Learn about the fact that forces come in pairs.
For example, if a baseball ball is applying force on a ball of 1000N and ball moves away. If the ball is applying the same force on the bat why doesn't the bat moves away? • (18 votes) Great question. There are two factors to consider. If you were to watch the collision from a car moving at v = +35m/s, you would see the bat initially at rest and finally moving at -15 m/s, so you would see it "moving away" from the collision. One final factor is that the player keeps pushing on the bat during the hit, so although the ball pushes on the bat equal and opposite to the bat pushing on the ball, there is additional force on the bat that tends to counteract the ball pushing on the bat. (47 votes) how did newton figure out his third law? • (24 votes) By noticing that an apple pushes downwards against his hand(because of gravity), as his hand pushes up on the apple(by holding it). (8 votes) Assume that I drop a ball from the 2nd floor to the ground. Why is it that the ball could not bounce all the way back to my position if Newton's third law state that the ground will exert an equal amount of force to the ball? • (15 votes) Energy is lost every time the ball bounces because of the air friction. In a (hypothetical) perfect vacuum where nothing acts on the ball except for gravity, the ball would bounce all the way back to the 2nd floor every time, forever (or until stopped by an external force). (14 votes) How does one differentiate between an action force and a reaction force if they are both "reacting" to each other? • (13 votes) Actually there is no difference between the two since they both occur at the same time. Action and reaction are just terms given for better understanding. (6 votes) Newton’s law states that if we apply force to an object, it will push back with the same amount of force in the opposite direction. So if I push a pen with 10 Newton it is supposed to push me back with 10 Newton too. And as the forces cancel out, the net displacement should be zero. Then how does the pen move? • (4 votes) Forces can only cancel out on a single object. In this scenario you experience a force of 10 N in one direction and the pen experiences a separate force of 10 N in the opposite direction. The net force on the pen is 10 N. There is no reason to add the 10 N on you to the pen because those 10 N are being exerted on you, not the pen. (10 votes) what if two boxes equal is mass and force collided in space head on • (1 vote) The answer to this depends on how elastic the collision is, see Coefficient of restitution (https://en.wikipedia.org/wiki/Coefficient_of_restitution). If kinetic energy is conserved, they bounce back with equal speed. If the collision has loss (i.e. they make a noise, their surfaces heat, they plastically deform etc), they bounce back with lower speed. Think of 2 cars in a head on collision. They crumple and don't bounce much at all - an inelastic collision. Note that momentum is maintained in all collisions: 2 objects the same mass travelling equal speed along the same line in opposite directions have a sum of 0 momentum. For momentum to be conserved they must have equal speeds in opposing directions after the collision. (7 votes) Why is lying in bed not an example of Newton's 3rd law? My text book says its because the forces acting on you come from a different interaction pair but doesn't explain further. • (4 votes) When you lie on your bed, you push on the bed and the bed pushes on you. That's Newton's third law. (5 votes) What would happen if Newton's Third Law didn't happen? Would a runner be able to run or would his force keep pushing at the ground with no result? • (5 votes) Yes this is true we are pulling up on the earth and the earth is pulling down in us so it’s and equal and opposite force we have the same magnitude but not mass at the earth (0 votes) If a push a table, that means it will apply a reaction force on my my hand.If the table moves forward, shouldn't my hand move backwards due to the force applied by the table? • (2 votes) No, because your hand is attached to your body, and your hand, like everything else, responds to the sum of all the forces on it, not just one particular force. (7 votes) "This law represents a certain symmetry in nature: Forces always occur in pairs, and one body cannot exert a force on another without experiencing a force itself. We sometimes refer to this law loosely as “action-reaction,” where the force exerted is the action and the force experienced as a consequence is the reaction." [ was seen in paragraph 3 ] • (3 votes) We can't know for sure, but we can be quite confident about it since but no one was able to disprove this theory up until now, with centuries of tries. (6 votes)Want to join the conversation?
First, the masses are different. The mass of a baseball is .145kg, while a bat has a mass of about 1.0 kg. So, from F = ma, this tells us that a = F/m, and so the acceleration of the ball will be about 7 times the acceleration of the bat. We know that the average acceleration is given by a = Δv/Δt, which tells us that Δv = a * Δt. Thus, since the Δt is the same for both, and the acceleration of the ball is 7 times bigger, the Δv of the ball will be 7 times bigger.
The Second factor is that the bat is already moving with a fairly high speed, and so its momentum is much greater than the momentum of the ball, at least in the frame of reference of the spectators. Thus, the bat is only slowed down, while the ball is turned completely around. Here is an example:
mass of bat = 1kg
mass of ball = (1/7)kg
initial velocity of bat = +35 m/s
initial velocity of ball = -35 m/s
final velocity of bat = +20 m/s
final velocity of ball = +70 m/s
You can see that the Δv for the bat = 20 - 35 = -15m/s, while Δv for the ball = 70 - -35 = +105m/s, which is 7 times as big as the Δv for the bat. 
At the same time, earth's gravitational force is pulling on you, and your gravitational force is pulling on earth. That's another example of 3rd law.
Note that 3rd law pairs have to be of the same type. contact force and contact force, in the bed/you situation. Gravity and gravity in the earth/you situation.
Now we can ask a different question: why are you stationary when you lie on your bed. That's because the contact force from the bed on you is equal to your weight. Here we have two different types of forces - the contact force and the gravitational force. The sum of those forces is zero, so you don't accelerate. That's Newton's 2nd law, F = ma.
QUESTION ; how do we know that forces always occur in pairs ? is it just due to Newton's third law ?
Also you can prove that this law holds for the types of forces we already know: gravitational, electromagnetic, weak and strong. The only way it could be wrong would be finding out new types of forces.
As an expert in physics and mechanics, I can confidently affirm the fundamental principle discussed in the conversation: Newton's Third Law of Motion. This law states that for every action, there is an equal and opposite reaction. The individuals in the discussion are grappling with various aspects of this law, and I will elucidate the concepts involved.
Let's break down the key points mentioned in the discussion:
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Forces Come in Pairs:
- Newton's Third Law asserts that forces always occur in pairs. This means that if object A exerts a force on object B, then object B simultaneously exerts an equal force on object A, but in the opposite direction.
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Example of Baseball and Bat:
- The question about a baseball hitting a bat is a classic illustration. The key to understanding why the bat doesn't move away as much as the ball lies in the concept of mass and acceleration.
- The formula (F = ma) (force equals mass times acceleration) is used to explain that, due to the difference in mass between the ball and the bat, the acceleration of the ball is much larger, leading to different final velocities.
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Newton's Third Law in Daily Life:
- Newton's Third Law is not limited to physics experiments. Everyday examples, such as lying on a bed or pushing a table, demonstrate the application of this law. When lying on a bed, for instance, the person exerts a force on the bed, and the bed exerts an equal and opposite force, keeping the person stationary.
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Conservation of Momentum:
- The concept of momentum is touched upon in the explanation of collisions. Even in scenarios where kinetic energy is not conserved due to factors like air friction, momentum remains conserved. The law of conservation of momentum is crucial in understanding the outcomes of collisions.
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Differentiating Action and Reaction Forces:
- A question is raised about how to differentiate between action and reaction forces. The response emphasizes that these forces occur simultaneously, making them challenging to distinguish in real-time. The terms "action" and "reaction" are essentially used for better conceptualization.
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Loss of Energy in Bouncing:
- Another participant highlights that energy is lost every time a ball bounces, primarily due to air friction. This brings attention to external factors influencing the outcome of physical interactions.
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Confidence in Newton's Third Law:
- The confidence in Newton's Third Law is expressed, stating that no one has been able to disprove the theory despite centuries of attempts. The law has been proven to hold for various types of forces, including gravitational, electromagnetic, weak, and strong forces.
In conclusion, the discussion encapsulates a range of concepts related to Newton's Third Law, from mass and acceleration to the conservation of momentum and the real-world applications of these principles in various scenarios. If you have any specific questions or need further clarification on these topics, feel free to ask.
