Momentum and movement basic GCSE Quest

[nonthesecond]

when a gun is fired a very large force acts on the bullet for a very short time.

formula that's given: force x time = change in momentum (kg m/s)

an average force of 4000 acts for 0.01 seconds on a bullet of 50g mass.

DONE THIS PART

b. the bullet is fired horizontally. in the short time that it takes for it to reach it's target, its horizontally has fallen by 80% of its initial speed.

explain why the speed of the bullet has decreased so quickly.

c. calculate the percentage of its original Kinetic energy the bullet still has when it reaches its target.

 

[tiny-tim]

hi nonthesecond!

b. the bullet is fired horizontally. in the short time that it takes for it to reach it's target, its horizontally has fallen by 80% of its initial speed.

explain why the speed of the bullet has decreased so quickly.

c. calculate the percentage of its original Kinetic energy the bullet still has when it reaches its target.

show us what you've tried, and where you're stuck, and then we'll know how to help!

 

[tal444]

I'm a little confused. What part have you done already? Also, please show us your attempt at a solution.

 

[nonthesecond]

hi nonthesecond!


show us what you've tried, and where you're stuck, and then we'll know how to help!

for b i think because it already has reached it's fastest speed it must slow down from then on. what's the best way to word this?

c. for this question i think it's o.5(0.005) x velocity^2 but i don't know how to work out velocity i know it's v=d/t.

 

[nonthesecond]

hi nonthesecond!


show us what you've tried, and where you're stuck, and then we'll know how to help!

I'm a little confused. What part have you done already? Also, please show us your attempt at a solution.

i've finished the first part (a)

i can't do b and c

for b i think because it already has reached it's fastest speed it must slow down from then on. what's the best way to word this?

c. for this question i think it's o.5(0.005) x velocity^2 but i don't know how to work out velocity i know it's v=d/t.

 

[tiny-tim]

hi nonthesecond!

for b i think because it already has reached it's fastest speed it must slow down from then on.

but why?

c. for this question i think it's o.5(0.005) x velocity^2 but i don't know how to work out velocity i know it's v=d/t.

you know that v_f/v_i is 0.8 …

so how much KE is lost? ​

 

[nonthesecond]

hi nonthesecond!


but why?


you know that v_f/v_i is 0.8 …

so how much KE is lost? ​

sorry but you have to dumb this down I'm doing GCSEs and i don't understand what you mean by the Vs, i also have no idea why it slows down

 

[tal444]

vf = final velocity, vi = initial velocity.

 

[tiny-tim]

sorry but you have to dumb this down I'm doing GCSEs and i don't understand what you mean by the Vs, i also have no idea why it slows down

ah, the question says that the horizontal speed has fallen to 20% of its initial value

ie v_f/v_i = 0.2, where v_i means initial speed and v_f means final speed

(i put v_f/v_i = 0.8 before, i misread the question)

 

[nonthesecond]

is the answer 40% ?

(0.5x 0.005) x 40^2

 

[Alpha Floor]

You have to model air friction right? If not I don't see how the bullet is supposed to loose speed (momentum)

Once the bullet is flying, the only horizontal force acting on it is air friction. This force is a function of the bullet's speed, and since the speed decreases constantly, the force also decreases constantly. The model you're supposed to use for air friction I don't know, but it's most probably a linear model, something like F= k · v

Use Newton's law of conservation of momentum to obtain an ODE (an easy one), solving that ODE will have your problem solved.

For the kinetic energy, I might be confusing the question because it's too easy, if they say the speed has decreased by 80%, than the energy has decreased by:

v1= v -> v2= 0,2 · v

E1 proportional to v1^2= v^2 -> E2 prop. to v2^2 = (0,2·v)^2 = 0.04·v^2

Thus the kinetic energy has decreased by 96% if I understood the question properly.