How can a shelf exert a force

The book then responds the only way a body subjected to an unbalanced force can - it accelerates in the direction of the force! The book moves down - but the table is is the way! The book moves down nonetheless - bending the table in the process. The book thus exerts a deforming force on the table, and the reaction to this is the restoring force that the table exerts on the book. It is this restoring force that we call the normal reaction. Being a reaction to the deforming force that the book exerts on the table, it is, indeed, equal and opposite to it.

But the question still remains - what makes it equal and opposite to the weight of the book? The answer to this is simple. As long as the normal force exerted by the table on the book does not balance the book's weight, the book will experience a net downward force - and it will continue to accelerate downwards.

This will bend the table even more - and with the increase in deformation, comes an increase in the magnitude of the restoring force. This process will continue until the normal force becomes equal and opposite to the weight. At this point the forces cease to change, and the book remains in equilibrium. Actually, things are a bit more complicated than that! By the time the normal force grows to match the weight, the book has already acquired some speed - so it overshoots.

This causes the normal force to grow further, exceeding the weight. So now, the net force is upwards, slowing down the book. It is easy to see that the book will move up and down a few times until dissipative forces will slow it down enough to halt it in the equilibrium position. All this is over in a very short time.

This is why we do not often notice this. Again, the deformation of the table in the process may be too tiny to notice unless someone is paying close attention. Of course, if you were to place the book on a soft bed instead of a table the deformation would be plainly visible.

Thus, the seat will constantly be changing the amount of reaction force throughout the day as students of different weight sit in it. A - All objects have inertia or a tendency to resist changes in their state of motion. This inertia is dependent solely upon mass and is subsequently not altered by changes in the gravitational environment.

To move an object horizontally, one must apply a force; this force will be resisted by the mass or inertia of the object. On the moon, the object offers the same amount of inertia as on Earth; it is just as difficult or easy to move around. C - A rope encounters tension when pulled on at both ends. The tension in the rope is everywhere the same. If team A were to pull at the left end, then the left end would pull back with the same amount of force upon team A. This force is the same everywhere in the rope, including at the end where team B is pulling.

Thus team B is pulling back on the rope with the same force as team A. So if the forces are the same at each end, then how can a team ever win a tug-of-war.

The way a stronger team wins a tug-of-war is with their legs. They push upon the ground with a greater force than the other team. This force upon the ground results in a force back upon the team in order for them to pull the rope and the other team backwards across the line.

For the next several questions, consider the velocity-time plot below for the motion of an object along a horizontal surface.

The motion is divided into several time intervals, each labeled with a letter. During which time interval s , if any, are there no forces acting upon the object? During which time interval s , if any, are the forces acting upon the object balanced? During which time interval s , if any, is there a net force acting upon the object?

During which time interval s , if any, is the net force acting upon the object directed toward the right? During which time interval s , if any, is the net force acting upon the object directed toward the left?

None - If an object is on a surface, one can be guaranteed of at least two forces - gravity and normal force. BDFH - If the forces are balanced, then an object is moving with a constant velocity. This is represented by a horizontal line on a velocity-time plot. ACEG - If an object has a net force upon it, then it is accelerating. Acceleration is represented by a sloped line on a velocity-time plot. CG - If the net force is directed to the left, then the acceleration is to the left in the - direction.

This is represented by a line with a - slope i. For the next several questions, consider the dot diagram below for the motion of an object along a horizontal surface. This is represented by a section of a dot diagram where the dots are equally spaced apart moving with a constant velocity or not even spaced apart at all at rest.

Once more, the Each object experiences a normal force equal to its weight since vertical forces must balance. So the friction forces for the 5. Using these F frict values and Newton's second law, a system of two equations capable of solving for the two unknown values can be written.

Note that the units have been dropped from Equations 3 and 4 in order to clean the equations up. Substituting this expression for F contact into Equation 3 and performing proper algebraic manipulations yields the acceleration value:.

This acceleration value can be substituted back into the expression for F contact in order to determine the contact force:. Again we find that the second approach of using two individual object analyses yields the same set of answers for the two unknowns.

The final example problem will involve a vertical motion. The approaches will remain the same. A man enters an elevator holding two boxes - one on top of the other. The top box has a mass of 6. The man sets the two boxes on a metric scale sitting on the floor. When accelerating upward from rest, the man observes that the scale reads a value of N; this is the upward force upon the bottom box.

Determine the acceleration of the elevator and boxes and determine the forces acting between the boxes. Both approaches will be used to solve this problem. The first approach involves the dual combination of a system analysis and an individual object analysis. For the system analysis, the two boxes are considered to be a single system with a mass of There are two forces acting upon this system - the force of gravity and the normal force.

The free-body diagram is shown at the right. The force of gravity is calculated in the usual manner using Since there is a vertical acceleration, the vertical forces will not be balanced; the F grav is not equal to the F norm value. The normal force is provided in the problem statement.

This N normal force is the upward force exerted upon the bottom box; it serves as the force on the system since the bottom box is part of the system.

The net force is the vector sum of these two forces. Now that the system analysis has been used to determine the acceleration, an individual object analysis can be performed on either box in order to determine the force acting between them. As in the previous problems, it does not matter which box is chosen; the result will be the same in either case. The top box is used in this analysis since it encounters one less force. The force of gravity is Since the acceleration is upward, the Fnet side of the equation would be equal to the force in the direction of the acceleration F contact minus the force that opposes it F grav.

F contact - Notice that the unrounded value of acceleration is used here; rounding will occur when the final answer is determined. Solving for F contact yields This figure can be rounded to two significant digits - 71 N. So the dual combination of the system analysis and the individual body analysis leads to an acceleration of 2. Now the second problem-solving approach will be used to solve the same problem.

In this solution, two individual object analyses will be combined to generate a system of two equations capable of solving for the two unknowns. We will start this analysis by presuming that we are solving the problem for the first time and do not know the acceleration nor the contact force.

Note that the F grav values for the two boxes have been included on the diagram. The contact force F contact on the top box is upward since the bottom box is pushing it upward as the system of two objects accelerates upward.

The contact force F contact on the bottom box is downward since the top box pushes downward on the bottom box as the acceleration occurs. These two contact forces are equal to one another since they result from a mutual interaction between the two boxes. The third force on the bottom box is the force of the scale pushing upward on it with N of force; this value was given in the problem statement.

Applying Newton's second law to these two free-body diagrams leads to Equation 5 for the 6. Now that a system of two equations has been developed, algebra can be used to solve for the two unknowns.

Equation 5 can be used to write an expression for the contact force F contact in terms of the acceleration a. This expression for F contact can then be substituted into equation 6. Equation 6 then becomes.

The same goes for the table. Now this is the important part - The weight isn't gravitational force. In the case of the table and the book, the gravitational attraction is absolutely negligible, since they are both so tiny. The force that the table experiences because of the book is what is being called normal force. The table then exerts an equal and opposite force.

This is also clearly seen, because if the table didn't exert an equal and opposite force, the book would be accelerating downward. But the whole system is at rest, therefore the total force on the book-table system must be zero. Basically normal force is only indirectly due to gravity. Khan Academy has a brilliant explanation of these concepts. So book-table has force pairs due to interaction forces, balanced and oppsite, call them normal due to book, normal due to table.

Both same kind. Book-earth has force pair due to gravity of each acting on other. Both same kind of forces, equal and opposite, and on different bodies. Table-earth, there is contact, which is electric interaction at electronic charge level. Equal, opposite yet same kind of force. Finally, each mass has gravity and the mass exerts force on other mass - NOTE: "on other mass!!!!

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 is a book on a table not an example of Newton's third law? Ask Question. Asked 9 years, 5 months ago. Active 5 years, 7 months ago. Viewed k times. Improve this question. The answer to this question is wrapped up in the same issues as the answer to your question about the ball.