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Need a demonstration of constant motion and balanced forces

  1. Oct 20, 2012 #1
    I'm a high school physics teacher and I need a way to demonstrate net force = zero when something is moving at a constant speed. Usually some get it right away, some don't care and a few just really can't get their mind around it. I tell them about the parachutist, their car going 60 mph in a straight line on a smooth road, and the airplane at level flight but some still insist more force needs to be applied to keep the airplane from falling out of the sky.
    I'm doing a demonstration Monday which shows balanced forces with the object at rest using spring scales, masses and pulleys. I'm also doing accelerated motion with a motion detector, car, and a mass hanging over the table, tied to the car and the string going over a pulley. But I can't think of a way to show balanced forces with constant, uniform motion.
    Any ideas? Even a computer simulation might help.
  2. jcsd
  3. Oct 20, 2012 #2


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    Just take the whole arrangement that shows balanced forces with the object at rest, and move it at a constant velocity.
  4. Oct 20, 2012 #3
    Thanks. Great idea and easy too.
  5. Oct 20, 2012 #4
    As AT has observed the acceleration has to be zero.

    Well you could have a sled or roller moving along a horizontal or inclined surface, with or without friction.

    The weight balances the normal reaction and the pull force balances the friction.

    All are directly measurable.
  6. Oct 20, 2012 #5
    We did a lab Friday where they found the coefficient of kinetic friction by pulling a sled horizontally at a constant speed and using a spring scale to measure the applied force which of course is equal to the frictional force.
    I'm not following the inclined plane. Wouldn't that cause the object to accelerate instead of move at a constant velocity?
    In the past, I've used the cart on an inclined plane to show acceleration and I've used a cart on an inclined plane with weights and pulleys to keep the cart at rest and but I haven't put the cart in motion. I'm concerned if I have different masses on the cart they will get more confused.
  7. Oct 21, 2012 #6
    Why an inclined plane?

    It really depends how complicated you want your balanced system of forces to be.

    The simplest is just two - the weight and the normal reaction on a horiz surface.

    You can add friction and a traction force.

    You can make it more complicated by inclining the plane.

    It's just a matter of choice. If you do it several ways you can show the result is not a fluke but always the same, no matter how complicated.
  8. Oct 21, 2012 #7
    You had mentioned horizontal or inclined. I was reading more into it. Thanks.
    I do the inclined plane with the car at rest because there is a test problem with a similar drawing stating the car is at rest, what can you conclude about the forces? (they are balanced).
    In the honors classes we get more into forces in two dimension but not much in the regular classes. Our state has decided all students must take physics so physics got less challenging.
  9. Oct 21, 2012 #8
    Yes the problem here is that all students must take the same physics (or whatever), regardless of ability. This reduces it to the lowest common denominator, which satisfies no-one, (except perhaps one person).

    go well
  10. Oct 21, 2012 #9
    LOL physics (or what ever). True.
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