Physics - Kinematics

• bonkysleuth

  9 Feb 08, 11:31

  I've got some questions that require some assistance from you guys. Appreciate your help.

   

  1. You walk a distance of 2km at an average speed of 6km/h. You then run for a further 10 minutes with an average speed of 12km/h. What is your average velocity for the whole journey?

   

  My answer's 8ms-1. Please check to see if you end up with the same as mine. If not, please highlight my errors. Thanks.

   

  2. A Toyota Corolla starts from rest and accelerates at 1.5m/s2. After what time would its velocity be 20m/s?

   

  I'm not pretty sure about this. But it should be something like 13.3333 secs when you divide 20 by 1.5? Or is it not?

   

  3a. When a velocity is increasing, acceleration is _______

  I wrote increasing

   

  3b. When velocity is decreasing, acceleration is ____

  I wrote decreasing

   

  3c. When the velocity is constant, acceleration is ____

  I wrote zero.

   

  Thanks for your guidance.

   

   

• HyuugaNeji

  9 Feb 08, 11:50

  Originally posted by bonkysleuth:

  I've got some questions that require some assistance from you guys. Appreciate your help.

   

  1. You walk a distance of 2km at an average speed of 6km/h. You then run for a further 10 minutes with an average speed of 12km/h. What is your average velocity for the whole journey?

   

  My answer's 8ms-1. Please check to see if you end up with the same as mine. If not, please highlight my errors. Thanks.8km-1

   

  2. A Toyota Corolla starts from rest and accelerates at 1.5m/s2. After what time would its velocity be 20m/s?

   

  I'm not pretty sure about this. But it should be something like 13.3333 secs when you divide 20 by 1.5? Or is it not?

   v=u + at

  20= 0+1.5t

  t=13.33

  3a. When a velocity is increasing, acceleration is _______

  I wrote increasing

  positive. It is not necessary increasing. Acceleration can be decreasing and yet, the velocity is still increasing.

   

  3b. When velocity is decreasing, acceleration is ____

  I wrote decreasing

   negative. similar case.

  3c. When the velocity is constant, acceleration is ____

  I wrote zero.

  zero

   

  Thanks for your guidance.

   

   

   

  answers in bold

• bonkysleuth

  9 Feb 08, 20:40

  I don't kind of get what you're trying to say. I heard of situations where acceleration decreases and yet velocity is increasing. But in what sort of circumstances does this happen?

• LatecomerX

  9 Feb 08, 20:55

  Originally posted by bonkysleuth:

  I don't kind of get what you're trying to say. I heard of situations where acceleration decreases and yet velocity is increasing. But in what sort of circumstances does this happen?

  When the acceleration is still positive, even if it is decreasing, the velocity is still increasing, but at a rate that is lower over time. The velocity graph would be something like half a neh. Acceleration is zero at the nipple, before it gets negative. See the first graph I found through Google below.

   

   

• LatecomerX

  9 Feb 08, 20:56

  Double-post.
  

   
• Darkness_hacker99

  11 Feb 08, 06:17

  Originally posted by bonkysleuth:

  I don't kind of get what you're trying to say. I heard of situations where acceleration decreases and yet velocity is increasing. But in what sort of circumstances does this happen?

  I am not sure if this example is the right one of not.
  
  Imagine a rocket in sky travelling downward towards an object straight down. With the rocket's booster working, the velocity of the rocket is increasing due to the accerleration cause by the booster. But when the booster is jettison, the acceleration decreases but velocity still keep on increase..
  
  Have to relate it back to gravity, mass,energy,weight, and accerlation due gravity.

• Darkness_hacker99

  11 Feb 08, 06:25

  By the way, TS, You are asking this questions at which academic level?

• Maith

  24 Feb 08, 22:13

  its sec 3 work,if im not wrong

• nightzip

  24 Feb 08, 22:46

  Originally posted by bonkysleuth:

  I've got some questions that require some assistance from you guys. Appreciate your help.

   

  1. You walk a distance of 2km at an average speed of 6km/h. You then run for a further 10 minutes with an average speed of 12km/h. What is your average velocity for the whole journey?

   

  My answer's 8ms-1. Please check to see if you end up with the same as mine. If not, please highlight my errors. Thanks.

  Average vel = total dist/total time Total dist = 2000m + 2000m = 4km Total time = 2/6hr+ 1/6hr = 20min+10min = 30min Average vel = 4/(1/2)=8 km/hr

   

  2. A Toyota Corolla starts from rest and accelerates at 1.5m/s2. After what time would its velocity be 20m/s?

   

  I'm not pretty sure about this. But it should be something like 13.3333 secs when you divide 20 by 1.5? Or is it not?

   

  3a. When a velocity is increasing, acceleration is _______

  I wrote increasing

  Wrong! When a velocity is CHANGING, acceleration is NON-ZERO. Thus, when a velocity is increasing, acceleration can also be a positive CONSTANT or an increasing value.

   

  3b. When velocity is decreasing, acceleration is ____

  I wrote decreasing

   

  3c. When the velocity is constant, acceleration is ____

  I wrote zero.

   

  Thanks for your guidance.

   

   

   

• Choya

  27 Feb 08, 23:21

  3a. When a velocity is increasing, acceleration is positive

  3b. When velocity is decreasing, acceleration is negative

   

• deathvale

  29 Feb 08, 20:25

  1. the answer is 8 km/h not 8 m/s

  2. i'm assuming acceleration is constant. so i use v=u+at

  20=0+1.5t

  t=13.3s (same answer as yours)