# 30 kg boy in a 1000 kg elevator moving up 5 m/s

1. Dec 17, 2015

### Nick63

1. The problem statement, all variables and given/known data
A 30kg boy is in a 1000kg elevator moving at +5m/s when it comes to a stop in 2 seconds. During the slow down...
1.what is the acceleration of the boy and the elevator?
2. what is the net force acting on the boy?
3.What is the force of the elevator on the boy?

2. Relevant equations
Acceleration=Net force on object/mass
Force in Newtons=mass*acceleration

3. The attempt at a solution
the boy and elevator have an acceleration of -2.5m/s^2

2. Dec 17, 2015

### PeroK

Okay. What about questions 2 & 3?

3. Dec 17, 2015

### Nick63

For #2 is the two forces on the boy Force of elevator on boy and Force of Earth on Boy(gravity)?

4. Dec 17, 2015

### PeroK

Yes.

5. Dec 17, 2015

### Nick63

I think I have the force of earth on boy at 144 N and the force of the elevator at -2500N, is that right?

6. Dec 17, 2015

### PeroK

If that were true, the net force on the boy would be $-2356N$, and hence the boy would accelerate at approximately $-80m/s^2$.

Do you know the boy's acceleration?

7. Dec 17, 2015

### Nick63

I thought it was -2.5m/s^2

8. Dec 17, 2015

### PeroK

It is.

Do you know the boy's mass?

9. Dec 17, 2015

### Nick63

Yes 30kg

10. Dec 17, 2015

### PeroK

So, you know the mass and the acceleration. What quantity are you trying to find? (For question 2.)

11. Dec 17, 2015

### Nick63

the net force on the boy

12. Dec 17, 2015

### PeroK

How are net force, mass and acceleration related?

13. Dec 17, 2015

### Nick63

so 30kg*(-9.8m/s^2+-2.5m/s^2)

14. Dec 17, 2015

### PeroK

That's not right. You may be confused by what net force means:

Suppose you were stuck in a room where the ceiling was pushing down on you and the floor was pushing up, so that you were getting squashed. In terms of kinematics (the study of motion) there is no net force on you. The two forces cancel out and you do not move. This situation is the same as if no forces were acting on you. That's the nature of forces and motion that you are currently studying. You might describe these as "external forces". There is no net external force in this case.

If you were studying internal stresses and strains on a body, then the two situations would be very different. But, this is not what you are looking for here. You're not measuring how much things are getting squashed, you're measuring how they are moving!

In any case, let me give you the key equation and explain it:

$F = ma$

$F$ is the net external force on a body. If two external forces are in opposite directions, then their action tends to cancel out. If you push a body to the right with force $F_1$ and someone else pushes it to the left with force $F_2$, then $F = F_1 - F_2$. And, of course, if you both push with the same force $F_1 = F_2$, then $F = 0$ and the body does not move. (You might be squashing it, but that's not what you're looking for here!)

15. Dec 17, 2015

### Nick63

so then its just 30kg*-2.5m/s^2?

16. Dec 17, 2015

Yes.