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Problems with the Force equation

  1. Dec 2, 2011 #1
    Ok I've been trying to figure this out for some time now
    We all know the formula F=ma
    now acceleration is measured a=(v-v0)/t

    now if there was an object in motion (lets say we measured the speed of a vehicle moving in a straight line at a constant speed of 10m/s over 30 seconds) this gives us v=10m/s and v0=10m/s and t=30s

    so a=(15-15)/30=0

    lets say the object weight 2000kg


    that gives us a F force of 0N

    so if this hypothetical vehicle is travelling at 15m/s it has a Force of 0N. But if it smashes into a wall that wall suffers damage due to applied force. Where does this force come from? Using Newton's equation the car has no force but we know from practicality that a car going 15m/s has a lot of force to exhert on a stationary object

    Is there another formula for F force if an object is at a constant velocity?
  2. jcsd
  3. Dec 2, 2011 #2
    That equation tells you that if the car is moving at a constant velocity then the sum of the forces acting on it are all canceling.

    It does have momentum, which is what will be transferred in a collision (Along with kinetic energy). Momentum is mv and kinetic energy is 1/2mv^2
  4. Dec 2, 2011 #3
    khemist said it. When something hits another object, you have an impulse. Momentum conservation states that momentum is always conserved. Because a car smashing into another object tries to transfer a large amount of momentum to the other object over a short period of time, a large force is transferred. Google Impulse-momentum change theorem.
    Another way to look at it is using Newton's third law. No net force is being exerted on the CAR, that is true. But the car exerts a force on the WALL. Those are two different things.
  5. Dec 3, 2011 #4
    The equation F = ma gives the RESULTANT force.
    It does not mean there is no force when the acceleration is zero. It means the forces 'cancel out'
  6. Dec 3, 2011 #5
    Here's a quote from a physics text I have:

    "A force acts on an object.

    Implicit in our concept of force is that a force acts on an object. In other words, pushes and pulls are applied to something - an object. From that object's perspective, it has a force exerted on it. Forces do not exist in isolation from the object that experiences them."

    You are trying to define your vehicle as a force existing in isolation from the object that experiences it. The reason you can't do that is because that same vehicle at that velocity can represent an infinite number of different forces depending on what it hits. It exerts one force on a stationary wall, another on a car heading right toward it at 60mph and yet another on a car heading away from it at 5 m/s, for example.

    Let's say the car hits the wall. NOW we have an acceleration. The car will go from 15 m/s to 0 in some short period of time. And whatever force that turns out to be, the wall will experience the same force.
  7. Dec 3, 2011 #6
    Cool thanks guys, I get it now. Makes perfect sense
  8. Dec 3, 2011 #7


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    Try Newton's First Law of Motion. No change - no force.
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