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B Can you please help me understand free body diagrams

  1. Oct 30, 2016 #1
    Free body diagrams are where you simplify each object to a point and draw all of the forces acting upon it.

    I am having difficulty understanding it entirely and it would really help to just have a bunch of examples of free body diagrams. If you have a link to a bunch that would be great.

    I also need help because I can not figure out how to draw free body diagrams for the following. If you can help explain principles that would help me draw them, draw them, or describe them then that would be great.
    1. A box that is on an incline but is not moving
    2. Box that is pushed by a human with horizontally applied force at constant speed
    3. A box is accelerating while moving down an incline
    4. Two boxes on other sides of pulleys, the system is accelerating.

    Also is normal force smaller if an object is being pushed even if the push is entirely horizontal? For example a rope that is pulling a box at an angle, the box has a smaller normal force then if it were just sitting there.
  2. jcsd
  3. Oct 30, 2016 #2


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    I'm not sure what you're looking for, but for most such problems, the important things to remember are:
    1. The normal force on an object sitting on a surface is always normal (perpendicular) to the surface, and is directed away from the surface.
    2. The friction force on an object sitting on a surface is always directed parallel to the surface, in the opposite of the direction of would-be motion. That's a little complicated to figure out sometimes. You have to figure out what direction the object would move in without friction, and then the force of friction is in the opposite direction from that.
    3. The magnitude of the normal force and the friction force is variable; you usually have to figure them out on a case-by-case basis.
    4. A rope or string can only pull. So the direction of the force is always in the direction the rope is running.
    5. For an ideal rope and pulley (ignoring friction and the weight of the rope), the tension in the rope is the same everywhere along the rope. The tension is equal to the force exerted due to the rope on whatever objects the rope is connected to.
    6. In every direction, [itex]F = ma[/itex] holds. That is, you have to compute the forces in each direction separately, and consider the acceleration in that direction.
    The free-body diagram looks the same, regardless of whether the object is accelerating or not, whether it is moving or not. But you have to take the acceleration into account in computing the magnitude of the forces.
  4. Oct 31, 2016 #3


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    See from 0 to 4min 55seconds.

    Note: This covers a stationary box pushed by a human against static friction (eg Constant velocity of zero!). In the case of a box moving at a non zero constant velocity the free body diagram is the same (Applied force = Friction) except now the friction force is that due to kinetic friction rather than static friction.

    More than one free body diagram might be needed. In this example they ignore the moment of inertia of the pulley (pulley is massless)..

    If the object is on a horizontal surface AND the push (Applied Force) is horizontal THEN the Applied Force does not change the Normal force. This is because they are orthogonal (90 degrees to each other) meaning the Applied Force does not have a component in the direction of the Normal force.

    IF the object is on an incline and the Applied Force is horizontal THEN the Applied Force will change the normal force (because the applied force has a component in the direction of the Normal force). Note the Applied force can increase OR decrease the Normal force depending on the set up.

    That depends on the angle of the rope. It is possible for the tension in the rope to increase the Normal force if the angles are right.
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