# Homework Help: Quick question about normal force

1. Oct 27, 2004

### jlshoop

This may be very simple... but here is the question.

This is the question..

A woman at an airport is towing her 20.0 kg suitcase at constant speed by pulling on a strap at an angle of (theta) above the horizontal. She pulls on the strap with a 35.0 N force, and the friction force on the suitcase is 20.0 N. What angle does the strap make with the horizontal? What normal force does the ground exert on the suitcase?

I got the first part. The vertical component of the angle is 26 N. So if the suitcase is 20 kg and she is pulling vertically 26 N, shouldn't she be lifting the suitcase? how do i figure out the normal force which the ground exerts?

2. Oct 27, 2004

### Staff: Mentor

What's the weight of the suitcase? (Don't confuse 20 kg with 20 N !)
Use the fact that the vertical forces on the suitcase are in equilibrium.

3. Oct 27, 2004

### stunner5000pt

Ok first of all draw a free body diagram. SINCE she is moving at CONSTANT velocity, a =0 that menas the X omponents of the force are equal

then $$35 cos \theta = 20$$

there you have theta now

Now, for the vertical components

the bag is NOT moving up or down that means the forces up and down are equal

the fac that she pulls the bag at an angle means that there is a vertical component for her force that points upward. in any case

$$N + 35 sin \theta - mg = 0$$

do you understand why N is not mg??

4. Oct 27, 2004

### chroot

Staff Emeritus
If the suitcase is moving at constant speed, the horizontal forces acting upon it must be equal and opposite. If there's a 20N friction force opposing its motion, the strap must be exerting 20N of force in the direction of motion.

Draw a free-body diagram. Look at the triangle made by the strap's force, and its components. Solve for theta:

$$\cos \theta = \frac{20}{35}$$

The horizontal component of the force is 20N. The vertical component of the force is not 26N.

- Warren

5. Oct 27, 2004

### jlshoop

Warren- I typed the wrong thing... i meant 29 N for the vertical component, right?

another confusion i have is, is 20 kg the weight, or mass?

6. Oct 27, 2004

### chroot

Staff Emeritus
7. Oct 27, 2004

### chroot

Staff Emeritus
The kilogram is a unit of mass. To obtain the weight, you must multiply the mass by the gravitational acceleration, 9.81 m/s^2, in accordance with F=ma.

- Warren

8. Oct 27, 2004

### jlshoop

so the weight is 216

N = 216 - 29 = 187

BY GEORGE THINK I'VE GOT IT....

yeah?

9. Oct 27, 2004

### jlshoop

Thanks very very very much Warren... you really truely rock.

10. Oct 27, 2004

### chroot

Staff Emeritus
So do stunner500 and Doc Al. :)

- Warren

11. Oct 27, 2004

### jlshoop

I agree with that... didn't mean to sell anybody short on their physics talent :^)