Conceptual Reasoning a. Suppose you place a rifle on a platform capable of glidi

Question

Conceptual Reasoning

a. Suppose you place a rifle on a platform capable of gliding with minimal resistance. When you pull the trigger, a bullet (mass 0.020 kg) shoots at high speed (300m/s) out of the barrel, and the rifle(mass 2.0 kg) recoils back in the opposite direction at speed 3.0 m/s. EXPLAIN

b. At the National Transportation Safety test facility, videos are made of 2 identical cars initially moving at 80 k/h (45 mi/h) toward each other. Immediately after the collision, the cars are at rest and stuck to each other. EXPLAIN

c. You are wearing ice skates and standing on a frozen pond. How might you start moving without pushing off on the ice? EXPLAIN

Explanation / Answer

Solution for (a). This can be explained by the conservation of momentum. Momentum p of a body is the product of its mass m and velocity v. According to the law of conservation of linear momentum, when the net external force acting on the system is zero, the total linear momentum of the system is conserved or remains constant.

In this case friction is the only external force. As the platform is capable of gliding with minimal resistance, the force due to friction is negligible. Hence the net momentum of the system is conserved or in other words the net momentum of the system before triggering is equal to the net momentum after triggering.

The mass of the bullet be M1=0.020kg

The mass of the rifle be M2=2kg

As the system is at rest before triggering, the velocity of the bullet U1 and rifle U2 is zero.

The net momentum before triggering is M1U1+M2U2. This is equal to zero as U1 and U2 are zero. It is important to note that the net momentum after triggering should also be equal to zero according to the conservation law.

   i.e. M1U1 + M2U2 = M1V1 + M2V2

Hence we write M1V1 + M2V2 =O. Where V1 and V2 are the final velocities of the bullet and rifle respectively.

   which gives

M1V1= - M2V2

M1V1 = 0.02 x 300 = 6

M2V2= 2 x 3 = 6

   The negative sign indicates that their direction of motion is opposite to each other.

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