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# Tag Archives: projectile motion

## Angle between velocity and acceleration

Discuss a circumstance in which angle between velocity and acceleration is less than 90.

Discuss a circumstance in which angle between velocity and acceleration lies batween 180 and 90.

(Hifsa Zafar asked this)

**Answer**:

In projectile motion when the object is moving up, the angle between velocity and acceleration (due to gravity g) is between 90 degree and 180 degree.

during the downward motion of a projectile, the angle between velocity and acceleration is between 0 and 90 degree.

## Kinematics Numerical

- A particle travels 20 m in 7th second and 24m in 9th sec. find initial velocity?
- in a projectile motion , a body thrown from the ground ,at what angle both the vertical height and range will be equal?
- The velocity v(cm/s) of a particle is given in terms of time t(in seconds) by the equation v = at+b/t+c. Dimension of a,b, and c are ?

## A Problem from Projectile motion

“A ball is projected vertically upwards with an initial velocity of 28m/s and is observed to pass a point 30m above the projection point,at what 2times does the ball pass these points?”

(Vimal Raj Answered)

ans : 1.356 sec and 1.444 sec .

sol:

from v=u+at we get t=2.8 . (time taken to reach the max height )

fm v^{2}-u^{2}=2aS we get S=39.2 m (max height reached)

observer at 30 m .

so , in btw distance btw observer and ball (at the top) =9.2 m

at top ball is at rest and time taken to reach the observer can be calculaed from S=ut + 1/2 at^{2} ,we get t=1.356s .

and next time can be calculated by 2.8 – 1.356 =1.444 sec .

**(The answer has not been scrutinized. If any error, teachers and visitors can post as comment)**

## Projectile Motion

Ramesh Asks:

“A body is projected with velocity u with an angle with the horizontal. What is its average velocity when it crosses half of its max height”

## A numerical from Projectile motion, friction and more

**Hale asked:**

One side of the roof of a building slopes up at 33.5°. A roofer kicks a round, flat rock that has been thrown onto the roof by a neighborhood child. The rock slides straight up the incline with an initial speed of 15.0 m/s. The coefficient of kinetic friction between the rock and the roof is 0.430. The rock slides 10.0 m up the roof to its peak. It crosses the ridge and goes into free fall, following a parabolic trajectory above the far side of the roof, with negligible air resistance. Determine the maximum height the rock reaches above the point where it was kicked.

**(The question is left for visitors to answer.)**