Some more Kinematics physics problems

[kingwinner]

1) A puck sliding across the ice at 20m/s[E] is struck and moves at 30m/s, at an angle of 120 degrees to its original path. Find its change in velocity.

 

[i know how to do this question, but there is one problem that confuses me. When it says 120 degrees to its original path...ok...but 120 degrees which way? I mean, would the direction of the final velocity W60N or E60N?]

 

2) A person walks up a stalled escalator in a department store in 90s. When standing still on the same escalator in motion, he is carried up in 60s. Could he walk down (same speed as he walked up) the escalator while it is moving up? If so, how long would that take?

 

[p=normal walking velocity of the person up the escalator

e=velocity of the escalator

d=displacement from bottom to top of escalator

t=time taken for the person walking down the escalator moving up

 

p=d/90

e=d/60

-p+e=-d/t

 

Using the 3 equations to solve for t, I finally got t=-180s...but I don't understand...why is the time interval negative?]

 

Thanks for your help! :)

[Qfwfq]

...ok...but 120 degrees which way? I mean, would the direction of the final velocity W60N or E60N?

If your teachers didn't specify which, but you have to specify direction, give'em both answers... :)

 

...but I don't understand...why is the time interval negative?

It's negative because the answer is no! :doh:

[snark1100]

But you do walk faster than the escalator moves - your 180 secs is right....

call Vw the speed at which you walk, and Ve the speed of the escalator -

it takes 90 secs to walk the distance of the escalator -

A) 90*Vw = D,

and 60 seconds if the escalator is running -

:) 60*(Vw+Ve) = D

subtarct these two equations to get

90*Vw - 60*(Vw+Ve) = 0, or

30Vw - 60Ve = 0

and so

Vw=2*Ve

that is, you walk twice as fast as the escalator moves, so, if

you really wanted to, you could walk down, at an effective speed of

Vw-Ve, or, since Ve = (1/2)Vw, at an effective speed of (1/2)Vw,

and so it would take twice as long as when the escalator was not

running, 180 seconds.

.

[TheBigDog]

1) 10 m/s. Velocity has nothing to do with direction. It went from 20 to 30. The change is 10.

 

2) No. If you are going slower than the escalator, you will never get to the bottom.

 

Bill

[CraigD]

... Velocity has nothing to do with direction ...

In the conventional terminology of mechanics, Velocity is a vector – direction is important. The scalar component of velocity is known as speed.

 

:) Kingwinner appears to have a good grasp of vectors and scalars, but we should still be careful not to post statements that could be confusing to students.

[arkain101]

If he walked the same speed as the escalator he would not move. Invision a treadmill. So in order to reach the other side he needs to walk faster than 60s of travel to make movment, but he doesnt.