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Few Questions from Observations

  1. Aug 21, 2011 #1

    I have a few questions that I have from observing some everyday phenomenon:

    1) I was wondering, when one hits a brake on the car, why does the car lean forward and compress the front brake?

    2) If I have a plastic rotorblade (like on propeller) and I apply a force perpendicular to the plane of the blades (like if I push the blade with my hand), does the amount of deflection change if the blade was rotating at different speeds?

    3) My bike's brake is acting strange; when I flip the bike upside down (so wheel not touching the ground) and rotate the peddle as fast as I can and brake, the wheels stop instantaneously, but if I ride the bike (on flat plane), the brakes basically do not work (I basically feel like I'm traveling at a slower, but constant velocity), I was wondering why this is?

  2. jcsd
  3. Aug 21, 2011 #2
    1) inertia will cause the car to want to continue to move forward so it leans forward after you jam it

    2)in theory there is no Y force , assuming blade is rotating in x direction. so the amount of deflection is the same as the amt of force you use , regardless of the blade rotating or not.

    but in reality, since there is inertia, the blade will want to continue rotating in the x-direction and not want to be deflected downwards in the y-direction

    3)it probably has to do with inertia again? since when on the road, the system that the brakes have to stop is heavier(you plus bike)

    on the other hand, when the wheel is suspended in mid air, the brakes only have to stop the wheel

  4. Aug 22, 2011 #3

    Thanks very much for the response

    I get that inertia will cause the car to continue moving forward, but why does it lean heavily toward the front when it stops and not lean evenly on both front and the back?

    I'm wondering what you mean by y force (and what direction x and y is)? Also, I'm confused about the effect of inertia in this scenario.

    Thanks very much
  5. Aug 22, 2011 #4
    1) imagine you are a passenger in a bus, when the bus jams stop, you rush forward right? thats inertia, which states you tend to continue in your movement. no part of yourself tends to lean backwards right? because your original movement was only towards the front. (you don't have a backwards component to your velocity)

    so now back to reality, the passenger that you imagined earlier is the tiny molecules that make up your bike and you. so when you jam the brakes, everything is caught to an abrupt stop from their original forward velocity.

    2) oh, the y force is the direction of force you apply perpendicular to the blades, where i assumed that the blades are rotating in the x-plane

    you meant something like this right?


    where the blades rotate in x-axis( horizontal plane), the perpendicular force downards in the y-axis (vertical plane)

    so likewise, inertia is defined as
    the faster the acceleration of the blades, the higher its tendency to resist change, so when you press down on the blades( which were rotating in the x-plane), you will need more force for a faster rotation.
  6. Aug 22, 2011 #5

    The last thing I'm confused about is how inertia is a factor here since when I press down on the blades, it still spins in the same plane and from the perspective of each individual point I don't see how anything has changed? Is it because the moment of inertia has changed?

    Also, is there a way to quantify the difference in force required to deflect the blade depending on its RPM.

  7. Aug 22, 2011 #6


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    When you apply the brakes, the ground pushes backwards where the tires contact the pavement, and there's a forwards reaction force at the center of mass of the car which is above the ground. This creates one of the torques (twisting forces) that causes the car to pitch downwards. The brakes generate the other torque that causes the car to pitch downwards.
  8. Aug 22, 2011 #7

    This seems a bit confusing to me. I was wondering if it's appropriate to just say that the inertia of the car body "shifts" the CoG forward and causes the leaning.

  9. Aug 23, 2011 #8


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    Unless the suspension is sloppy, the CoG isn't going to significantly shift in any direction. The load (weight) applied to the tires does shift though. The leaning is due to torque. If the suspension was raised, the torque force would increase and the car would lean more. If it was high enough, like a monster truck, the car could do a "stoppie" (raise the rear tires off the ground).
  10. Aug 23, 2011 #9
    well, you see, there is contact friction between the blades and your finger which you pressed down.

    so in essence, your force is not actually perpendicular, but rather it will be diagonally downward to counteract the direction of the rotation of blades. so that your finger remains at the same spot and not get "washed away with the flow of the rotation". as such , you provide a horizontal (x-plane) force to counteract the spin and hence your resultant force is diagonally downward.

    with regards to quantifying the force , i am not sure , but you can try in terms of energy of the rotation instead.


    the link : scroll down to 'Scalar moment of inertia for single body'

    so by measuring how much the angular rotation shows down, you can find the change in energy, which would sort of quantify the problem? ??

    i don't know, some other pros probably have to come in to help you. :X
  11. Aug 23, 2011 #10


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    Place a box on a flat surface. Push one of the corners in direction along the edge. The box turns as it moves. That's because force you applied isn't along the line with center of mass.

    Same thing happens with the car. The braking force is applied bellow the center of mass in direction parallel with the ground. That cause the car to rotate forward.
  12. Aug 23, 2011 #11

    So if I only consider the component of the force perpendicular to the plane of rotation, it would be the same regardless of RPM? And also, since friction is independent of velocity, the applied force would only differ between the propeller moving and not moving?

    I'm wondering if I'm understanding this correctly: when a brake is applied, the brake grips the wheel and applies a force opposite to the direction of motion. The forward reaction force on the brake system/rest of the car causes a torque that is counteracted by the front suspension system and ground, is this correct?

    Also, with a brake like :


    would there be an downward reaction force on the car as well?

    Thanks very much
  13. Aug 23, 2011 #12


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    It's the braking force itself that's causing the torque. Under that torque, the car rotates forward, increasing compression of front suspension and reducing compression of rear suspension. That increases force on the front, keeping the sum of forces still equal to car's weight. Since now there is more force in the front, there is a torque to tilt the car back, counterbalancing the torque from the braking force.
  14. Aug 23, 2011 #13

    Can you go into more detail on this? I'm quite confused on how the braking can cause torque? What about the reaction force on the braking system itself?

  15. Aug 23, 2011 #14


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    You might be trying to picture it in more detail than necessary. Forget the wheels actually rolling and the torque brakes apply on the disks/drums. Just picture the braking force as a an external force opposing direction of travel and applied at wheels. What's important is that the point where the force is applied is not at the center of mass, and the force does not point straight at/away from center of mass. Such a force will always produce torque.
  16. Aug 23, 2011 #15

    So I should picture the whole car as one body but the braking force as external; also, this is the reason for the weight transfer to the sole cause of the weight transfer to the front wheel during braking and to the back during acceleration?

    I'm also wondering what is the effect of the reaction to the braking force?

    Thanks very much for helping me out
    Last edited: Aug 24, 2011
  17. Aug 24, 2011 #16

    yes,the applied force would only differ between the propeller moving and not moving, assuming your propeller is rotating at a constant speed

    because F=ma.
    if v was changing, a would change.
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