = ΔV / t [m/s²]; acceleration = current velocity - start velocith divided by time

Given:
V = 100 km/h = 27.78 m/s //car speed
V(0) = 0 km/h = 0 m/s //car end speed
m = 70 kg //my mass in car
t = 1 s //crash speed, from top speed to car stopping

To find out:
Q = ?

Calculations:
Q = gm + am = 9.8 x 70 + 27.78 x 70 = 2630 kg = 2.6 tons (wow!)
a = ( 27,78 - 0 ) / 1 = 27.78 [m/s²]

Is this really so that my weight is 2.6 tons at the moment of crash?

#### steve10

##### Full Member level 3 Re: Simple physics

The weight of any object has been clearly defined to be the product of the mass(m) and the gravity acceleration (g). It won't change whichever state the object is at, staying, moving, accelerating, ... even thought it DOES vary slightly for different places on the earth.

Let's forget about the reletivity effect. According to what you said, I think what you are asking is the force that is moving the object right at the crashing. You know the force is a vector and, therefore, you can't simply add the two terms mg and ma together, especially in the situation stated in your question. When the car craches into a wall, the direction of a is along the horizontal direction while the g in the vertical direction. Therefore, at the crashing, your weight does not change, but you have another force along the horizontal direction rushing you into the wall.

#### shiv.emf

##### Newbie level 5 Re: Simple physics

yes totally agreed with steve ....
weight will change when u r moving in lift i.e m(g±a)
it should b along the direction of g or against g

#### neils_arm_strong

##### Full Member level 5 Simple physics

Yes steve is perfectly right.U can't add two vectors algebraically.The wieght of any object will be the same(neglecting change ing from place to place)
unless it is accelerating in vertical direction.

#### qratman

##### Newbie level 2 Re: Simple physics

So the weight remains the same?

Hmm.. thats strange. I've heard people talking in media like howmuch the weight of some object is when car crashes. (Trafic) Police warns people not to put bottles (especially full) under the cars rear glass, because when crashing its weight is multiplied by xx factor (depending on speed) and it flies with such a great speed to front that it could kill driver. So maybe they talk it just for people as comparsion for them to understand.

But... I am thinking that when driving directly into the wall then the angle between g and a is 90° . So what about calculating the weight Q1 = gm. Then Q2=am and using Pythagorean theorem like so: Q = √(Q1² + Q2²) = √(700² + 1945²)=2067 kg = 2.1 tons.

My question is now, can we talk about the change in weight at all (and calculating it correctly using vectors) or the weight remains the same (but then i think it wouldn't also change when accelerating with lift - so it must change in car crash ).

edit: having read your posts through for the second time I'm thinking that it might be wrong too. Also the weight is not measured in kg's (or tons) but it should come as a x m [kg x m/s²]. Oh boy.. #### flatulent

##### Advanced Member level 5 Re: Simple physics

There are two other aspects if you are considering the destruction caused.

One is that the energy of the object 0.5 x mass x velocity squared is absorbed by the object it hits. It takes as little as 50 ft-lbs to kill someone if it is applied in a vulnerable spot of their body such as the side of the head or if it is a small object that can penetrate the body and land in the heart.

The other is the force exerted on the item the moving object impacts. force x delta time = mass times delta velocity. This force can exceed the limits of bones and break them.

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