9: Exam Questions

Question 1

Using momentum explain how an engine ejecting exhaust gases in a downward direction is able to accelerate the rocket upwards

Answer 1

total momentum of rocket + fuel is constant; 2) fuel gains downwards momentum; 3) so rocket gains upwards momentum; 4) so (upwards) momentum of rocket increases with time;

Question 2

Explain how helium particles in a balloon from collapsing

Answer 2

1)particles bounce off balloon; 2) exerting outwards force/pressure; 3) because of change of momentum of particles / transfer of momentum to balloon;

Question 3

Estimate the rate at which air molecules collide with an oject, area of 56m^2 to provide this pressure. P=1*10^5 Pa, temp=20 degrees, mass given and r.m.s.s already calculated. State an assumption

Answer 3

any one of the following assumptions 1. elastic collisions 2. molecules impact surface at right angles to it 3. all molecules moving at rms speed 1) evidence of use of F = PA (= 1.0x105 x 0.56 = 5.6×104 N); 2) Force = momentum/area = 2nm*r.m.s.s/1 so n= 1.27 *10^27 per second