1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Center of Mass, Equilibriums help!

  1. Jul 22, 2007 #1
    1. The problem statement, all variables and given/known data
    Hey, I'm just doing a physics assignment and I seem to be having some issues. Its all written answers and no calculations are needed. We are meant to learn this topic by ourselves (over the holidays) and that's why I'm having some trouble understanding some of the concepts. I need some help elaborating and explaining the topics. Like why does lowering the center of mass help a jumper ect.

    Q1. Research and explain 3 types of equilibrium. (are mine correct?)
    Q2. Explain why a truck standing still on an inclined road may or may not tip over. You need to discuss "restoring" and "non-restoring" torques.
    Q3. Explain why High Jumpers can reach greater heights doing the "Fosbury Flop" rather than the more traditional "scissor" or "straddle" jumping styles.

    2. Relevant equations
    No equations. No calcs needed just explanations using diagrams and theorems.

    3. The attempt at a solution

    http://img508.imageshack.us/img508/3073/stablecopyza7.jpg [Broken]
    (picture not displaying for some reason...)

    As we can see from the above picture, if the center of mass lies outside the range of the base of support the system is unstable and in this case will topple.
    If a system is applied torque and this causes the system to topple or reach a new equilibrium (unstable equilibrium) then this torque is classified a non-restoring torque. i.e. After applying a non-restoring torque the center of mass moves below the base of support. For example, if a person was to lean over to tie up their shoes they can do so without falling over as long as their center of mass stay above their base of support, the feet.
    If a system is applied torque and this causes the system to return to its original equilibrium (stable equilibrium) this is known as a restoring torque.


    The Fosbury Flop is been the technique of choice for high jumpers since it was introduced into the sport in 1968. The athlete sprints diagonally towards the bar, jumps off his/her outside foot, then curves and leaps head-first, back downwards over the bar in a rolling motion keeping as much of the body as possible below the bar. When high jumpers perform this jump, they bend their body in such a way that it is possible for the athlete to clear the bar while his or her center of mass does not.
    The main reason the flop is superior to previous high jump techniques (straddle, scissors, Eastern cutoff and Western roll) is that it allows the athlete to jump with a lower center of gravity, meaning more height can be cleared from the same energy input. It's possible for a jumper's center of gravity to pass beneath the bar using the Fosbury Flop technique.


    Mechanical Equilibrium (Static Equilibrium):
    For a system to be in mechanical equilibrium the force and reaction are balanced and the system remains unchanged over time. Or in other words the net forces and moments must equal 0 (i.e. no rotational or translational acceleration).

    Stable Equilibrium:
    The system is said to be in stable equilibrium when after disturbed the system displays tendencies to return to its original equilibrium.
    e.g. A square tool (used for checking right angles in many jobs) tapped slightly returns to its original stable position after wobbling (returns to equilibrium after disturbance). The center of mass after the disturbance was higher than it was originally allowing the square to return to its original equilibrium.

    Unstable Equilibrium:
    A system is in unstable equilibrium when after a disturbance, the system return to a new but different equilibrium.
    e.g. The same square tool used in the stable equilibrium is positioned differently and then tapped which causes it to topple and stop laying flat on the table. A new equilibrium has been reached. The center of mass in this example has moved downwards after the disturbance which causes the square to fall reaching a new equilibrium.
    Last edited by a moderator: May 3, 2017
  2. jcsd
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Can you offer guidance or do you also need help?
Draft saved Draft deleted