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### Course: MCAT>Unit 9

Lesson 3: Force and Newton's laws of motion

# Newton's third law of motion

Newton's third law states that for every action there is an equal and opposite reaction. The "action" and "reaction" refer to forces; if Object A exerts a force on Object B, then Object B exerts an equal amount of force on Object A in the opposite direction. Examples include pushing an object, stepping on the ground, and rockets. Created by Sal Khan.

## Want to join the conversation?

• If there is an equal and opposite reaction for every action (force), what exactly is an unbalanced force? Are they just two separate ideas?
• This is a common misconception when the idea of action/reaction pairs is introduced. The point is that there is an equal and opposite reaction to every action, but these two forces are acting on different objects! So, for instance, if I kick a ball, I apply an unbalanced force to the ball, and the ball will accelerate in the direction of the applied force. The same force, in the opposite direction, will be applied by the ball on my foot. What will happen to my foot will depend on how firm is my standing on the football field ;-)
• I am confused about the last part of the video? why can not we move in space?? suppose if we try to push ourselves in some direction, why can not we??? i am very confused :( i mean where does my force go that i use in the attempt to move?
• There is no friction or air resistance that will support you to move. Whereas ,on earth we have friction while walking, air to sail a boat etc which is not there in space. Therefore, we are not able to push our self in space. Its just emptiness in space, Nothing is there to hold onto.
Hope it helps.
• the horse pulls the cart forward and the cart pulls the horse with an equal and opposite force both forces cancel each other neither the cart nor the horse should move but why does the cart move
• The cart and the horse move because the push of the horse on the earth is greater than the cart's friction with the earth. Also, if you look at the horse and the cart as a system, the total force on the cart and the horse is in the direction the system is going.
• Let there be a rock on a grassy surface. You applied a force on it and it started moving. Now, when you push the rock, the rock will also apply an equal and opposite force on your hand. This will result in a zero net force. Now, if there is zero net force on the body, how can it move? Please clear my doubt. Thanks
• Actually, guptaakshat0505, I believe you are incorrect. According to Newton's third law, the magnitude of the force that you exert on the rock is always exactly equal to the magnitude of the force of the force that the rock exerts on you. The rock moves because there is a net force acting on it. Zero net force occurs when two forces act in equal magnitudes in opposite directions on a single object. Here there are two objects each of which experiences the same magnitude force. Therefore, the rock will move. Does this help?
• ok, so why does this equal and opposite reaction happen? you explained only how it happens.
• We can't really answer why that happens. In our universe, one of the laws of physics is that momentum is conserved (which implies equal and opposite forces). Asking why this happens is like asking why masses attract other masses, or why positive charges attract negative, or why energy is conserved. Why are the laws of physics what they are? We can't say.
• So in space, why does it have to be a massive object that you throw to be propelled back to where you want to be? Won't any object do? Because regardless of whether you exert 100N of force on a 10kg object or on a 100kg object, it will still exert 100N of force back on you right? So why does it have to be the most massive object on you?
• Because a small object will very quickly accelerate out of reach of your hand, so it won't be able to exert force on you for very long, and therefore it won't accelerate you very much in the opposite direction.

Try it for yourself. Hold a small ball in your hand, jump in the air, and throw it. You didn't go backwards very much. Now stand next to a building, jump in the air, and push the building away from you. What happened to you?
• `If you are pushing the brick, the brick is exerting an equal and opposite force on you. That means you are being pushed back. How can you still push the brick?`
• You are correct in noting that when you push a brick, the brick exerts an equal and opposite force on you, as described by Newton's third law of motion. This phenomenon is often summarized as "action and reaction."

The reason you can still push the brick despite the equal and opposite force acting on you is due to the difference in mass and acceleration between you and the brick. Your force on the brick depends on your strength and the force you apply, while the brick's force on you depends on its mass and acceleration.

Consider Newton's second law, which states that force (F) is equal to mass (m) times acceleration (a), or F = ma. If you apply a force to the brick and it is heavier (has more mass) than you, it will experience less acceleration than you when you exert that force. In other words, the brick resists your force more because of its greater mass.

As a result, you feel the equal and opposite force from the brick, but it doesn't accelerate you much because you have much less mass than the brick. You can still push the brick if you apply a force greater than the force of static friction between the brick and the ground. Overcoming static friction allows you to set the brick in motion.

In summary, you can still push the brick because the equal and opposite force you feel from the brick doesn't accelerate you significantly due to the difference in mass and acceleration between you and the brick. Your force, if greater than the force of static friction, can set the brick in motion.
• Does the 3 laws of motion work in the space.
• Yes, all three of Newtons laws apply on earth as well as in space.