Basic Kinetics Question [Secondary]

  • Thread starter glasgowm
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In summary, a ball is projected horizontally from the edge of a table which is 0.8 m high. The ball lands 0.2 m from the table. The ball takes 0.4 s to fall to the ground. The ball was projected with a horizontal speed of 0.5 ms, but whenever I work this out I get 4 m/s. :(
  • #1
glasgowm
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Okay I suck at physics and if someone could help me out it would be greatly appreciated.

  • A ball is projected horizontally from the edge of a table which is 0.8 m high, as shown below. The ball lands 0.2m from the table
  • How long does the ball take to fall to the ground?

Worked it out to be 0.4s using s = ut + 1/2 at^2

0.8m = 1/2 * 9.8t^2
1.6/9.8 = t^2
t=0.4s

  • Hence show that the ball was projected with a horizontal speed of 0.5ms

but whenever I work this out I get 4m/s. :(

v = u + at
v = 9.8 * .4
v = 4m/s

Dont think this place is really for Secondary level Physics but I couldn't find anywhere else :cry:
 
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  • #2
glasgowm said:
but whenever I work this out I get 4m/s. :(

v = u + at
v = 9.8 * .4
v = 4m/s
The horizontal motion is not accelerated!
 
  • #3
ok... the problem is you are using the vertical acceloration as the horizontal acceloration. In the horizontal direction the ball is NOT accelerating. you know that it took 0.4 s to land and in that time it went a horizontal distance of 0.2 m... speed = distance/time
 
Last edited:
  • #4
Think of the different forces affecting the object in the x and y direction, each by itself.
 
  • #5
I don't get what you mean? :-/

Could you direct me to some notes or something...I've tried the ones from hsn.uk.net but I can't figure it out from them

----
Edit : thank you dmoravec, I tried that formula as well the first time but I was putting in the wrong numbers :-p
 
Last edited:
  • #6
think of it this way... once the ball leaves the table what is happening? gravity is pulling it straight down, but there is nothing pulling it in the horizontal direction. Therefore with no force in the horizontal direction there is no acceleration.
 
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  • #7
(g*t^2)/2 = 0.8 => t = 0.4 s ;
v*t = 0.2 => v = 0.2/0.4 = 0.5 m/s
 
  • #8
Yeah thanks all, was using the Horizontal component when I should of been using the vertical :-/

2 more questions please :smile:

1) Can someone point me to a site that explains Horizontal and Vertical speed/time graphs. I've tried google but found nothing

2) How do I find the horizontal velocity of projection/vertical velocity it has on reaching the ground.
Would the horizontal velocity be found by distance over time?
i.e 25m/2.5 = 10m/s

Would the vertical velocity be t * -9.8m/s?
ie. 2.5seconds x -9.8m/s = -24.5m/s

cheers :!)
 
Last edited:
  • #9
Bump, edited the last post a bit so that I might get some answers :-(
 

1. What is basic kinetics and why is it important in science?

Basic kinetics is the study of how chemical reactions occur and how fast they proceed. It is important in science because it helps us understand and predict how reactions will happen, which is essential in fields such as chemistry, biology, and environmental science.

2. What factors affect the rate of a chemical reaction?

The rate of a chemical reaction can be affected by several factors, including temperature, concentration of reactants, presence of catalysts, and surface area of the reactants. Changes in these factors can speed up or slow down the rate of a reaction.

3. How is the rate of a chemical reaction measured?

The rate of a chemical reaction can be measured by tracking the changes in concentration of reactants or products over time. This can be done through experiments or by using mathematical equations such as the rate law.

4. What is the difference between a reversible and irreversible reaction?

A reversible reaction is one in which the products can react to form the original reactants. An irreversible reaction, on the other hand, produces products that cannot be converted back to reactants. Reversible reactions tend to reach an equilibrium state, while irreversible reactions proceed to completion.

5. Can the rate of a chemical reaction be changed?

Yes, the rate of a chemical reaction can be changed by altering the factors that affect it. This can be achieved by adjusting the temperature, concentration of reactants, or using a catalyst. The rate of a reaction can also be changed by changing the reaction conditions, such as pressure or pH.

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