Cecily Zamora
Newton's third law states that every action has an equal and opposite reaction. This means that if object A exerts a force on object B, object B will exert the same force on object A, but in the opposite direction. Given this, it's reasonable to ask how anything can move at all. If you push a table with your finger, for example, the table pushes back with the same force, and you feel the opposing force. But why, then, can you push a box across a table? Shouldn't the box just push back with the same force, resulting in no movement? The answer is that the third law doesn't make movement impossible. It simply states that a force applied to an object will result in an opposite force being exerted by that object. The force needed to start movement, or to change movement (acceleration or deceleration), will result in an opposite force, but the object will still move.
| Characteristics | Values |
|---|---|
| Forces acting on different bodies | The force that body B applies back to body A acts on body A, so there is no cancellation of forces as they are acting on different objects |
| Action and reaction are not cause-effect | They are simultaneous |
| Acceleration of an object | Depends on its mass and the net sum of forces acting upon it |
| Net force | When you apply two equal and opposite forces to an object, the net force is zero, and nothing happens |
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What You'll Learn
Forces act on different bodies
Newton's first law of motion states that an object will not change its motion unless a force acts on it. This tendency to resist changes in a state of motion is called inertia. Newton's second law defines a force to be equal to the change in momentum (mass times velocity) per change in time.
Newton's third law states that for every action (force) in nature, there is an equal and opposite reaction. If object A exerts a force on object B, object B exerts an equal and opposite force on object A. In other words, forces always exist in pairs, acting on different bodies.
For example, when you push a table with your finger, the table applies the same force onto your finger, but in the opposite direction. However, the forces do not cancel each other out because they are acting on different objects. This is why you can still move the table, despite the equal and opposite forces at play.
The acceleration of an object depends on its mass and the net sum of forces acting upon it. It does not depend on the forces exerted by the object on other things. For instance, when you push a box on a table, you are applying a force that outbalances the force of the box on your finger and the friction between the box and the table. As a result, you can move the box, even though the forces are not technically cancelled out.
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Action and reaction are not cause-effect; they are simultaneous
Newton's Third Law of Motion states that for every action, there is an equal and opposite reaction. This means that if object A exerts a force on object B, object B will exert an equal force on object A, but in the opposite direction. This is often referred to as "action-reaction", but this terminology can be misleading as it implies a cause-and-effect relationship, with a lag between the two. Instead, these forces are simultaneous and act on different bodies.
For example, if you push a table with your finger, the table pushes back with an equal force in the opposite direction. This is not a force cancelling out the other, as they are acting on different objects. If the forces cancelled each other out, nothing would move. Instead, the table pushes back on your finger, and your finger pushes back on the table with an equal force in the opposite direction, and so on. This is why you feel resistance when pushing against an object.
The action-reaction forces are equal because the velocities of the objects involved change in proportion to their masses. This is in accordance with Newton's definition of force as the change in momentum (mass times velocity) per change in time. Therefore, the force exerted by an object depends on its mass and acceleration, not on the forces it exerts on other objects.
The "action-reaction" terminology is misleading because it suggests that one force is the initiator and the other is the response, when in reality, they occur at the same time. This is why some physicists prefer to refer to them as "third law pairs" or "force pairs".
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Acceleration depends on mass and net force
Newton's third law of motion states that for every action (force) in nature, there is an equal and opposite reaction. In other words, forces always exist in pairs. However, this does not mean that the forces cancel each other out and prevent motion. This is because the forces described by Newton's third law act on different bodies. For example, when you push a table with your finger, the table applies an equal and opposing force onto your finger. But this does not prevent you from being able to push a box on the table because the forces involved are acting on different objects.
Now, according to Newton's second law, a force can be defined as the change in momentum (mass times velocity) per change in time. Mathematically, this can be expressed as:
> F = (m1 * V1 - m0 * V0) / (t1 - t0)
Where F is the force, m is mass, V is velocity, and t is time.
Furthermore, acceleration is defined as the change in velocity divided by the change in time. For objects with a constant mass, Newton's second law can be simplified to:
> F = m * (V1 - V0) / (t1 - t0)
This equation demonstrates that force is directly proportional to mass and acceleration. Therefore, when mass is held constant, an increase in force will result in a higher acceleration.
Additionally, it's important to understand that the acceleration of an object depends not only on the force applied but also on the net force acting on the object. The net force is the vector sum of all the forces acting on an object. For example, if multiple forces are acting on an object in different directions, the net force will be the result of combining these forces while taking their directions into account.
In conclusion, while Newton's third law states that forces exist in equal and opposite pairs, this does not prevent motion because the forces act on different objects. Acceleration, on the other hand, depends on both the mass of an object and the net force acting upon it. The relationship between force, mass, and acceleration is described by Newton's second law, which shows that force is directly proportional to mass and acceleration.
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The reaction force is due to inertia
Newton's third law of motion states that for every action (force) in nature, there is an equal and opposite reaction. This means that if object A exerts a force on object B, object B will exert an equal and opposite force on object A. For instance, if you push a table with your finger, the table will push back on your finger with an equal force in the opposite direction.
Now, this leads to an interesting question: given Newton's third law, why is there motion at all? Shouldn't all forces cancel each other out, resulting in no movement? The answer lies in the concept of inertia and the fact that forces act on different objects.
When you apply a force to an object, you will feel the opposing force dictated by the third law. This "reaction force" is due to the inertia of the object. Inertia is an object's tendency to resist changes in its state of motion. In other words, an object at rest wants to stay at rest, and an object in motion wants to stay in motion. The magnitude of the reaction force depends on the mass of the object, with more massive objects having greater inertia and, therefore, exerting a stronger reaction force.
For example, if you push on something 100 times more massive than you, you will feel the reaction force for a long time because the object accelerates very slowly due to its high inertia. On the other hand, if you push a lightweight object like a ping-pong ball, you might not feel any reaction force because it accelerates quickly and the force is only present for a short time.
It's important to note that the forces described by Newton's third law act on different objects. So, when you push a box on a table, the force you apply to the box is an external force to the box-table system. The box then applies a force back to your finger, but these forces do not cancel each other out because they are acting on different objects. Therefore, motion is possible despite Newton's third law.
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Forces are vector quantities
Newton's third law states that for every action (force) in nature, there is an equal and opposite reaction. This means that if object A exerts a force on object B, object B will exert an equal force on object A but in the opposite direction. This is often referred to as "action-reaction".
However, this does not mean that the forces cancel each other out and prevent motion. Forces can only cancel out when they act on the same object. In the case of Newton's third law, the forces are acting on different objects, and therefore do not cancel each other out. For example, when you push a table with your finger, the table pushes back with an equal force, but in the opposite direction. This prevents your finger from passing through the table, but it does not prevent motion.
Now, onto the next part of your request: "Forces are vector quantities".
Forces are indeed vector quantities. Vector quantities have both a magnitude and an associated direction. This is in contrast to scalar quantities, which only have magnitude. Velocity, force, acceleration, and momentum are all vector quantities, as they have both a magnitude and a direction associated with them. For example, a force of 20 newtons to the left has a magnitude of 20 newtons and a direction of left.
Vector diagrams can be used to represent Newton's third law. These diagrams show the two forces in a third-law pair acting in opposite directions on different objects. The length of the arrows in the diagram represents the magnitude of the force, while the direction of the arrows represents the direction of the force.
In conclusion, while Newton's third law states that forces exist in equal and opposite pairs, this does not prevent motion because the forces act on different objects and therefore do not cancel each other out. Additionally, forces are vector quantities, which means they have both a magnitude and a direction associated with them.
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Frequently asked questions
Newton's third law states that for every action, there is an equal and opposite reaction. This means that if object A exerts a force on object B, object B exerts an equal force in the opposite direction on object A. These forces act on different bodies and do not cancel each other out. Therefore, the object can be moved.
Forces can only cancel themselves out when they act on the same object. In the case of Newton's third law, the forces are acting on different objects, so they do not cancel out.
The acceleration of an object depends on its mass and the net sum of forces acting upon it. When a force is applied to an object, it will accelerate more slowly if it has a greater mass. For example, it is easier to push a matchbox than a table because the table has a larger mass.
When a force is applied to an object, we feel the opposing force due to the object's inertia, which prevents it from being instantly displaced. The reaction force depends on the mass of the object, with larger masses resulting in a longer-lasting reaction force.
