M1 question. Not exam style.
So when a ball hits a wall straight on, it rebounds with the same speed due to the consv.of linear mom. Something causes it's velocity to fall significantly though and decelerate quickly. Something more than gravity and air resistance. I haven't done much m2, so I was wondering if it had something to do with lost energy, perhaps elasticity or something?
Any info would help. Not looking for any actual maths, just the idea of why it would lose velocity.
Any info would help. Not looking for any actual maths, just the idea of why it would lose velocity.
Original post by Student10011
So when a ball hits a wall straight on, it rebounds with the same speed due to the consv.of linear mom. Something causes it's velocity to fall significantly though and decelerate quickly. Something more than gravity and air resistance. I haven't done much m2, so I was wondering if it had something to do with lost energy, perhaps elasticity or something?
Any info would help. Not looking for any actual maths, just the idea of why it would lose velocity.
Any info would help. Not looking for any actual maths, just the idea of why it would lose velocity.
The wall provides an impulse that causes its momentum to change and momentum=mv so its velocity decreases.The wall exerts an impulse because impulse=force*time and every reaction(the ball acting on the wall) has an equal and opposite reaction(the wall acting on the ball) so this impulse is that force*the time the ball is in contact with the wall.
Well assume the ball is initially travelling in the positive direction. After it has rebounded from the wall, it is travelling in the negative direction. Hence it's velocity has "fallen" and it has undergone rapid deceleration. Of course if you define your axis such that the ball is initially travelling in the negative direction the opposite of this is true (velocity increases, rapid acceleration), but it amounts to the same thing. Is this what you mean?
If it has the same speed, it can't really have lost energy can it?
If it has the same speed, it can't really have lost energy can it?
Original post by Dalek1099
The wall provides an impulse that causes its momentum to change and momentum=mv so its velocity decreases.The wall exerts an impulse because impulse=force*time and every reaction(the ball acting on the wall) has an equal and opposite reaction(the wall acting on the ball) so this impulse is that force*the time the ball is in contact with the wall.
Momentum hasn't changed though. mv = mu. Mass is constant, speed remains the same, so momentum hasn't changed. At the instance the ball leaves the wall its speed is equal to the speed with which it hit the wall.
Original post by Student10011
Momentum hasn't changed though. mv = mu. Mass is constant, speed remains the same, so momentum hasn't changed. At the instance the ball leaves the wall its speed is equal to the speed with which it hit the wall.
In the M1 questions on Edexcel the wall always exerts an Impulse which causes the balls momentum to change
Original post by Student10011
Momentum hasn't changed though. mv = mu. Mass is constant, speed remains the same, so momentum hasn't changed. At the instance the ball leaves the wall its speed is equal to the speed with which it hit the wall.
v=final VELOCITY
u=initial VELOCITY
Original post by Dalek1099
In the M1 questions on Edexcel the wall always exerts an Impulse which causes the balls momentum to change
Consider a particle of mass 3kg, hitting a wall with speed 4ms, and say it rebounds at 4ms. The impulse is 24NS (mv-mu). An impulse has occured but speed hasn't changed. Don't worry about it anyway, holidays are almost over, I'll just ask the teacher when we get back.
Thanks anyway though
Original post by Student10011
Consider a particle of mass 3kg, hitting a wall with speed 4ms, and say it rebounds at 4ms. The impulse is 24NS (mv-mu). An impulse has occured but speed hasn't changed. Don't worry about it anyway, holidays are almost over, I'll just ask the teacher when we get back.
Thanks anyway though
Thanks anyway though
The speed has stayed the same but the velocity has changed for the same reasons as I originally described and the fact that the speed has stayed the same is an unusual fluke.You may also have been confused because momentum is only conserved if no forces act and forces act here so it isn't conserved.
(edited 10 years ago)
Dude if its travelling in the opposite direction it's velocity has changed ie. 3 is different to -3.
It's like uniform circular motion in a way that its SPEED (scalar) is the same but it's direction is constant changing so its velocity is changing (and therefore it is accelerating)
It's like uniform circular motion in a way that its SPEED (scalar) is the same but it's direction is constant changing so its velocity is changing (and therefore it is accelerating)
Original post by Student10011
Momentum hasn't changed though. mv = mu. Mass is constant, speed remains the same, so momentum hasn't changed. At the instance the ball leaves the wall its speed is equal to the speed with which it hit the wall.
momentum is a vector, p=mv
it's proportional to the velocity, not the speed. the magnitude of the momentum may be the same because the magnitude of the velocity (i.e. the speed) is the same; but the momentum vector is not the same; it's in the opposite direction
Original post by Student10011
Consider a particle of mass 3kg, hitting a wall with speed 4ms, and say it rebounds at 4ms. The impulse is 24NS (mv-mu). An impulse has occured but speed hasn't changed. Don't worry about it anyway, holidays are almost over, I'll just ask the teacher when we get back.
Thanks anyway though
Thanks anyway though
there is an impulse hence a change of momentum (since impulse is a change of momentum by definition). speed hasn't changed, but velocity and momentum have as per my last post.
Speed is a scalar, so direction is not important. So, speed has remained constant.
However we are considering velocity, which is a vector.
This means it has magnitude AND direction.
I would set the initial direction of motion as positive (->+), after it hits the wall, it's direction will be <-, which means velocity is negative.
This means mv=-mu
However we are considering velocity, which is a vector.
This means it has magnitude AND direction.
I would set the initial direction of motion as positive (->+), after it hits the wall, it's direction will be <-, which means velocity is negative.
This means mv=-mu
Speed is a scalar, so direction is not important. So, speed has remained constant.
However we are considering velocity, which is a vector.
This means it has magnitude AND direction.
I would set the initial direction of motion as positive (->+), after it hits the wall, it's direction will be <-, which means velocity is negative.
This means mv=-mu
[
However we are considering velocity, which is a vector.
This means it has magnitude AND direction.
I would set the initial direction of motion as positive (->+), after it hits the wall, it's direction will be <-, which means velocity is negative.
This means mv=-mu
[
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