# Couple of Dynamic Questions

1. Jan 12, 2005

### Affirmed

Boredom...

I'm just plain stupid... (Don't know which equations to use...)

If a 256 kg spacecraft weights 436 newtons when it is on the moon, what is the gravitational field strength (in N/kg) there?

To slide a 9.0 kg object at constant speed along the ground requires a force of 14.3 N. What is the coefficient of friction?

You are pulling a 50 kg mass over a rough surface of friction coefficient µ = 0.15 and you are pulling the mass using a spring of force constant = 200 N/m. How much would the spring stretch?

How much does a 55 kg girl compress the spring in a pogo stick when she stands on it? You are given that the spring constant is 78 N/cm.

Last edited: Jan 12, 2005
2. Jan 12, 2005

### Curious3141

You should post any ideas you have. I'm sure you've studied *some* equations, otherwise you wouldn't need to be doing these.

3. Jan 12, 2005

### Curious3141

So what you're saying is that you're expected to solve all these questions without having ANY knowledge of the first thing about dynamics ? Because that's the impression you're trying to sell.

It's weak, dude. Unless you show that you've taken some effort, you'll get no help here.

4. Jan 12, 2005

### Affirmed

If a 256 kg spacecraft weights 436 newtons when it is on the moon, what is the gravitational field strength (in N/kg) there?

I would divide 436 N by 256 Kg = 1.703 N/Kg, unless you need to include the gravity for the moon.

To slide a 9.0 kg object at constant speed along the ground requires a force of 14.3 N. What is the coefficient of friction?

I would start off with µ = Fn/Ff
Fn = 9.0 * 9.8 = 88.2
Ff = 14.3 N

Then solve the equation as, µ = 88.2/14.3 = 6.17

As far for now...

$$\mu = \frac {FN} {Ff}$$

Last edited: Jan 12, 2005
5. Jan 12, 2005

### Curious3141

This answer and method is correct. You don't need to do anything more.

Your equation for $\mu$ is the wrong way around. Try solving it again, it should be less than one.

Use the same principles as in the previous question to find the kinetic frictional force resisting motion. Use Hooke's law $F = kx$ where k is given in the problem to determine $x$, the extension.

What is the weight of the girl in Newton ? Use Hooke's law again.

6. Jan 12, 2005

### Affirmed

To slide a 9.0 kg object at constant speed along the ground requires a force of 14.3 N. What is the coefficient of friction?

Yes I got it now, µ = Fn/Ff
µ = 14.3/88.2 = 0.16
(Thanks for telling me that the coefficient has to be less than 1)

You are pulling a 50 kg mass over a rough surface of friction coefficient µ = 0.15 and you are pulling the mass using a spring of force constant = 200 N/m. How much would the spring stretch?

Ff = µFn
Ff = (0.15)(490)
Ff = 73.5

F = kx
x = F/k
x = 73.5/200
x = 0.3675 m, 36.75 cm

How much does a 55 kg girl compress the spring in a pogo stick when she stands on it? You are given that the spring constant is 78 N/cm.

55 kg girl = 539 N
F = kx
x = F/k
x = 539/78
x = 6.91 cm

7. Jan 12, 2005

### Curious3141

Yup, they all look right.

8. Jan 13, 2005

### HallsofIvy

Staff Emeritus
You did the calculation correctly but the formula is &mu;= Ff/Fn.