# Questions from our Physics Periodical Exam

1. Jan 23, 2010

### yik-boh

First Question:

A 0.45 N block is given on initial velocity of 5.3 m/s along a horizontal surface. If the coefficient of friction between the block and the surface is 0.25, how far will the block move before coming to rest? How long will it take the block to stop?

Second Question:

A hospital attendant exerts a horizontal force of 60.0 N on a gurney with a patient on it and accelerates at 0.400m/s$$^{2}$$. The patient is 65.0 kg and the gurney is 30.0 kg. What is the magnitude of the frictional force opposing the motion?

Third Question:

A military plane wants to drop supplies to some survivors on a rocky ridge. The plane flying horizontally at an altitude of 235 m with a speed of 72.5 m/s. How far in advance must the relief goods be released so that they arrive precisely at the survivors' location?

Fourth Question:

The coefficient of sliding friction between a metal block and the inclined plane surface over which it will slide is 0.200. If the surface makes an angle of 20.0 degrees with the horizontal and the block has a mass of 80.0 kg, what force is required to slide the block at constant speed up the plane?

I know the formulas but I do not know how to use them properly. I mean I know how to derive from this to this but in this cases, these are problems I'm not familiar with so I don't know how to do. In usual cases, after my coach or teacher has solved them, I would just realized that I know how to do it and I know that type of solution but I just don't know that it's the one we're looking for or it's just that kind of solution I need.

I hope you could help me. :) Thanks!

2. Jan 23, 2010

### rl.bhat

First Question:
i) What is the mass of the block?
ii) What is the normal reaction of the force?
iii) What is the frictional force?
iv) What is retardation of the block?
v) What is the initial velocity of the block?
vi) What is the final velocity of the block?
vii) What is the relation between the initial velocity, final velocity, displacement and acceleration?
And finally
viii) What is the relation between the initial velocity, final velocity, acceleration and time?

3. Jan 23, 2010

### yik-boh

i) 0.460 kg
ii)
iii)
iv) I think I don't know what retardation of the block means. Sorry if I sound silly.
v) 5.3 m/s
vi) 0
vii) 3rd law $$V_{f}$$=$$\sqrt{V_{i}^{2}+2ad}$$
viii) We can get acceleration from $$a=\frac{V_{f}-V_{i}}{t}$$

Last edited: Jan 24, 2010
4. Jan 23, 2010

### yik-boh

5. Jan 23, 2010

### rl.bhat

ii) What is the normal reaction of the force?
fn = mg =0.45 N
iii) What is the frictional force?
fr = μ*fn
iv) What is retardation of the block?
Retardation means negative acceleration = fr/m.

6. Jan 24, 2010

### yik-boh

Now I've answered the First Question:

Displacement = 5.7 m and Time = 2.2 s

Thanks rl.bhat for helping! :)

Onto the next questions. :) Help! :D

Edit:

I tried solving the Second Question:

I came up with two answers that I'm not sure with.

first try: 22.0 N

What I did was I get the mass of the hospital attendant by $$\frac{60.0 N}{0.400 m/s^{2}}=150 kg$$ then I subtracted the mass total mass of the gurney and patient which is 95.0 kg from the mass of the hospital attendant which is 150 kg so, $$150 kg - 95 kg = 55 kg.$$ Then I multiplied 55 kg to the acceleration. $$55.0 kg \times 0.400 m/s^{2} = 22.0 N$$

second try: 98.0 N

Instead of subtracting the masses, I added them. $$150 kg + 95 kg = 245 kg.$$ Then I multiplied 245 kg to the acceleration. $$245.0 kg \times 0.400 m/s^{2} = 98.0 N$$

If what I did was silly, please tell me, I understand. :) Just please also correct me. :D Thanks!

Last edited: Jan 24, 2010
7. Jan 24, 2010

### rl.bhat

22 N is the correct answer. But the method is wrong.
F(net) = F(applied) - F(friction)
95*(0.400) = 60N - Fr.

8. Jan 24, 2010

### yik-boh

Why is the $$F_{w}$$ (Weight) of the gurney and patient is the Net Force?

Could you help me with the 3rd and 4th? :) Thanks. :D

9. Jan 24, 2010

### rl.bhat

Net force is m*a, not Fw(weight)
Show your attempt to 3rd and 4th problems. At least wright down the relevant equations.

10. Jan 24, 2010

### yik-boh

I know that net force is not the force weight. But why did you use the mass of the gurney and patient as the net force?

I'll try the 4th later. But with the 3rd one, I'm really having a hard time at projectile. My notebook is not here with me. My teacher is checking it.