Physics Final reading questions – Flashcards

The papers will move together because the pressure of the faster moving air between them is less than the pressure of the still or slower moving air outside them so there is a net inward force.

As the air blows over the house it moves faster and the pressure decreases. The pressure outside the roof is then lower than the pressure inside the house, there is a net upward force, and the roof is pushed off the house (see the figure).

The lift generated by a wing depends on the speed of the air relative to the wing. By taking off into the wind the relative speed is higher and the lift necessary to take off occurs at a lower ground speed.

As the water falls it moves faster. Since water is (nearly) incompressible, the equation of continuity shows that as the water flows faster the cross sectional area must decrease.

As the ships move, the water between them moves with them, lowering the water pressure between the ships. The water outside the ships has a lower relative speed and therefore is at a higher pressure and there is a net force on each ship toward the other ship, risking collision.

You can push the blunt end of a pen very hard against your skin without it penetrating while a much smaller force will cause the pin to penetrate your skin. Since the pin has a much smaller point than the pen, for the same force the pressure of the pin is much greater. It is pressure, not net force which determines whether your skin is cut.

When the water boils the can fill up with steam. After the can is removed from the heat and the lid put on the can cools and the steam condenses back to liquid water, leaving a partial vacuum in the can. Atmospheric pressure crushes the can since the force on the outside of the can is now much greater than the force on the inside.

If the blood pressure is measured at a position lower than the heart then the measured blood pressure will be higher than the blood pressure at the heart, due to the effects of gravity on the blood in the blood vessels. If the pressure is measured at a higher position the measured blood pressure will be low for the same reason. To measure the blood pressure at the heart the measurement must be at the same level as the heart.

Since the ice cube floats, the density of ice is less than the density of water. The mass of the ice displaces a volume of water which has the same mass as the ice. The mass of the ice doesn't change as the ice melts, so the volume displaced remains the same whether its is solid or liquid. So the level of the water in the glass remains the same as the ice melts and the water doesn't overflow.

Iron is more dense than water so a solid ship of iron would sink. But ships aren't solid iron, they have large open spaces filled with air. And air is much less dense than water so the average density of the ship is less than the density of water and the ship floats.

(a) The weight of the wall exerts the torque to keep it upright. (b) The lever arm for the wall in (a) is small (half the width of the wall) so the torque due to its weight is small. For the wall in (b), in addition to the weight of the wall there is a torque due to the weight of horizontal part of the wall and the soil above it. This is a much larger force and has a much larger lever arm so the horizontal force exerted by the ground on the vertical part of the wall would have to be many tines larger in order to overturn it,

It is more likely to slip when the person stands near the top since her lever arm will be greater than when she is near the bottom.

The mass of the meter stick is equal to the mass of the rock. Since the meter stick is uniform its center of gravity is at its geometric center, i.e. the 50 cm mark. The lever arm for the rock is 25 cm and the lever arm for the weight of the meter stick is also 25 cm. Since the meter stick is in equilibrium the net torque must be zero, and since the lever arms are the same, the forces must be the same magnitude.

When you rise on your tiptoes your center of mass shifts forwards. But with your nose and abdomen against the door your CM can't shift forward and gravity exerts a torque on you which returns your feet to the floor.

At A the ball is in unstable equilibrium, at B it is in stable equilibrium, and at C it is in neutral equilibrium.

As seen from Example 8-13, the speed of a sphere rolling down an incline is independent of both its mass and its radius so they have the same speed at the bottom and reach the bottom at the same time. The more massive sphere has twice as much gravitational potential energy a the top of the incline so it has twice as much total kinetic energy at the bottom of the incline.

The sphere reaches the bottom first and has the greatest speed at the bottom since it has a smaller moment of inertia than the cylinder and therefore has less rotational KE and more translational KE. Both objects have the same amount of gravitational potential energy at the top of the incline since they have the same mass, so they have the same total kinetic energy at the bottom of the incline. But since the cylinder is moving slower it has less translational KE and more rotational KE than the sphere.

Momentum is only conserved if the net external force on the object is zero and angular momentum is only conserved if the net external torque on the object is zero. For single objects this is almost never the case since external resistive forces such as friction and air resistance act on the objects causing them to slow down and eventually stop.

Neglecting friction and air resistance and assuming that your arms don't move when you drop the masses your angular velocity will stay the same. This is somewhat surprising since it seems that your rotational inertia decreases when you drop the masses. Before you drop the masses the total rotational inertia of the system (you, the stool, and the masses) is your moment of inertia plus the moment of inertia of the stool plus the moment of inertia of the masses. But dropping the masses doesn't change their moment of inertia so your moment of inertia doesn't change unless you change the position of your body. Since your moment of inertia doesn't change your angular velocity doesn't change. Another way to think about this is in terms of angular momentum. At the instant you drop the masses they are moving tangent to the circle and so they have the same angular momentum they had just before you dropped them. So you have the same angular momentum after dropping them as before and therefore the same angular velocity. Finally we can think about his in terms of work and energy. You do no work on the masses when you drop them (since you just let go of them, you don't move them through a distance) so your rotational kinetic energy doesn't change and your angular velocity doesn't change.

The hollow sphere will have a larger moment of inertia than the solid sphere since all its mass is far from the axis of rotation. So any experiment that involves the spheres rotating will be able to distinguish them. For example, roll the spheres down a rough incline, starting together from the same height. The solid sphere will reach the bottom first.

Yes. Since torque is force times lever arm, a small force with a large enough lever arm can exert a greater torque than a larger force with a smaller lever arm.

To do a sit-u your must rotate your upper body about an axis through your hips. With your arms behind your head your moment of inertia is greater than with your arms stretched out in front of you so it takes a larger torque to rotate your upper body.

With the mass concentrated close to the body the legs have a smaller moment of inertia than if the mass were uniformly distributed. Thus less torque will be required to have a given angular acceleration, or, alternatively, a higher angular acceleration can be developed for the same torque. Thus the animal can run fast.

With the beam, the moment of inertia of the system is greater than that of the tightrope walker alone. If the walker gets off center, gravity will exert a torque on the walker. With the beam the angular acceleration will be smaller and it will be easier for the walker to get centered again and keep from falling.

Not necessarily in either case. For example in a couple (top diagram) the net force is zero but the net torque is not zero. The object will rotate counterclockwise without any translational motion. Similarly, in the bottom diagram, the net torque is zero but the net force is not zero. The object will move downward without rotating.