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[caprica1]

Hey everyone,

I am having trouble figuring out a couple practice physics word problems. Could you guys help me out? You dont have to do the actual equations. Just tell me what formulas to use. Thanks in advance.

1) A bullet is fired with a certain velocity at an angle θ above the horizontal at a location where g =
10.0 m/s2. The initial x and y components of its velocity are 86.6 m/s and 50.0 m/s respectively.
How long does it take before the bullet hits the ground?
A) 10.0 seconds
B) 15.0 seconds
C) 20.0 seconds
D) 5.0 seconds
E) None of the other choices is correct.


2) A ball is thrown horizontally with an initial velocity of 40 m/s from a height of 10 m. How long
will it take for the ball to touch the ground? Use g = 10 m/s2.
A) 1.4 seconds
B) 1.0 second
C) 2.0 seconds
D) 0.70 second
E) 2.4 seconds


3) A person throws a ball horizontally from the top of a building that is 40.0 m high. The initial
velocity of the ball is 100 m/s. What is the horizontal distance that the ball travels before hitting the
ground?
A) 572 m B) 286 m C) 367 m D) 325 m E) 100 m

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[Farmercyst]

Hey everyone,

I am having trouble figuring out a couple practice physics word problems. Could you guys help me out? You dont have to do the actual equations. Just tell me what formulas to use. Thanks in advance.

1) A bullet is fired with a certain velocity at an angle θ above the horizontal at a location where g =
10.0 m/s2. The initial x and y components of its velocity are 86.6 m/s and 50.0 m/s respectively.
How long does it take before the bullet hits the ground?
A) 10.0 seconds
B) 15.0 seconds
C) 20.0 seconds
D) 5.0 seconds
E) None of the other choices is correct.


2) A ball is thrown horizontally with an initial velocity of 40 m/s from a height of 10 m. How long
will it take for the ball to touch the ground? Use g = 10 m/s2.
A) 1.4 seconds
B) 1.0 second
C) 2.0 seconds
D) 0.70 second
E) 2.4 seconds


3) A person throws a ball horizontally from the top of a building that is 40.0 m high. The initial
velocity of the ball is 100 m/s. What is the horizontal distance that the ball travels before hitting the
ground?
A) 572 m B) 286 m C) 367 m D) 325 m E) 100 m

Figure out the effect of gravity on the ball and calculate the time it takes for gravity to pull the ball to the ground. Then calculate how far the ball can travel in that time. I don't have the equations in front of me (it's been 5 years+/-) but that should be the appropriate way to approach the problem.

 

[mochiko]

It's been a few years since taking Physics, so I don't remember the exact kinematic equations to use. But here are the main things to consider for these questions:

1) For projectile motion, always split the velocity into its X and Y components. In this case they've already done it for you, but you can use simple trig (sin/cos/tan) to split it if they only give you velocity and an angle. The next thing you need to find out is how long (duration) the bullet travels. You can assume the X velocity to have 0 acceleration because it's usually negligible compared to the effect of gravity on its vertical movement. Thus, use the Y velocity of 50 m/s and calculate the time it takes for it to go up then hit the ground with an acceleration of g.

2) The ball is thrown horizontally, so the Y component of the ball's movement is essentially free-fall with a starting Y velocity of 0. Simple plug and chug.

3) Another projectile problem. Again, the best approach to this type of problem is to split velocity into its 2 components and treat them as two 1-dimensional motion problems. The only common number between both X and Y in projectile motion problems is time, so you will solve for that in one direction and use it to find something about the other. Find the time it takes for the ball to hit the floor, multiply by the X velocity and you have your answer.

 

[caprica1]

thanks so much to both of you! i was able to figure out the problems from all the detail in your posts. and the test wasnt as bad as i thought it would be! yay!