Projectile motion and mass

[Jengreef]

I've never been able to fully grasp this concept. But I keep reading that ideal projectile motion doesn't depend on mass, however when we're talking springs, air resistance, or transfer of energy from the source to the projectile, mass is a considerable factor (such as in football, where the energy from the foot transfers to the ball, apparently mass of the ball is significant here).

That last scenario always throws me off. When is projectile motion ever not the result of the transfer of energy? If you throw a ball, energy transfers from your arm to the ball. You do anything to project any object into the air, the energy to launch that object needs to come from some source as you don't see objects spontaneously launching themselves without energy input. Then is mass always a considerable factor? Obviously not, but I can't figure out the line at which mass needs to be considered and where it shouldn't be.

At this point it just seems like questions arbitrarily state that mass is important for this particular projectile motion problem, yet in another one mass is irrelevant.

Are there a finite number of scenarios where mass must be taken into consideration for projectile motion? or is this really as arbitrary as it appears to me?

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[BB8730]

When is projectile motion ever not the result of the transfer of energy?

Projectile motion never deals with the transfer of energy. Projectile motion only deals with the motion of the projectile. It doesn't matter how that projectile got there or what it does when the trajectory stops - projectile motion only deals with the path it takes.

At this point it just seems like questions arbitrarily state that mass is important for this particular projectile motion problem, yet in another one mass is irrelevant.

Are there a finite number of scenarios where mass must be taken into consideration for projectile motion? or is this really as arbitrary as it appears to me?

Mass only matters when there are external forces (other than gravity) on the object, such as air resistance.

 

[Jengreef]

If projectile motion never deals with energy transfer, why is it that in slingshot or spring-loaded projectile problems, mass must be considered to determine the distance traveled by the projectile?

 

[BB8730]

If projectile motion never deals with energy transfer, why is it that in slingshot or spring-loaded projectile problems, mass must be considered to determine the distance traveled by the projectile?

Those problems are much more than just projectile motion. A slingshot or spring will apply a force to the object, so mass needs to be considered.

A projectile motion question would be something like this: "A cannonball is launched at a 30 degree angle at 50 m/s on level ground. Ignoring air resistance, how far does it travel?" In a question like that, there are no forces (besides gravity) making it a projectile motion question where mass is irrelevant.

 

[Jengreef]

Ok, I think I'm almost understanding it now, just maybe one more question.

I get the feeling that I need to read between the lines a bit to know whether or not 'a force is applied to a projectile'. Are there definite flags that define when this is the case? Because it appears to me that a force needs to be applied to launch a cannonball just the same as anything else. And only examples seem to define whether a force is applied or not (meaning now I have memorize 'force applied to object' scenarios and those that aren't, and if I come across a particularly special case, I'm screwed). Is there a general statement that could better define these conditions?

 

[milski]

A force has been applied at some point in all projectile motion problems. In a pure PMP you're given the result of what the force did - the projectile is launched at such and such speed and such angle from the ground and you don't need to worry exactly how strong that force was, how strong the spring in the cannon was, how heavy the projectile is, etc. In more complicated problems, you'll have to determine that initial velocity from some other information that was given to you - mass of the projectile, direction of the spring, spring coefficient, etc.

Once the projectile is launched and you know its initial velocity, it does not matter how the launch was done - it would normally have only one force acting on it, gravity, and will behave the same way.

 

[BB8730]

Ok, I think I'm almost understanding it now, just maybe one more question.

I get the feeling that I need to read between the lines a bit to know whether or not 'a force is applied to a projectile'. Are there definite flags that define when this is the case? Because it appears to me that a force needs to be applied to launch a cannonball just the same as anything else. And only examples seem to define whether a force is applied or not (meaning now I have memorize 'force applied to object' scenarios and those that aren't, and if I come across a particularly special case, I'm screwed). Is there a general statement that could better define these conditions?

The question will provide you with some kind of energy or force data if it's meant to be more than just projectile motion - in which case you'll need to consider the mass. If the question only provides you with motion data, like velocity, angles, time, etc, and says "ignore air resistance" then it's projectile motion and you don't need to consider mass.