Momentum and Torque Flashcards
If an object has a net force and net torque of zero, is the object necessarily at rest?
No, it just means object is NOT accelerating but it could either be at rest or be moving at a constant velocity
In an elastic collision in the absence of a net
external force, both kinetic energy and momentum are conserved or not conserved?
Both are conserved.
When someone who is standing still on a skateboard starts to walk forward, what happens to the skateboard?
It will move in direction opposite to person’s motion. Initially, the system is at rest—the momentum of the system is zero. When person walks forward, the skateboard must move backwards, in a direction opposite to person’s motion. These two motions give us a net final momentum of zero, which we must have by conservation of momentum.
If there is conservation of momentum, what happens to the final speed, when you either both double or both half the mass of the two initial objects/masses?
Final speed stays the same as before because only the relative masses matter and since both objects increased or decreased the same amount the relative masses stay the same. This works because when solving the equation mass variable cancels out and only the relative mass matters.
When a bullet hits a block, does it give more momentum and impulse to the block if it was an elastic collision or inelastic collision?
Elastic collision because all the momentum from bullet can be transferred to block and also the bullet rebounds.
During inelastic collisions, kinetic energy is lost to?
Friction and deforming the objects
To maximize work, you want force in what direction in relation to distance?
How about to maximize torque?
You want parallel force to distance moved for work.
HOWEVER, you want perpendicular force to radius/lever arm for torque.
When a bullet becomes embedded in a block and starts to swing back and forth like a pendulum, is energy and momentum conserved?
Only energy is conserved.
If we take as our system the bullet plus block, then momentum is not conserved because there
is an external force acting on this system, the force due to gravity. As a result, the velocity changes throughout its
oscillation, so its momentum is continually changing, which implies it is not conserved. The total energy of the system in harmonic oscillation is conserved. Gravitational potential energy is converted to kinetic energy and back again as the block and bullet swing back and forth.
Is mechanical energy conserved in elastic or inelastic collisions?
Elastic
When collisions are elastic, all of the mechanical energy
(i.e., kinetic + potential) before the collision goes into the mechanical energy after the collision. When collisions are inelastic, some of the initial mechanical energy goes into heat and deformation. Since it is practically impossible to calculate the exact amount of heat and deformation, you cannot use energy conservation in an inelastic collision problem.
Why is it a good design to make the counterweight more massive than maximum load on a pan balance?
So adjustments to counterweight’s position to compensate for variations in the mass of the load are minimal.
In an inelastic collision, what does final velocity of the two objects depend on?
It depends on the initial moving object’s initial speed and mass, and also on the initial stationary object’s mass because:
m1v1 + m2v2 = (m1+m2)vf
m1v1 + 0 = (m1+m2)vf
vf = (m1v1)/(m1+m2)
For two objects (one elastic, one inelastic) with equal initial speed and mass, does the elastic one or inelastic one cause a bigger change in momentum and therefore bigger change in speed?
the elastic one will exhibit the greater change in momentum, when an object collides and rebounds elastically, there is a direction change in the object’s velocity. The incoming and outgoing velocities may have the same magnitude, but the two velocities
are pointing in different directions. This means that the object may experience a velocity change that is twice its initial velocity = v - (-v) = 2v.
How do air bags work to protect drivers in the front seats?
They reduce the force on the driver because there would be an increase in time of collision since it would take longer to stop the driver compared to the driver just hitting glass or something. However, momentum change is the same either way because same initial and final velocity with or without air bag.
If cars collided elastically, would there be more or less damage to the drivers?
More because there would be more force on the drivers since there would be a greater change in momentum, but there would be no damage to the car because momentum is conserved so no kinetic energy will be converted to damage.
If someone is rotating on a stool holding weights with arms outstretched, what happens if they drop the weights?
They fly off from that circle in a tangential fashion, in opposite directions. Together, they exhibit no net momentum, so the skater cannot experience a change in angular momentum. Dropping the weights does not change the moment of inertia for the system
(although its total momentum is split between two systems now).
