View Full Version : A physics question

Brady Yoon

2004-May-13, 07:32 PM

In my science class, we're learning about forces and vectors, and I see a paradox. If Newton's 3rd law is true (it obviously is), and each force has an opposite and equal force, why do objects move? For example, if you kick a soccer ball, and the soccer ball pushes back, wouldn't it remain still?

I'm seriously confused... :)

The real question is, shouldn't you both move?

daver

2004-May-13, 07:43 PM

In my science class, we're learning about forces and vectors, and I see a paradox. If Newton's 3rd law is true (it obviously is), and each force has an opposite and equal force, why do objects move? For example, if you kick a soccer ball, and the soccer ball pushes back, wouldn't it remain still?

I'm seriously confused... :)

You apply a force to the soccer ball, the soccer ball applies a force to you. The soccer ball was motionless before your foot hit it, now it's heading out of bounds. Your foot was moving before it hit the soccer ball, now it's moving slower.

Mars

2004-May-13, 07:50 PM

When you kick the ball your leg is pushed back with the same amount of force you put into the ball, therefore slowing your foots motion. Since the energy you put into your leg is usually greater then what is needed to push the ball forward, your leg continues moving forward until it is stopped.

When you compact a spring it pushes back, when you release the spring to spring outward it does so with the same amount of energy you put into the coil. The spring doesn't stay compacted once you smash it.

bobjohnston

2004-May-13, 07:53 PM

The key is to remember that the two forces the third law is talking about are NOT acting on the same object. You push me, I push you back. We both get pushed but in opposite directions.

Eta C

2004-May-13, 08:06 PM

You omitted two more forces that apply here. One is the normal force that supports you on the planet's surface (think about it, gravity pulls you down, but you don't, usually, fall through its surface.) The second is friction. What's happening is that the ball is exerting a force on you, but that force is matched by the static friction between you and the ground. So as the ball pushes against you, the surface matches that force and keeps you in place.

Now the maximum amount of force static friction can provide is determined by the surfaces involved. Someone in cleats on a soccer field can have a lot. Try the same thing on ice (or on wet grass in street shoes) and you're likely to slip.

The best way to avoid seeming paradoxes like this is to draw what's called a free body diagram. Put in every force that acts, do the vector sums, and you'll come up with any net force.

Mars

2004-May-13, 08:11 PM

=D>

Sam5

2004-May-13, 08:35 PM

In my science class, we're learning about forces and vectors, and I see a paradox. If Newton's 3rd law is true (it obviously is), and each force has an opposite and equal force, why do objects move? For example, if you kick a soccer ball, and the soccer ball pushes back, wouldn't it remain still?

I'm seriously confused... :)

It has to do with the amount of your mass and the ball's mass.

The soccer ball moves because you are much bigger and weigh more than the soccer ball.

This is why in a car crash, the bigger more massive car doesn’t move around very much, while a little car moves a lot and may go tumbling down the highway.

If two cars of the same size and mass, both traveling 30 mph, hit each other head on, they will pretty much both come to a dead stop at the same time. But if a big heavy fully-loaded Mack truck hits a tiny little light-weight car, the little car will go flying off through the air while the truck will just absorb the force from the little car and not move out of place too much.

If you kicked a big heavy 4-foot wide soccer ball that is filled with lead, the ball will not move very much, but you will rebound considerably. So you must consider the overall mass of the two objects that meet at high speed. The one with the less mass will generally do the most moving.

Kullat Nunu

2004-May-13, 08:54 PM

The product of mass and velocity is called momentum:

p = m*v

A small exercise:

A car (mass m1 = 1,000 kg) hits a truck (m2 = 50,000 kg, v2 = 80 kph).

How fast the car has to drive to stop the truck? (Or, that they have same momentum?)

You see that mass is much more important.

Sam5

2004-May-13, 09:04 PM

The product of mass and velocity is called momentum:

p = m*v

A small exercise:

A car (mass m1 = 1,000 kg) hits a truck (m2 = 50,000 kg, v2 = 80 kph).

How fast the car has to drive to stop the truck? (Or, that they have same momentum?)

You see that mass is much more important.

Interesting, thanks. That explains why a 12 oz soccer ball can knock a 200 pound person to the ground if the soccer ball hits the person while traveling 200 mph. Whereas if the 200 pound person kicks the 12 oz soccer ball while the person is running at 10 mph, the ball goes flying through the air.

AZgazer

2004-May-13, 09:14 PM

The product of mass and velocity is called momentum:

p = m*v

A small exercise:

A car (mass m1 = 1,000 kg) hits a truck (m2 = 50,000 kg, v2 = 80 kph).

How fast the car has to drive to stop the truck? (Or, that they have same momentum?)

You see that mass is much more important.

A little faster and this Mini-Cooper (http://news.bbc.co.uk/2/hi/americas/3613715.stm) would have made it. Funny that I found this article, I had just described this problem to my oldest (Who I had solve the problem.) as a Mini Cooper hitting a Tank when she asked for relative examples of the car and truck. I googled for M3 speed and found a Mini that can go that fast! :wink:

pi is exactly 3

2004-May-13, 09:15 PM

Eta C. THats some good advice. I think if I asked my physics teacher brady's question he would have said word for word what you said. Personally I don't like using FBD's (free body diagrams) if I don't have to. But if you are having trouble visualizing how the forces interact then FBD's can really help you. Eta C, your real last name doesn't happen to start with a D. does it? #-o

George

2004-May-13, 10:37 PM

Shucks, everyone has already answered this one and in one of the few areas I understand (Newton). [After the 17th century I get a little fuzzy]

Once you see it, it's pretty simple thereafter. If you would like more, just holler.

Taibak

2004-May-14, 12:58 AM

Not to take away from the way everyone else explained this, but, if it helps, you might want to consider a situation where nothing is moving. Think about what happens if you balance a book on your head (try it!). You feel the weight of the book pushing on your head. However, your head is pushing back on the book, holding it up. That force is exactly as strong as the gravity pulling the book down, it's just pushing up.

And like bobjohnston said, the trick is remembering that third law pairs act on different objects. The book's weight is pushing on your head, your head pushes the book up. Your kick applies a force to the soccer ball, the soccer ball applies a force right back to your foot. Personally, I don't like free body diagrams either (if possible, I'd much rather look at these in terms of momentum), but if they work for you, by all means use them.

Normandy6644

2004-May-14, 03:22 AM

Another interesting (albeit somewhat off topic, but it still fits) consquence is that of impulse which is equal to the force multiplied by the time interval over which the force is applied. This is actually equal to the momentum! So we have

F(delta)t=m*v

Since the momentum of the object will be constant (after the initial conditions are applied), the only way to lessen the force is to lengthen the time. This is principle behind airbags. Your head has a certain momentum moving forward, and so the only way to lessen the force on your head (and thus save you a mean headache) is to increase the time. So you go right into the bag, taking more time, and decreasing the force. The same principle is behind why you should bend your knees as you hit the ground when you jump from a ledge or something.

Eta C

2004-May-14, 12:46 PM

Eta C. THats some good advice. I think if I asked my physics teacher brady's question he would have said word for word what you said. Personally I don't like using FBD's (free body diagrams) if I don't have to. But if you are having trouble visualizing how the forces interact then FBD's can really help you. Eta C, your real last name doesn't happen to start with a D. does it? #-o

Nope, a T (being cryptic). When I was in grad school at the U of Illinois I was a teaching assistant in the classical mechanics course Physics 106. The class started with statics and drawing FBDs was a big part of getting the visualization of a problem right. I don't know how many times I uttered that phrase (or something like it) during that year.

George

2004-May-14, 01:06 PM

It might be helpful, at this point, to take separate basic approaches to problems involving dynamic action and problems that can be taken as static problems. Impulse and momentum are essential when motion is involved but a simple free body diagram works great for static situations. Dynamics involves the 2nd law, whereas, the third law handles statics, such as the prior book on the head example. "Statics and Dynamics" is usually the name of one of the main foundational engineering textbooks.

There is a possibility that a more complete explanation will be found in the foreseable future for all three of Newton's laws. The "why" behind Newton's 3 laws may be just around the corner. When your head starts hurting from the weight of the book and you are taught that no "work" is being done (no movement), it doesn't seem right to me.

swansont

2004-May-14, 05:26 PM

When your head starts hurting from the weight of the book and you are taught that no "work" is being done (no movement), it doesn't seem right to me.

Because, like many terms, there is a specific physics definition and a lay definition for the word, and they aren't the same. e.g. in everyday use, velocity and speed are generally interchangeable, and acceleration means "speeding up." Physics has a much more specific definition for each of those terms.

SiriMurthy

2004-May-14, 07:19 PM

Another good example would be to push the train while being inside the train. Can't make the train move this way.

George

2004-May-14, 09:18 PM

When your head starts hurting from the weight of the book and you are taught that no "work" is being done (no movement), it doesn't seem right to me.

Because, like many terms, there is a specific physics definition and a lay definition for the word, and they aren't the same. e.g. in everyday use, velocity and speed are generally interchangeable, and acceleration means "speeding up." Physics has a much more specific definition for each of those terms.

My hope is that new discoveries in physics will reveal the inner workings behind these forces. As knowledge grows, so do terms to describe it but this may cause greater initial confussion for the uneducated and certainly more puns. For example, it's a no-brainer that M-Theory must deal with p-branes (like me). :) :wink:

Another good example would be to push the train while being inside the train. Can't make the train move this way.

You also know the example about the 500lb. Canary in a plane?

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