Opening a Curtain

Wow! Physics certainly is in everything we do. I say this because opening a curtain is one of those things that we do everyday without thinking about how or why we are able to do it.

In fact, opening a mini-blind has a lot to do with physics. First, the curtain is set on a pulley that allows a person to open the blind from the ground and with less force than

would be required without the pulley. This pulley shows the physics concepts of tension and Newton's second law, net force equals mass times acceleration. In order to open the curtain a

person must apply force to their side of the pulley that is greater than the downward force

of the mass times gravity (weight) of the mini-blind. The force creates tension on the rope and because the rope is the same in the handle and the rest of the curtain the tension transfers to the curtain and as soon as the force becomes more than the weight, the curtain will begin to accelerate upwards.

Because curtains have a large surface area, the rope that serves as a handle on one side of the pulley must be evenly distributed throughout the curtain. This explains the web of string

you see in a curtain as the evenly distributed string

allows the upward acceleration due to tension to be supplied

throughout

the curtain, thus making both sides accelerate upwards at the same time.

Chariot Collisons

This chariot colliding into a wall and then into a pillow is an excellent example of impulse and momentum, two concepts that we are now learning in Physics. When I was younger, I used to love playing with legos and toy cars, and now with knowledge of physics I can better explain what happens when they collide with a still object.

When the chariot collides with the wall, the force of its impact is greater than the force of its impact

when it collides with the pillow. This is because force equals the impulse (change in momentum) divided by

the change in time. When the chariot hit the wall the time it takes to change momentum is less than when it hits the pillow because the pillow slows the time that the chariot takes to impact, and therefore it changes momentum at a slower rate. Because the impulse is the same in both scenarios, when the chariot collides with the wall, its force of impact is greater than when it collides with the pillow because the impulse is divided by a smaller time, therefore making the force greater.

Pushing A Vacuum

Pushing a vacuum, though we may not think so, requires a lot of physics. First, the vacuum itself provides an excellent opportunity to draw a free body diagram where all of the forces on the vacuum are

labeled (see diagram).

When pushing a vacuum, you must first apply enough force (which must be compnonentized to find the horizontal component, since you apply force to a vacuum at an angle) to overcome the force of static friction (Fsmax=us(coefficient of static friction) x Fn). This will cause

the vacuum to accelerate forwards as the force of kinetic friction comes into play (kinetic friction opposes the direction of motion of the vacuum). The acceleration of the vacuum can be found using Fnet=ma, F-fk=mass x acceleration, acceleration=(F-fk)/mass.

Also because I am moving the vacuum forward, I am doing work on the vacuum. Work= force x displacement. Because I am applying force at an angle I must componentize the force vector to find the horizontal component of force, so Work=Force(cos(theta)) x Displacement. However, if I were applying force on the vacuum, but it was not moving, even though it may feel as though I am working hard at trying to move the vacuum, I would not be doing any work because their is no displacement of the vacuum. In order for work to occur, their must be both displacement and force.

Raising a Flag

This week as I was driving home I realized that there are about ten United States flags within a mile of my house, and all of them are raised and lowered everyday. Why is it possible to raise these flags while standing on the ground? Pulleys of course, I've always answered myself, but in physics these past few weeks I have come to understand how and why a pulley works. In general, a pulley sits at the top of an object, in this case a flag pole, and has a rope on top of it, crossing to either side (in the case of a flag the ends of the rope are connected to make a circle so that the flag will slide straight up and down the pole, but

this has a minimal effect on the applicable physics concepts). In this case a flag is attached to one end of the rope and a person holds onto the other. In order to raise the flag the person must apply force to

his side of the rope. The application of this force creates tension in the rope. In order

to make the flag accelerate upwards, the tension in the rope must be greater than the weight (mass times gravity) of the flag (assuming that the rope has negligible mass). This can be explained by the equation net force=mass x acceleration. When the tension has overcome the weight of the flag, the flag will accelerate upwards until the person stops putting force on the rope, at which time their will no longer be tension on the rope and the flag will stop moving. Fnet=ma, 0=ma, a=0.