# Homework Help: Newton's 2nd Law Problems

1. Sep 11, 2008

### confusedguy87

I have a question:

Al and Bert stand in the middle of a large frozen lake (frictionless surface). Al pushes on Bert with a force of 20 N for 1.2 s. Bert's mass is 114 kg. Assume that both are at rest before Al pushes Bert.
(a) What is the speed that Bert reaches as he is pushed away from Al?
wrong check mark m/s

(b) What speed does Al reach if his mass is 72 kg?
m/s

For part A I did
F = MA
20N = 114kg * A
A = .18 m/s^2
.18 m/s^2 * 1.2s = .21 m/s
Is this correct?

As for Part B
Since 20N is applied to Bert, 20N is also applied to Al from Newton's 3rd law.
So F = MA
20N = 72kg * A
A = .18 m/s^2
.28 m/s^2 * 1.2s = .33 m/s
Is this correct?

Also another question

A person in an elevator is holding a 11 kg block by a cord rated to withstand a tension of 157 N. When the elevator starts up, the cord breaks. What was the minimum acceleration of the elevator?

Use Newton's Second Law of Motion
F=ma
therefore 157/11 = a = 14.27 m/s squared
This came out to be wrong, how come?

2. Sep 11, 2008

### Topher925

Is that a typo? If it is, then it looks correct to me.

As for the elevator question, it is wrong because you are not considering the force of gravity. The total force on the string will the gravitational and inertial force on the block.

Accel of gravity is 9.81 m/sec^2 btw.

3. Sep 11, 2008

### confusedguy87

Oh yeah it is a typo. Thanks!

Im not too sure what you mean by this sentence
"The total force on the string will the gravitational and inertial force on the block. "

what is the inertial force?

4. Sep 11, 2008

### confusedguy87

5. Sep 12, 2008

### alphysicist

Hi confusedguy87,

The F in Newton's second law is the total force, the sum of all the forces. You have included the tension, but what other force is acting on the block?

6. Sep 12, 2008

### confusedguy87

(50N + 9.81N)/7kg = 8.5 m/s^2?

7. Sep 12, 2008

### alphysicist

What do the factors 50N, 9.81N, and 7kg represent? I don't see where they come from.

In my last post I said that in Newton's second law

$$\vec F_{\rm net}=ma$$

the left side is the vector sum of all the forces. You have to decide what forces are acting on the object and put them all on the left hand side. There are two forces acting on the block, but you only had the tension on the left side. Which force are you missing? Once you fix that (being careful with signs) you should get the right answer.

8. Sep 12, 2008

### confusedguy87

50N is the force of the tension
-9.81 is the force of gravity
7kg is the mass of the block

Since is the sum of the forces is (50N - 9.81N)/7kg?

Am I missing any forces? Does the force of the elevator going upwards count?

9. Sep 12, 2008

### alphysicist

Did you move on to another problem? I was talking about the second problem in your original post:

where the tension was 157 N and the block's mass was 11kg. You do need the force of gravity, but it is not -9.81N. The acceleration of gravity is -9.81m/s^2, so what is the force of gravity on a 11kg block (in Newtons)? In other words, what is the weight (in Newtons) of an 11kg mass?

10. Sep 12, 2008

### confusedguy87

It was the same problem, Im sorry I confused you.

The weight is M * G.
So the weight of the 11kg mass is 11kg * -9.81m/s^2 which is -107.91N.

So the sum of forces are 157N - 107.91N = 49.09N
49.09N/11kg = 4.46 m/s^2

Is this correct?

11. Sep 12, 2008

### Redbelly98

Staff Emeritus
Yes.