Question

a projectile is thrown in the upward direction making an angle of 60 degree with the horizontal direction with a velocity of 147 metre per second then the time after which its inclination with the horizontal becomes 45 degree is ????


My thinking:
Uy= 147sin(60) and. Vy= 147sin(45)
Using Vy=Uy-gt
What is wrong in my thinking. ?????

Answer 1

The only about your thinking is where you take Vy=147sin(45)

Because we want to find the time when the inclination become 45 with the horizontal. We know that at theeta=45 the horizontal component and the vertical component are the same.(sin45=cos45).

The horizontal component of a projectile is constant throughout the motion ie. Ux=constant=147cos(60)=73.5 m/s.
At 45 degrees Vy=Ux=73.5 m/s
Uy=147sin(60)=127.306 m/s

Vy=Uy-gt gives you the time....