Everyday Example [1] In the diagram below, we see a person placing a bag of dog food on a table. The person holding the bag of dog food must supply an upward force equal in size and opposite in direction to the force of gravity on the food. The card table sags when the dog food is placed on it, much like a stiff trampoline. Elastic restoring forces in the table grow as it sags until they supply a normal force equal in size to the to the weight of the load.
Image credit: University Physics. Reinforcement Exercises: Normal Force. OpenStax CNX. Air Force Chief Master Sgt. Previous: Center of Gravity. Next: Slipping. Share This Book Share on Twitter. Sometimes questions like this are asked to make the point that mechanical equilibrium ME should not always be assumed as a given.
And also that the definition of ME states that both the net forces and net torques must total to zero. The question does not explicitly state that the system is in mechanical equilibrium, and it says nothing about the shape of the box.
If the system is not in ME, then there is no reason for the forces to have to be equal. The box could be in a state of translational or rotational acceleration.
Note there may be no ME even if the normal force on the floor and the weight are equal and opposite. The box could be tipping over because it has an asymmetric shape that causes the force normal to the floor to not be directly in line with the center of mass. The resulting net torque on the box could be tipping it over. I voted up the previous answer because I liked its elegance stack on a second box, but decided to post this anyways because I suspect hope the question was suppose to be more about physics than a trick.
Sign up to join this community. The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Learn more. Why might the normal force on a box not be equal to its weight? Ask Question. Asked 9 years ago. Active 9 years ago.
Viewed 10k times. If an object at rest on a table weighs , what force does the table exert on the object? Normal force is defined as the force that a surface exerts on an object.
If the object is at rest, net force on the object is equal to zero; therefore, the downward force weight must be equal to the upward force normal force. Since weight acts in the downward direction it will be negative. The total sum of the forces must be zero, in order for the object to be at rest.
This is also in accordance with Newton's third law, which suggests that the normal force will be equal and opposite the force of weight. A wooden cube is resting on a table. The cube has a mass of kg. What is the magnitude and direction of the normal force? Normal force is the force that the table pushes onto the block. If the force on the block from gravity is pushing the block downward, then the normal force has to push straight upward to keep the block at rest.
Since the block is not moving, these two forces must be equal and opposite to produce a net force of zero. Since normal force is pushing upward, it will be positive. The force of gravity would be negative. A box sits on a ramp at an incline of 30 degrees. If the box has a weight of 50N, what is the normal force experienced by the box? The correct answer is 43N. Since the box is on an incline, normal force balances with the component of gravity that is perpendicular to the surface of the incline.
Note that the normal force is in the upward positive direction, while gravitational acceleration and the force of gravity are in the downward negative direction. Normal force always acts perpendicular to the contact surface.
It can act counter to gravity or to any other force that pushes an object against a surface. Think of normal force as the force that keeps a surface solid; without normal force, an object would pass right through a surface. With this in mind, consider a book that you push against a wall in the horizontal direction such that it is in equilibrium and does not slide down the wall.
In this case, normal force is acting counter to your pushing force along the horizontal axis and perpendicular to the force of gravity. Another example is any object placed on an inclined plane; in this system the normal force will act perpendicular to the incline and will only counter a portion of the gravitational force. If you've found an issue with this question, please let us know. With the help of the community we can continue to improve our educational resources.
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Hanley Rd, Suite St. Louis, MO Subject optional. Email address: Your name:. Example Question 1 : Understanding Normal Force. Possible Answers: None of these laws. Correct answer: Newton's third law.
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