# Homework Help: Finding mass from F=ma

1. Feb 19, 2015

### Drew552

1. The problem statement, all variables and given/known data
Well i have the force which equals 500N and I don't have a given acceleration so should I be substituting 9.81 as the acceleration? or is their another way to find it given that I also have the friction which equals 125N. So which way is the right way of going?

2. Relevant equations
F=Ma

3. The attempt at a solution
Well what I've done is 500=m x 9.81, which goes to 500/9.81=m, which then gives me 51.02kg, so is that correct because it doesn't seem right to me.

Any help would be much appreciated,

Thanks.

2. Feb 19, 2015

### Staff: Mentor

Can you please state the exact problem you are asked to solve, word for word.

3. Feb 19, 2015

### Drew552

Its a fill in the chart question so its not really a word for word one, i have all the values but this one. If i can provide anything else please tell me.

4. Feb 19, 2015

### Staff: Mentor

You need to describe the situation a bit more. You have the force on what? What is it doing?

5. Feb 19, 2015

### Drew552

Well its on a friction simulator online. I'm pushing on a unknown mass with 500N of applied force. The friction force comes out as 125N and the sum of the forces is 375N to the right direction. I am asked to find the mass and the weight of the object. I want to know the mass of the object which in turn will let me work out the weight with W=mg.

Thanks.

6. Feb 19, 2015

### Staff: Mentor

OK. Across a horizontal surface?

So the only horizontal forces involved are the applied force and the friction?

Are you given any additional information? Such as the acceleration or the coefficient of friction?

7. Feb 19, 2015

### Drew552

Yes it is being pushed on an horizontal surface. And yes the only horizontal forces are the force and friction. No i am not given the acceleration but im assuming it could possibly be 9.81 and it doesn't give me the coefficient of friction.

8. Feb 19, 2015

### Staff: Mentor

That's the acceleration due to gravity. No reason to think the mass has that acceleration.

I do not see that enough information has been given to solve the problem. Does the diagram or chart have any other info in it? Or a previous problem?

9. Feb 19, 2015

### Drew552

Am i allowed to attach a picture of the chart and also the picture of the simulator??

10. Feb 19, 2015

### Staff: Mentor

11. Feb 19, 2015

### Drew552

.

12. Feb 19, 2015

### Drew552

13. Feb 19, 2015

### Staff: Mentor

So you have plenty of other info! Hint: Use the data for the other masses to figure out the coefficient of friction. (In fact, if you understand the chart, you don't even need to calculate the coefficient. You can just read off the mass that you need.)

14. Feb 19, 2015

### Drew552

I do? Well point taken then, thanks for the hint, ill try to to do my best at getting the answer but still unsure how to get the friction coefficient

15. Feb 19, 2015

### Staff: Mentor

Hint: What's the basic relationship for kinetic friction that uses the coefficient of friction?

16. Feb 19, 2015

### Drew552

F(kinetic)= u(coefficient) x F(normal)??

17. Feb 19, 2015

### Staff: Mentor

Exactly! And in these examples, what creates the normal force?

18. Feb 19, 2015

### Drew552

a entity pushing against something thus making the kinetic energy?

19. Feb 19, 2015

### Drew552

so if i had a 20N force pushing a against a box the normal force would equal 20N right?

20. Feb 19, 2015

### Staff: Mentor

The normal force is the force pushing each mass against the surface. What is that force?

21. Feb 19, 2015

### Drew552

The force is the 2 entities rubbing together?

22. Feb 19, 2015

### Staff: Mentor

Sure, but what supplies that force? It's not the applied force, since that force is horizontal. Hint: What's the only vertical force acting on the masses?

23. Feb 19, 2015

### Drew552

gravity?

24. Feb 19, 2015

### Staff: Mentor

Of course!

Now pick any one of the masses and use the given data to figure out the coefficient of friction (which is a constant for this problem).

25. Feb 19, 2015

### Drew552

so it would be 40=u+9.8 Assuming 40 is the mass