Newton's Third Law
TLDRIn this AP Physics essentials video, Mr. Andersen explains Newton's third law, the principle of action and reaction. He illustrates how forces are exerted in pairs, equal in magnitude and opposite in direction, using examples like pushing a wall and a bowling ball. The video also demonstrates the concept in space, where the absence of a ground leads to different outcomes. To help understand these interactions, Andersen introduces free-body diagrams, which depict only the body of interest and its external forces, excluding internal forces and forces exerted by the body. The video aims to teach students how to use these diagrams to identify action-reaction pairs and calculate acceleration.
Takeaways
- 🚀 Newton's third law, also known as the law of action and reaction, states that for every action, there is an equal and opposite reaction.
- 🔍 Action-reaction pairs are always equal in magnitude and opposite in direction, and can be identified using a free-body diagram.
- 🤔 It can be challenging to identify action-reaction pairs in certain scenarios, but free-body diagrams help visualize and understand these forces.
- 🧱 When you push against a wall, the wall exerts an equal and opposite force on you, but you don't move because of the reaction force from the ground.
- 🌌 In space, without a ground to push against, applying a force to an object would cause both you and the object to accelerate away from each other.
- 🏃 While walking, the force you apply to the ground is met with an equal and opposite force from the ground, resulting in no net acceleration.
- 🎳 When pushing a bowling ball, the ball moves due to the force applied, but you don't move because of the frictional force between you and the floor.
- 📐 A free-body diagram includes the body of interest and all the external forces acting on it, excluding internal forces and forces exerted by the body itself.
- 🚫 Students often make the mistake of including forces exerted by the body in the free-body diagram, which should be avoided.
- 👨🦯 For a person standing on the ground, the free-body diagram includes gravity and the normal force, representing the action-reaction pair.
- 🤸♂️ In scenarios where there is no gravity, such as in space, the free-body diagram will not include a normal force, and the net force and acceleration will be determined by the remaining external forces.
- 📉 The acceleration of an object is in the direction of the net force acting on it, which can be determined through the analysis of a free-body diagram.
Q & A
What is Newton's third law of motion?
-Newton's third law of motion, also known as the law of action and reaction, states that for every action, there is an equal and opposite reaction. When one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.
Why does a cannon recoil when it fires a cannonball?
-The cannon recoils because of Newton's third law. When the cannon exerts a force on the cannonball to launch it, the cannonball exerts an equal and opposite force on the cannon, causing it to recoil.
What are action-reaction pairs in the context of Newton's third law?
-Action-reaction pairs refer to the forces that two interacting objects exert on each other. The action is the force exerted by one object, and the reaction is the equal and opposite force exerted by the other object.
How can a free-body diagram help in understanding Newton's third law?
-A free-body diagram can help identify the forces acting on an object and determine the overall acceleration. It includes the body and any external forces acting on it but excludes other bodies, internal forces, and forces exerted by the body itself.
What happens when you push against a wall?
-When you push against a wall, you apply a force (action) to the wall. According to Newton's third law, the wall applies an equal and opposite force (reaction) on you. However, since you are standing on the ground, there is no net force causing you to move.
What would happen if you pushed a wall in space where there is no gravity?
-In space, without gravity, there would be no normal force from the ground. If you pushed a wall, it would apply an equal and opposite force on you, causing you to accelerate away from the wall.
Why do you not move when pushing a bowling ball?
-When you push a bowling ball, you apply a force to it, and it applies an equal and opposite force on you. However, you do not move because you are applying a force against the floor, which in turn applies a force back on you, resulting in no net force and no acceleration.
What would happen if you pushed a bowling ball in space?
-In space, with no ground to apply a force against, when you push a bowling ball, it would apply an equal and opposite force on you. Both you and the ball would accelerate away from each other.
Why do you not move as fast as the bowling ball when pushing it in space?
-In space, when you push a bowling ball, both you and the ball accelerate away from each other. However, you do not move as fast as the ball because you have a greater mass, and acceleration is inversely proportional to mass when force is constant.
How does walking or running across the floor relate to Newton's third law?
-When you walk or run, you apply a force to the floor. According to Newton's third law, the floor applies an equal and opposite force back on you, which propels you forward. Without this reaction force, you would not be able to move.
What should be included in a free-body diagram?
-A free-body diagram should include the body in question and all the external forces acting on it. It should not include other bodies, internal forces, or forces exerted by the body itself.
What is the common mistake students make when drawing a free-body diagram?
-A common mistake students make is including forces exerted by the body itself in the free-body diagram, which should only include external forces acting on the body.
Outlines
🚀 Newton's Third Law: Action and Reaction
This paragraph introduces Newton's third law, which is the principle of action and reaction. Mr. Andersen explains that when a force is applied to an object, like launching a cannon ball, an equal and opposite force is exerted back on the object applying the force, causing recoil. This concept is exemplified by the large ropes on cannons to prevent them from moving. The paragraph also discusses the use of free-body diagrams to identify action-reaction pairs and calculate overall acceleration. Examples given include pushing a wall, which results in the wall pushing back with an equal force, and the scenario of being in space where the lack of a ground to push against results in acceleration away from the wall.
🎳 Forces and Acceleration in Bowling and Walking
This paragraph delves deeper into Newton's third law through the examples of pushing a bowling ball and walking or running. When pushing a bowling ball, the ball exerts an equal and opposite force on the person, but the person does not move due to the force applied to the ground and the reaction force from it. In space, without a ground, both the person and the ball would accelerate away from each other, with the person moving slower due to their greater mass. The paragraph also touches on the concept of constructing explanations using Newton's third law and the importance of identifying external forces in a free-body diagram, which should not include internal forces or forces exerted by the body itself.
Mindmap
Keywords
💡Newton's Third Law
💡Action and Reaction
💡Free Body Diagram
💡Force
💡Acceleration
💡Magnitude
💡Direction
💡Mass
💡Frictional Force
💡Normal Force
Highlights
Newton's third law, also known as the law of action and reaction, states that forces between two objects are equal and opposite.
The recoil effect of a canon is explained by Newton's third law, where the canon exerts a force on the ball and the ball exerts an equal and opposite force on the canon.
Action-reaction pairs are always equal in magnitude and opposite in direction, which can be visualized using a free body diagram.
A free body diagram helps in identifying forces and calculating overall acceleration.
When pushing a wall, the wall exerts an equal and opposite force, but you don't move because of the reaction force from the ground.
In space, without a ground to push against, applying a force to a wall results in both you and the wall accelerating.
Pushing a bowling ball on Earth results in the ball moving due to the force you apply, but you don't move because of the floor's reaction force.
In space, when pushing a bowling ball, both you and the ball accelerate away from each other due to the absence of external forces like gravity or the ground.
The difference in acceleration between you and the ball in space is due to your greater mass.
Walking or running involves applying a force to the floor, which in turn applies an equal and opposite force, allowing you to move.
In the absence of a floor in space, no action-reaction pair occurs, leaving you stranded without movement.
Free-body diagrams should include the body and all external forces acting on it, excluding other bodies and internal forces.
A common mistake is including forces exerted by the body itself in a free-body diagram, which should be avoided.
Examples of constructing free-body diagrams include a person standing on the ground and a person pushing against a wall.
In a free-body diagram for a person pushing a wall, include gravity, the normal force, and the wall's reaction force.
In space, a free-body diagram for a person pushing a wall would only include the force exerted by the wall due to the absence of gravity and normal force.
For a bowling ball, a free-body diagram would include gravity and the person's pushing force, with the acceleration directed down and to the right.
The video aims to teach viewers to construct explanations using Newton's third law, identify action-reaction pairs, and use free-body diagrams to analyze forces.
Transcripts
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