# Kinematics

• Sep 28th 2006, 11:53 PM
classicstrings
Kinematics
A particle moving in straight line with constant acceleration of 4m/s^2 has initial velocity of 11m/s

How far does it travel in 2nd second of its motion?
How far does it travel in 8th second of its motion?

An object moving with a velocity of 42m/s is stopped in 7 seconds.
How far has it travelled?

Isn't that 42+36+30+24+18+12+6 = 168? But answer is 147.
• Sep 29th 2006, 01:10 AM
CaptainBlack
Quote:

Originally Posted by classicstrings
A particle moving in straight line with constant acceleration of 4m/s^2 has initial velocity of 11m/s

How far does it travel in 2nd second of its motion?
How far does it travel in 8th second of its motion?

v=4t+11
s=2t^2+11t

(assuming it starts from 0)

At start of 2nd second (t=1) it is at 13m, at end of 2nd second (t=2) it is at
30m, so it has moved 30-13=17m.

Now you should be able to do the 8th second part.

Quote:

An object moving with a velocity of 42m/s is stopped in 7 seconds.
How far has it travelled?

Isn't that 42+36+30+24+18+12+6 = 168? But answer is 147.
Average speed is 21m/s, distance travelled in 7 seconds is 7*21=147.
(using the aversge speed works here if we assume constant decelleration).

RonL
• Sep 29th 2006, 01:44 AM
classicstrings
Sorry how did you get average speed is 21m/s?
• Sep 29th 2006, 02:35 AM
CaptainBlack
Quote:

Originally Posted by classicstrings
Sorry how did you get average speed is 21m/s?

start speed 42m/s decreasing linearly to 0m/s - average is 1/2 the starting speed.

(Altenativly think of it this way: the distance travelled is the area under the speed/time
graph - this is a triangle of height 42 and base 7 so area is 42*7/2)

RonL