Projectile motion question

Please can someone do this question for me!!! please state the equation of motion use!!!! THANKS IN ADVANCE

A ball is projected horizontally from the top of a cliff on the surface of the earth with a speed of 40 m/s.Assuming that there is no air resistance, what will its speed be 3 s later?

A 30 m/s

B 40 m/s

C 50 m/s

D 60 m/s

E 70 m/s

You'll need to use a couple of formulae v = u + at and Pythagoras' Theorem.

The answer is 'None of the above' unless the question also asks you to round to the nearest whole number or use a value for g that is rounded to 1 s.f.!

Reply 2

jinx007

OP

its a multiple choice question...i think it must be round up...the answer is C in the answer booklet by the way where should i use the Pythagoras equation

Reply 3

steve2005

17

jinx007

Please can someone do this question for me!!! please state the equation of motion use!!!! THANKS IN ADVANCE

A ball is projected horizontally from the top of a cliff on the surface of the earth with a speed of 40 m/s.Assuming that there is no air resistance, what will its speed be 3 s later?

A 30 m/s

B 40 m/s

C 50 m/s

D 60 m/s

E 70 m/s

Taking g as 9.81 I get the speed as 49.6601 m/s so my choice is C

Ok it is against the rules to give full answers but I think you may need it in this case.

When an object is projected and there is no air resistance, the x component stays the same (in this case it always travels at 40 m/s in the horizontal direction).

Now consider the y component. Gravity acts downward here (they want you to use 10 metres per second squared for a). The starting velocity (u) in that direction is zero m/s. You can get the final speed in that direction by using v = u + at so v = 0 + 10 x 3 = 30 m/s.

You now have a right angled triangle with 40 m/s in the x direction and 30 m/s in the negative y direction. The speed is the hypotenuse of this triangle (it is a 3,4,5 triangle) so the speed is 50 m/s.