Question on Phys

[Anonymous_]

Let’s say a ball (or any projectile) has a force of 500N travelling leftward in a linear direction. Let’s also say the opposing wind resistance force is 300N or something; why does the ball slow down and stop moving? Shouldn’t the ball still be travelling leftward since it’s still got 200N of force heading leftward?

[jamonwindeyer]

Let’s say a ball (or any projectile) has a force of 500N travelling leftward in a linear direction. Let’s also say the opposing wind resistance force is 300N or something; why does the ball slow down and stop moving? Shouldn’t the ball still be travelling leftward since it’s still got 200N of force heading leftward?

In your scenario, the ball would keep moving, and in fact, accelerating!! Usually though, a projectile doesn't have a force pushing it forward continually - You apply the force to get it going and from there no forces act in the horizontal direction except air resistance! So that 300N of air resistance (or however much) decelerates and eventually stops the projectile

[Anonymous_]

In your scenario, the ball would keep moving, and in fact, accelerating!! Usually though, a projectile doesn't have a force pushing it forward continually - You apply the force to get it going and from there no forces act in the horizontal direction except air resistance! So that 300N of air resistance (or however much) decelerates and eventually stops the projectile

Thanks for your response. So let’s ignore gravity and other resistances except for air resistance and the projectile’s force. So if the ball travelled leftward with a force of 500N and the air resistance force was 300N, how much would the force of the ball go down? Like what I mean is, 0 net force will mean that the object remains at constant velocity, but if 500N gets an opposing force of 300N as soon as it travels; does the projectile’s force decrease from 500N to 200N and then to -100N meaning it travels the other way? And how does negative force actually work?

[jamonwindeyer]

Thanks for your response. So let’s ignore gravity and other resistances except for air resistance and the projectile’s force. So if the ball travelled leftward with a force of 500N and the air resistance force was 300N, how much would the force of the ball go down? Like what I mean is, 0 net force will mean that the object remains at constant velocity, but if 500N gets an opposing force of 300N as soon as it travels; does the projectile’s force decrease from 500N to 200N and then to -100N meaning it travels the other way? And how does negative force actually work?

You can't say it is travelling with 500N of force because the force isn't there (besides gravity, and if you want, air resistance) after the launch. But the speed that it has obtained in the horizontal direction will decrease due to the retarding force of air resistance. So let's say the horizontal speed is 50 metres per second, and let's say the mass is 10kg - That 200N force will cause the object to decelerate by \(a=\frac{F}{m}=\frac{200}{10}=20\) metres per second. So one second later, it will be travelling 30 metres per second, then 10 metres per second.

However, it never travels the other way - Air resistance doesn't work that way. It isn't a constant force - It is a force that gets bigger the faster you go. So in reality that 200N will decrease as the object slows down!

You can sort of see why we don't consider air resistance in HSC Physics - It gets a little complex