How to understand Newton's 3rd Law

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In summary: So, even though the car travels at constant speed, the car is accelerating because it's direction of travel is constantly changing. It's constantly "turning" because it's constantly following the curve of the earth.The first car (the one that gets hit), is not moving, so when it gets hit it's just like you getting hit by a car while standing still. Your body accelerates backwards. The car accelerates backwards.When I throw a rock I cause a force on it and a rock causes equal force on me. So if I would stay at frictionless surface would I move backward when throwing rock?Yes.
  • #36
The same example as above:

u-velocities before interaction
v-velocities after interaction

So I know
m1=100kg u1=0m/s
m2=10kg u2=10m/s


\begin{equation}

v_1 = \frac{u_1(m_1-m_2) + 2m_2u_2}{m_1+m_2};\\
v_2 = \frac{u_2(m_2-m_1) + 2m_1u_1}{m_1+m_2}; \end{equation}

\begin{equation}
v_1 = \frac{0(100-10) + 2*10*10}{100+10};\\
v_2 = \frac{10(10-100) + 2*100*0}{100+10}; \end{equation}

\begin{equation}
v_1= \frac{200}{110}=1.(81)\frac{m}{s}
\end{equation}

\begin{equation}
v_2= \frac{-900}{110}=-8.(18)\frac{m}{s}
\end{equation}

So does it mean first planetoid will now travel at 1.(81)m/s in left direction and second at -8.(18)m/s in right direction (I guess "-" mean it will reflect and travel into direction opposite to the direction before impact) ?

These are very different results from attempt I made before and I used the same data ( I can't get it why previous method gives such different results)
 
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  • #37
ultrauser said:
So does it mean first planetoid will now travel at 1.(81)m/s in left direction and second at -8.(18)m/s in right direction (I guess "-" mean it will reflect and travel into direction opposite to the direction before impact) ?
Exactly. The smaller mass bounces back.

These are very different results from attempt I made before and I used the same data ( I can't get it why previous method gives such different results)
The given data (masses and initial speeds) are insufficient to determine the final speeds. Here, we assumed an elastic collision and got the results consistent with that assumption. Earlier you assumed an impulse (force X time) during the collision that would stop the second planetoid and thus got results consistent with that assumption. (If you work out the speeds for that earlier case you will find that energy is not conserved.)
 
  • #38
Just for fun. For the elastic collision case, assume that the collision took exactly 1 ms (just like your previous example). Figure out the average force during the collision and compare that value to what you had earlier.
 
  • #39
Doc Al said:
Just for fun. For the elastic collision case, assume that the collision took exactly 1 ms (just like your previous example). Figure out the average force during the collision and compare that value to what you had earlier.

I don't know how to do it (what formulas should I use) ?

What I calculated earlier was average force needed to stop second planetoid (F= m*v/t ?) instead of force of actual impact?
 
  • #40
ultrauser said:
I don't know how to do it (what formulas should I use) ?
You'd find the average force using mΔv/Δt.

What I calculated earlier was average force needed to stop second planetoid (F= m*v/t ?) instead of force of actual impact?
Right. You calculated the average force assuming that the second planetoid was stopped.
 
  • #41
F=10*(10 - -8.18)/0.001 = 181.8/0.001= 181800N ? (probably not xP)
 
  • #42
ultrauser said:
F=10*(10 - -8.18)/0.001 = 181.8/0.001= 181800N ?
Right. When it bounces back elastically, the average force (with our simple assumption) is almost twice as much as before. (Of course there's no real reason to think the collision time would be the same, but it makes the point.)
 
  • #43
There is one more thing I would like to know, what conditions needs to be met so the small planetoid m=10kg would hit big planetoid m=100kg and mantain it's direction (instead of being reflected)
 
<h2>1. What is Newton's 3rd Law?</h2><p>Newton's 3rd Law states that for every action, there is an equal and opposite reaction. This means that whenever an object exerts a force on another object, the second object exerts an equal and opposite force on the first object.</p><h2>2. How does Newton's 3rd Law apply to everyday life?</h2><p>Newton's 3rd Law can be observed in many everyday situations, such as when you push against a wall and feel the wall push back on you. It also explains how rockets are able to launch into space by pushing against the ground with a powerful force.</p><h2>3. What is an example of Newton's 3rd Law in action?</h2><p>An example of Newton's 3rd Law is when you jump off a diving board. As you push down on the diving board, the diving board pushes back up on you with an equal force, propelling you into the air.</p><h2>4. How is Newton's 3rd Law related to the other laws of motion?</h2><p>Newton's 3rd Law is one of three laws of motion created by Sir Isaac Newton. It is closely related to the other two laws, as they all describe the relationship between forces and motion. Newton's 1st Law states that an object will remain at rest or in motion unless acted upon by an external force, and Newton's 2nd Law explains how the acceleration of an object is directly proportional to the net force acting on it.</p><h2>5. Why is it important to understand Newton's 3rd Law?</h2><p>Understanding Newton's 3rd Law is important because it helps us to understand how forces work and how objects interact with each other. It also allows us to make predictions about the motion of objects and to design technologies that utilize this law, such as airplanes and rockets.</p>

1. What is Newton's 3rd Law?

Newton's 3rd Law states that for every action, there is an equal and opposite reaction. This means that whenever an object exerts a force on another object, the second object exerts an equal and opposite force on the first object.

2. How does Newton's 3rd Law apply to everyday life?

Newton's 3rd Law can be observed in many everyday situations, such as when you push against a wall and feel the wall push back on you. It also explains how rockets are able to launch into space by pushing against the ground with a powerful force.

3. What is an example of Newton's 3rd Law in action?

An example of Newton's 3rd Law is when you jump off a diving board. As you push down on the diving board, the diving board pushes back up on you with an equal force, propelling you into the air.

4. How is Newton's 3rd Law related to the other laws of motion?

Newton's 3rd Law is one of three laws of motion created by Sir Isaac Newton. It is closely related to the other two laws, as they all describe the relationship between forces and motion. Newton's 1st Law states that an object will remain at rest or in motion unless acted upon by an external force, and Newton's 2nd Law explains how the acceleration of an object is directly proportional to the net force acting on it.

5. Why is it important to understand Newton's 3rd Law?

Understanding Newton's 3rd Law is important because it helps us to understand how forces work and how objects interact with each other. It also allows us to make predictions about the motion of objects and to design technologies that utilize this law, such as airplanes and rockets.

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