A ball is thrown downward with an initial speed of 10 m/s from a height of 20 m.

A ball is thrown downward with an initial speed of 10 m/s from a height of 20 m. How long will it take to hit the ground? (Assume g = 10 m/s²)

A ball is thrown downward with an initial speed of 10 m/s from a height of 20 m. How long will it take to hit the ground? (Assume g = 10 m/s²)

Correct Answer: 2 seconds

Step 1: Identify the variables in the problem. The initial speed (u) is 10 m/s, the height (h) is 20 m, and the acceleration due to gravity (g) is 10 m/s².
Step 2: Write down the equation of motion that relates height, initial speed, time, and acceleration: h = ut + 0.5gt².
Step 3: Substitute the known values into the equation: 20 = 10t + 0.5 * 10 * t².
Step 4: Simplify the equation: 20 = 10t + 5t².
Step 5: Rearrange the equation to set it to zero: 5t² + 10t - 20 = 0.
Step 6: Use the quadratic formula or factor the equation to find the value of t. The quadratic formula is t = (-b ± √(b² - 4ac)) / 2a.
Step 7: In this case, a = 5, b = 10, and c = -20. Calculate the discriminant: b² - 4ac = 10² - 4 * 5 * (-20) = 100 + 400 = 500.
Step 8: Calculate t using the quadratic formula: t = (-10 ± √500) / (2 * 5). Since we are looking for a positive time, we take the positive root.
Step 9: Simplify to find t = 2 seconds.

Kinematics – The question tests the understanding of the equations of motion, specifically how to apply them to calculate the time of flight for an object under constant acceleration.
Quadratic Equations – The problem requires solving a quadratic equation, which is a key skill in physics and mathematics.