Projectile Motion Example How long to hit the ground

Launch a projectile from the top of a building. How long does it take to hit the ground? What does the negative time solution correspond to?

[alexiluther]

I can't chew gum and walk at the same time. He's writing, left-handed, backwards, and explaining physics so clearly. Well done professor. Subscribing.

[letsstudyeverything]

I have watched every video of your playlist. I am in highschool and it helped me a lot. Thanks Dr A.👍💚

[anelemjwara]

thank you prof. all love from south africa🙏

[victrolavince]

4:01 couldnt you just solve for Vf (by using Vf^2=Vi^2+2aX) but this time with angles in the y direction (because we know the most about y), and then rearrange the equation Vf=Vi+at to t=(Vf-Vi)/a to find the time?

[fizixx]

Fascinating interpretation of that neg time solution!

[HDitzzDH]

The -2.1s simply comes from the fact that it's a parabola (a quadratic equation) and this means that we will have two solutions, in this example we sort of ignored the left side of the parabola and only focused on the positive solution (when the ball hits the ground/x-axis) which took 4.21 seconds, but you can imagine if the ball was thrown in the complete opposite direction, then it would've taken 2.1 seconds for it to hit the ground which makes sense from the illustration, the negative sign comes from the fact that it's the opposite (just like when you multiply a vector by -1, it points in the opposite direction with the same magnitude).

[nuhibrahim]

Thank u so much! Great job professor! Just subscribed!

[parabola]

But the time period we are finding here is not the timeperiod till which the ball stayed in the air, it is calculated using displcement right????, please correct me dr.A

[frankgama]

I don't understand the negative time part...

[arjunkbiju]

Hello professor.. My name is Arjun .. I had a doubt .. Two ball are projected at different angles from the same place and with same initial speed of 50 m/s. Both ball have Same rage of 216m . The difference in their time of flight is close to ?... sir please help me ...?

[ajeesha]

How to find at what angle will it hit the ground?

[Tommo]

What was the time it took from the apex to the ground though?

[dannybangs]

When you were mentally trying to figure out the value of sin30, I heard you mentioning sines 60 and 90. Is there a way to mentally check their own values too without a calculator. Is there some sort of formula or technique

[KennyShizzle]

At marker 3:05 you multiply by two "just for kicks".. is that just to get rid of the 1/2 and make the equation more simple to solve?

[jaysilverheals]

he drew normally and simply mirrored the video. Note the buttons on his shirt and pocket--they are on the wrong side. also he is drawing left handed but odds are far higher is right handed. also note the ring is on the wrong hand.

[arleighjanpangatungan]

Hi Sir can you help me with my homework
Question 1. A 35 g bullet was fired horizontally from a gun with a velocity of 320.0 m/s. If the barrel is 1.1 meter from the ground, how long will it take for the bullet to reach the ground? (ROUND YOUR ANSWER TO 1 DECIMAL PLACE, Ex. 100.0)
Question 2 . A 35 g bullet was fired horizontally from a gun with a velocity of 315.0 m/s. If the barrel is 1.5 meter from the ground, how far will the bullet reach as it first hit the ground? (ROUND YOUR ANSWER TO 1 DECIMAL PLACE, Ex. 100.0)

THank you so much