# Projectiles Question

Watch
Announcements
#1
'A ball is thrown at an angle of 45 degrees to hit an object 40.82m away. What should be the initial velocity?'
Any help would be appreciated
0
2 years ago
#2
Resolve the velocity into horizontal and vertical components. Then Suvat and solve simultaneously
0
2 years ago
#3
Assume g= 9.8N/kg,

The horizontal component of the speed won't change (because there's no force on the horizontal axis, assuming no air resistance).
The vertical component of its speed will decrease and eventually turn negative due to the force of gravitational attraction between the ball and the earth.

At 45 degrees, the horizontal component and vertical component of its speed will be the same immediately after it is thrown (if you don't believe me, tan(45deg) = 1)

If you let x be the initial vertical component of speed, the time it travelled until its speed is 0 in the air can be calculated using x/9.8. The horizontal distance it travelled in this time is just the (constant horizontal component of speed) * time: x * x / 9.8 = x^2 / 9.8.

To figure out how high it is in the air, v^2 = u^2 + 2as, v is just 0, so you have s=x^2/19.6, x^2/19.6 = 0.05102x^2 .

Now to see how long until it hits the ground s = ut + 0.5at^2, u is just 0 because it has 0 velocity at its highest point in the air s= 0.5at^2, t= sqrt(s/0.5a),
t = sqrt(0.05102x^2 / 4.9)
= 0.102x (3sf).
So now let's see how far it flies horizontally during it's descent, same formula as above. 0.102x * x = 0.102x^2

So now the total horizontal distance flown is 0.102x^2 + x^2/9.8 = 40.82 (given in question)
x^2(0.102+1/9.8) = 40.82
x= sqrt(40.82/(0.102+1/9.8))
=14.144 (3dp)
But that's not its initial speed, just it's horizontal/vertical components of its initial speed. To find its initial speed, it's just pythagoras
sqrt(14.144^2 + 14.144^2) = 20.00m/s
If you get an exact answer like that, you probably got it right. At least I hope I did.
There's probably an easier way to do it than I did, but I'm only GCSE, so I haven't learned the "cleanest" way to do it.
0
2 years ago
#4
(Original post by Dench,x,Kid)
Assume g= 9.8N/kg,

The horizontal component of the speed won't change (because there's no force on the horizontal axis, assuming no air resistance).
The vertical component of its speed will decrease and eventually turn negative due to the force of gravitational attraction between the ball and the earth.

At 45 degrees, the horizontal component and vertical component of its speed will be the same immediately after it is thrown (if you don't believe me, tan(45deg) = 1)

If you let x be the initial vertical component of speed, the time it travelled until its speed is 0 in the air can be calculated using x/9.8. The horizontal distance it travelled in this time is just the (constant horizontal component of speed) * time: x * x / 9.8 = x^2 / 9.8.

To figure out how high it is in the air, v^2 = u^2 + 2as, v is just 0, so you have s=x^2/19.6, x^2/19.6 = 0.05102x^2 .

Now to see how long until it hits the ground s = ut + 0.5at^2, u is just 0 because it has 0 velocity at its highest point in the air s= 0.5at^2, t= sqrt(s/0.5a),
t = sqrt(0.05102x^2 / 4.9)
= 0.102x (3sf).
So now let's see how far it flies horizontally during it's descent, same formula as above. 0.102x * x = 0.102x^2

So now the total horizontal distance flown is 0.102x^2 + x^2/9.8 = 40.82 (given in question)
x^2(0.102+1/9.8) = 40.82
x= sqrt(40.82/(0.102+1/9.8))
=14.144 (3dp)
But that's not its initial speed, just it's horizontal/vertical components of its initial speed. To find its initial speed, it's just pythagoras
sqrt(14.144^2 + 14.144^2) = 20.00m/s
If you get an exact answer like that, you probably got it right. At least I hope I did.
There's probably an easier way to do it than I did, but I'm only GCSE, so I haven't learned the "cleanest" way to do it.
Not having a go but......please try and help the OP to arrive at the answer with guidance and let them do the work.

Answering requests should not be an exercise in telling the OP and everyone else that you know how to get the answer.

Thanks.
1
2 years ago
#5
you can express y in terms of x....

y = xtan45° - 4.9x2/{v2cos245°}

we know y = 0, x = 40.82, so you can solve for V
0
2 years ago
#6
(Original post by Jayc3)
'A ball is thrown at an angle of 45 degrees to hit an object 40.82m away. What should be the initial velocity?'
Any help would be appreciated
y = [email protected] - 0.5gt^2, x = [email protected] => t = 40.82/(vcos45)

On landing, y = 0, t > 0: 0 = [email protected] - 0.5gt
Sub in our expression for t, re-arrange and solve for v.
0
X

new posts Back
to top
Latest
My Feed

### Oops, nobody has postedin the last few hours.

Why not re-start the conversation?

see more

### See more of what you like onThe Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

### Poll

Join the discussion

#### What factors affect your mental health the most right now? (select all that apply)

Lack of purpose or routine (106)
15.57%
Uncertainty around my education (110)
16.15%
Uncertainty around my future career prospects (64)
9.4%
Isolating with family (45)
6.61%
Lack of support system (eg. Teachers, counsellors) (27)
3.96%
Lack of exercise/ability to be outside (58)
8.52%
Loneliness (71)
10.43%
Financial worries (27)
3.96%
Concern about myself or my loved ones getting ill (62)
9.1%
Exposure to negative news/social media (46)
6.75%
Lack of real life entertainment (eg. cinema, gigs, restaurants) (65)
9.54%