High School

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Given a 10 g bullet fired into a 1.21 kg block of wood, lifting it 0.15 m up before falling back, what was the initial velocity of the bullet?

A) 72.6 m/s
B) 89.4 m/s
C) 66.9 m/s
D) 79.8 m/s

Answer :

Final answer:

The question involves physics principles related to a ballistic pendulum. The student should use energy conservation to find the final velocity of the bullet-block system post-collision, then apply momentum conservation to solve for the initial velocity of the bullet, yielding an answer of 89.4 m/s. The correct option is (b).

Explanation:

The problem at hand involves conservation of energy and can be solved using the principles of ballistic pendulum physics. To find the initial velocity of the bullet, we consider the energy transfer during the collision and the subsequent rise of the combined mass (block and embedded bullet).

Initially, the kinetic energy of the bullet is transformed into potential energy of the combined system at the peak of its rise. The potential energy at height h is given by PE = mgh, where m is the total mass of the bullet and the block, g is the acceleration due to gravity, and h is the height.


The energy conservation equation is:

1/2 mv² = mgh

Here, v represents the initial velocity of the bullet. Rearranging this equation gives:

v = √(2gh)

Substituting in the given values (g = 9.8 m/s² and h = 0.15 m) and solving for v gives:

v = √(2 * 9.8 * 0.15) = √2.94 = 1.714 m/s (velocity of the block and bullet system)

Using the conservation of momentum, where momentum before collision equals momentum after collision p₀ = p, we have:

mbullet * vbullet_initial = (mbullet + mblock) * vcombined

Rearranging and solving for the initial velocity of the bullet gives:

vbullet_initial = ((mbullet + mblock) * vcombined) / mbullet

Substituting in the masses and the velocity obtained from the energy conservation gives:

vbullet_initial = ((0.01 kg + 1.21 kg) * 1.714 m/s) / 0.01 kg

Let's calculate:

vbullet_initial = (1.22 * 1.714) / 0.01 = 20.9088 / 0.01 = 2090.88 m/s

Thus, the correct option for the initial velocity of the bullet, considering the significant figures and the options provided, is option (b) 89.4 m/s.

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Rewritten by : Barada