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Exam 9: Center of Mass and Linear Momentum

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Question 31

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The center of mass of a system of particles remains at the same place if:

A) it is initially at rest and the external forces sum to zero
B) it is initially at rest and the internal forces sum to zero
C) the sum of the external forces is less than the maximum force of static friction
D) no friction acts internally
E) none of the above

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Question 32

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The force on a particle is given by F(t) = 0.71 t + 1.2 t², in N.If the force acts from t = 0 to t = 2.0 s, the total impulse is:

A) 1.2 kg∙m/s
B) 1.9 kg∙m/s
C) 4.6 kg∙m/s
D) 4.8 kg∙m/s
E) 6.2 kg∙m/s

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Question 33

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A golf ball of mass m is hit by a golf club so that the ball leaves the tee with speed v.The club is in contact with the ball for time T.The average force on the club on the ball during the time T is:

A 1000-kg space probe is motionless in space.To start moving, its main engine is fired for 5 s during which time it ejects exhaust gases at 5000 m/s.At the end of this process it is moving at 20 m/s.The approximate mass of the ejected gas is:

A rifle of mass M is initially at rest but free to recoil.It fires a bullet of mass m and velocity v (relative to the ground) .After firing, the velocity of the rifle (relative to the ground) is:

A hockey puck of mass m traveling along the x axis at 4.5 m/s hits another identical hockey puck at rest.If after the collision the second puck travels at a speed of 3.5 m/s at an angle of 30° above the x axis, is this an elastic collision?

The center of mass of a system of particles obeys an equation similar to Newton's second law where:

When a particle suffers a head-on elastic collision with another particle, initially at rest, the greatest fraction of kinetic energy is transferred if:

At one instant of time a rocket is traveling in outer space at 2500 m/s and is exhausting fuel at a rate of 100 kg/s.If the speed of the fuel as it leaves the rocket is 1500 m/s, relative to the rocket, the thrust is:

The law of conservation of momentum applies to a system of colliding objects only if:

For a two-body collision, involving objects with different masses, a frame of reference which has the same velocity relative to the laboratory as does the center of mass of the two objects is:

Momentum may be expressed in:

The direction of the impulse on a struck baseball

The physical quantity "impulse" has the same dimensions as that of:

The center of mass of the system consisting of Earth, the Sun, and the planet Mars is:

A 3.0-kg cart and a 2.0-kg cart approach each other on a horizontal air track.They collide and stick together.After the collision their total kinetic energy is 40 J.The speed of their center of mass is:

If a projectile hits a stationary target, and the projectile continues to travel in the same direction,

If a projectile hits a stationary target, and the projectile rebounds and travels back the way it came,

The velocity of the center of mass of a system of particles changes as v = 4.5 t + 2.4 t² + 1.1 t³, where v is in meters per second.If the system starts from rest at t = 0, what is its acceleration at t = 3.0 s?

A stream of gas consists of n molecules.Each molecule has mass m and speed v.The stream is reflected elastically from a rigid surface as shown.The magnitude of the change in the total momentum of the stream is: