Projectile question help (1 Viewer)

[abdog]

A projectile is fired horizontally at 10m/s from the top of a cliff, 55m high, into the water. What is the magnitude of its velocity as it impacts the water?
A. 32.8m/s
B. 1078m/s
C. 1175.8m/s
D. 34.3m/s

If g is -9.8, is delta Y negative too? If not, when is it negative?

 

[Rathaen]

Don't need to worry about g being positive or negative so long as you're consistent with the directions. No need to even think about delta y, just plug into equations.

X component (V_x) remains constant at 10m/s

Using kinematic equation for Y component: (Initial V = 0, Displacement = 50, Acceleration = 9.8m/s² Final V = ?)
-Since the particle is fired horizontally, there is no initial velocity in the Y axis-
V_y² = U² + 2AS
V_y² = 0 + 2 (9.8) (55)
V_y² = 1078 m/s
V_y = 32.8 m/s (approx, doesn't matter since we're using pythagoras in the next step)

Use pythagoras to find the resultant vector (ie. the magnitude of the velocity of the particle)
V = sqrt(V_x² + V_y²)
V = sqrt(100 + 1078)
V = 34.3 m/s

So the answer should be D (Assuming I haven't gone completely retarded since I last did physics.)

As with all kinematics questions, look at what values they've given you, what value they're asking for, and which kinematics equation you can plug them all into.

EDIT: Just note that the question is asking for the magnitude of the velocity, which is the scalar value normal people call speed (in case you didn't notice). Hence why we find the resultant of the X and Y component vectors of the particle.

 

[abdog]

Don't need to worry about g being positive or negative so long as you're consistent with the directions. No need to even think about delta y, just plug into equations.

What do you mean by "consistent"? Because I always take g as -9.8, so does this mean delta Y is always negative as well? Also, unfortunately the answer is C.

 

[D94]

Also, unfortunately the answer is C.

The answer is D. You are forgetting to take the square root of V_x² + V_y²