Question

You accidentally drop an eraser out the window of an apartment 17 m above the ground.

Part A:

How long will it take for the eraser to reach the ground?

Express your answer to three significant figures and include the appropriate units.

Part B:

What speed will it have just before it reaches the ground?

Express your answer to three significant figures and include the appropriate units.

Part C:

. If you multiply the time interval answer from a and the speed answer from b, why is the result much more than 15 m?

Answer 1

Part (A) : Time taken for the eraser to reach the ground which will be given as -

using equation of motion 2, we have

y = v₀ t + (1/2) g t²                                        { eq.1 }

where, v₀ = initial speed of an eraser = 0 m/s

g = acceleration due to gravity = 9.8 m/s²

y = height of the window from ground = 17 m

inserting the values in eq.1,

(17 m) = (0.5) (9.8 m/s²) t²

t² = (17 m) / (4.9 m/s²)

t = 3.46 m²/s²

t = 1.86 sec

Part (B) : Before it reaches the ground, speed which will be given as :

using equation of motion 1,

v = v₀ + g t                                                              { eq.2 }

inserting the values in eq.2,

v = (0 m/s) + (9.8 m/s²) (1.86 s)

v = 18.2 m/s

Part (C) : If you multiply the time interval answer from a and the speed answer from b, then we get a distance which is given as -

using an equation,   D = v t                                      { eq.3 }

inserting the values in eq.3,

D = (18.2 m/s) ((1.86 s)

D = 33.8 m

The result is much more than 15 m.