Newtons 3rd law says that an object exerts the same force back. Then why is it that if I push hard enough then the object will move? Is there a name for the total amount of force an object can put against me?
In: Physics
The object does push back just as hard, at least if you consider how hard it’s pushing as force. But what you need to keep in mind is that the rate you move/accelerate is based on your mass, and that the force is only applied for as long as you are in contact.
So when you throw a ball you are applying a force to the ball, the ball applies force back. While the ball is in contact with you it keeps getting accelerated forward, while you are accelerated back. This stops as soon as you release the ball. You, however can tansfer thas force into the ground effectively combining your mass with the mass of the earth. Since that combined mass is many many times larger than the mass of the ball you move backward very little, and don’t really feel it, but it’s there.
The effect is a bit more apparent if you have two things of similar mass on a slippery surface, like if you were to push of someone else while standing on ice. Both you and the person pushed would end up moving away from your original spot at about the same speed.
According to the third law of motion, every action has an equal and opposite reaction. What I understand from your question is, if there’s an equal opposite reaction, why doesn’t the force get cancelled out? Why do things move?
Even though there is an equal opposite reaction, these two forces are not acting on the same object.
Since Force = Mass x Acceleration,
And newton’s third law states F1 = -F2,
Even if both the forces are equal, the masses aren’t equal, therefore the acceleration is adjusted to keep F1=-F2.
For example, if you jump on the ground, you exert some force on the ground and the ground exerts the same force on you. But the Earth’s mass is way more than yours, that’s why the Earth doesn’t move, you do.
I think you’re confused about where the force are applied. When you push against an object, you exert a force on the object, but the object push a reaction force against YOU, not the object. So both move. From the point of view of a stationary outsider, the entire you-object can be considered a single system, in which case the total force on the system is indeed 0, and in fact the system doesn’t move (its center of mass is stationary), it merely stretches out.
I feel a major component that’s being missed here is friction. You can always push an object and you will feel an equal force back, but the friction you have with the ground can make it hard to notice this. For instance, if you try and push something while on ice, you will go backwards.
Also, to piggy back on the ball throwing example someone else gave, if you were floating in space and you threw a baseball, both you and the baseball would push off with an equal force, however, F=ma. If you are 100 times the mass of the ball, then 100*a1 = 1*a2, where your acceleration is a1 and the ball’s acceleration is a2. All equations aside, your acceleration is going to be 100 times less than the baseball because your mass is 100 times more.
Another example where you’ll notice this kickback more is while bowling, especially in socks. The bowling ball is closer to your mass, so you can feel its effects easier, even with friction.
Think of a Newton’s cradle with two spheres, one larger and one smaller.
You pick up the smaller of the two and drop it (acceleration due to gravity). It will swing into the larger sphere and the larger sphere will move away from the now-stopped smaller sphere. The larger mass will make that distance moved quite a bit smaller than the original smaller sphere. When the larger sphere swings back to the smaller one, it imparts all of its momentum and the smaller sphere moves again, measurably farther than the large one.
F=ma. This means that the original force you applied by lifting and dropping the small sphere is constant. The mass of the small sphere is constant. The mass of the larger sphere is constant. The only thing left that is changing is the acceleration. The force applied to the small sphere moves the smaller mass further.
Another example. Throw a baseball. Your mass is large, the baseball’s mass is small. You are able to throw it very far because your muscles and tendons and torque and mass all combine to allow a certain amount of force and we know through experience that is more than enough force to launch it nicely.
Now go put a car in park on a flat surface, run up behind it as fast as you can and slam your shoulder into it (don’t do this for real). You have a small mass compared to the car, so the momentum imparted to the car is not going to be enough to do more than maybe rock it a bit.
Newton’s third law is about the interaction between two objects
When you push on a box with a force of 100 Newtons, the box must push back with a force of 100 Newtons or your hand would be pushing through the box though the resistance from the box will always match the force on the box but it’ll be something under 100
Consider putting a weight on a big foam block, what happens? The foam squishes until it’s pushing up with as much force as the weight pushes down
As for why objects are able to move, newton’s third law only looks at the forces at the interface but you need to look at the forces on the object as a whole. There’s 100 newtons pushing the box to one side, if friction isn’t matching that then there’s a net force on the box and it will slide. At each interface (hand/box and floor/box) forces are equal and opposite, but the friction doesn’t have to match the hand force, they’re not the same action
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