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PHYSICS: ACCELERATION QUESTIONS

1) An egg is thrown vertically downward from a window. If the egg was released with a velocity of 10.0m/s and it strikes the ground at a velocity of 25.0 m/s, how long did it take the egg to reach the ground ?

2) An object is dropped from the roof of a building. If the object takes 2.5s to reach the ground, what was the velocity of the object when it reached ground?

Thank you so much. :P

And for this question

An object accelerates uniformly at a rate of 1.50m/s^2 [E] for 10.0s. If the velocity of the object reaches 25.0m/s [E] at this time, what was the initial velocity?
Apparently the answer for that is 10.0m/s [E], but I keep getting -10.0

Thank you so much to anyone who could help!

Answer :

AL2006

1). The 'acceleration due to gravity' is about 9.8m/s².  That means
that the downward speed of a a falling object increases by 9.8 m/s
every second it falls.

The egg was thrown downward at 10 m/s, and was falling at 25 m/s
when it went splat on the ground.  Gravity increased its speed by
(25 - 10) = 15 m/s.  That takes (15/9.8) = 1.53 seconds.

2).  The 'acceleration due to gravity' is about 9.8m/s².  That means
that the downward speed of a a falling object increases by 9.8 m/s
every second it falls.

After falling for 2.5 sec, its downward speed is (2.5 x 9.8) = 24.5 m/s
faster than it was when the object began falling.

3).  The 'acceleration due to gravity' is about 9.8m/s².  That means
that the downward speed of a a falling object increases by 9.8 m/s
every second it falls.
(Have you noticed a pattern among these questions yet ?)

Acceleration = (change of speed) / (time for the chage).

change of speed = (end speed) minus (start speed) = (25m/s - start speed).

Acceleration = (25m/s - start speed) / (time for the change)

          1.5 m/s² = (25m/s - start speed) / (10 sec)

Let's multiply each side by  10sec :    15 m/s = 25m/s - start-speed

Add  start-speed  to each side:    start-speed + 15 m/s = 25 m/s

Subtract  15 m/s  from each side:  Start-speed = (25-15) m/s = 10 m/s

========================

Quicker & easier way:
(Caution ... requires thinking)

Accelerate uniformly at 1.5 m/s² for 10 sec ===>  gain 15 m/s of speed

Moving at  25 m/s  after gaining 15m/s  ===> must have started out at 10m/s.


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