How Things Work: An Introduction to Physics week 6

In this blog you will find the correct answer of the Coursera quiz How Things Work: An Introduction to Physics week 6 mixsaver always try to brings best blogs and best coupon codes

Week- 6

Wheels

1. You are walking at constant velocity on a sidewalk that slopes gently uphill. What force(s) is the sidewalk exerting on you?

A frictional force directed uphill, parallel to the sidewalk’s surface, and a support force directed perpendicular to the sidewalk’s surface.
A ramp force directed downhill.
A support force directed perpendicular to the sidewalk’s surface, but no frictional force.
Zero force, since you are at constant velocity.

2. For millennia, people have ground grain into flour by placing a thin layer of grain between two surfaces that slide across one another. Those two surfaces are usually stacked on top of one another and the surface above the grain is that of a very heavy object, such as a millstone. When motionless, the heavy millstone will exert large support forces on the grain and thereby crush that grain to some extent, but it won’t make fine flour. Spinning the millstone while keeping the surface below the grain motionless, however, will grind the grain as fine as you like. What role does the millstone’s weight play in this grinding process?

The millstone’s weight reduces the effects of friction so that it is easier to keep the millstone turning steadily.
The millstone’s great weight keeps the grain from bunching up as the millstone rotates.
The heavy millstone continues to exert large support forces on the grain, even as the millstone spins, so that the support forces continue to crush the grain into flour. Frictional forces are unavoidable nuisances that only make the grinding process more difficult.
The grinding process uses both support forces to crush the grain and sliding frictional forces to wear the grain. Since frictional forces are approximately proportional to support forces, the millstone’s weight enhances both the crushing and the wearing processes.

3. Walking on ice can be treacherous, but some techniques are safer than others. In general, you are less likely to slip and fall if you lower your foot vertically into place on the ice as you take each step than you are if you slide your foot horizontally into place on the ice as you take each step. Why is the vertical landing method of walking on ice usually safer than the horizontal sliding method?

The vertical landing method increases the support forces the ice exerts on you and larger support forces can prevent you from slipping sideways and falling.
The static frictional forces you can obtain from the ice using the vertical landing method are larger, and more effective at preventing sideways slips, than the sliding friction forces you are likely to obtain from the ice using the horizontal sliding method.
The vertical landing method increases your mass and makes it harder for you to accelerate sideways and slip.
The vertical landing method increases your weight and makes it harder for you to accelerate sideways and slip.

4. Most automobiles have mechanical brakes on all four of their wheels. Each of these brakes consists of two surfaces—one surface that rotates with the wheel and one surface that doesn’t rotate. When you put your foot on the brake pedal in such an automobile, those two surfaces begin to slide across one another. The harder you press on the brake pedal, the more tightly those surfaces are pressed against one another. Why do the brakes permit the two surfaces to slide across one another, rather than locking those two surfaces together so that they don’t slide across one another?

The purpose of the brakes is to waste the moving automobile’s kinetic energy, using sliding friction, and thereby slowing the automobile safely. Locking the brakes would result in static friction in the brakes and prevent the brakes themselves from wasting more than a tiny fraction of the automobile’s kinetic energy.
The purpose of the brakes is to waste the moving automobile’s kinetic energy and thereby slowing the automobile safely. While either static or sliding friction between the brake surfaces wastes energy, static friction offers more control as it wastes energy than sliding friction does as it wastes energy.
Locking the brakes would stop the automobile instantly.
Locking the brakes would stop the automobile much faster than unlocked brakes, but not instantly.

5. The harder you press on an automobile’s brake pedal, the greater the support forces that the two surfaces in each brake exert on one another. Why does this increase in support forces in the brakes result in more rapid deceleration (acceleration opposite its velocity) of the automobile?

As the backward support force that the brake exerts on the car increases, so does the car’s backward acceleration.
As those support forces increase, the backward force the car exerts on itself increases and the car slows more rapidly.
The sliding frictional forces the two brake surfaces exert on one another are approximately proportional to their support forces on one another. As the support forces increase, the sliding frictional forces also increase; they waste the automobile’s kinetic energy faster, so that it slows more rapidly.
As those support forces increase, the car’s mass increases and its speed must decrease as a result.

6. The front wheel of your bicycle spins freely on its axle and rotates at almost constant angular velocity if nothing outside the bicycle exerts a torque on it. Suppose the front wheel is motionless as you stand next to your bicycle. You get on the bicycle and pedal forward. As the bicycle begins to move forward, why does its front wheel begin to rotate?

The ground exerts a forward static frictional force on the bottom of the front wheel to help the bicycle accelerate forward. That frictional force, exerted at a lever arm from the wheel’s center of rotation, produces the torque that causes the wheel to begin rotating.
The ground exerts an upward support force on the bottom of the front wheel to prevent that wheel from falling into the ground. That support force, exerted at a lever arm from the wheel’s center of rotation, produces the torque that causes the wheel to begin rotating.
The ground exerts a backward static frictional force on the bottom of the front wheel to prevent that wheel from sliding across the ground. That frictional force, exerted at a lever arm from the wheel’s center of rotation, produces the torque that causes the wheel to begin rotating.
The force you exert on a pedal, exerted at a lever arm from the front wheel’s center of rotation, produces the torque that causes the front wheel to begin rotating.

7. When you drive a car on a level (horizontal) road that is slippery with ice, you usually have no problems except when you try to speed up, slow down, or turn. Why does the icy road make those three actions hazardous? [neglect any effects due to air]

Each action involves a horizontal acceleration and requires a horizontal force. The only forces that level pavement can exert on the car are support forces and the brittle, fragile ice reduces those support forces.
When the car is moving at constant velocity, its wheels are experiencing static friction with the pavement. When it is speeding up, slowing down, or turning, its wheels are experiencing sliding friction. Since ice exerts zero sliding friction, those three actions are not possible.
When the car is moving at constant velocity, its wheels are experiencing sliding friction with the pavement. When it is speeding up, slowing down, or turning, its wheels are experiencing static friction. Since ice exerts zero static friction, those three actions are not possible.
Each action involves a horizontal acceleration and requires a horizontal force. The only forces that level pavement can exert on the car are frictional forces and the slippery ice reduces those frictional forces.

8. Pedaling your bicycle provides power to its rear wheel and propels your bicycle forward. What force(s) is principally responsible for the bicycle’s forward acceleration as you pedal your bicycle forward from rest on a level (horizontal) road?

The pavement exerts a forward support force on the bottom of the rear wheel.
The pavement exerts a forward frictional force on the bottom of the rear wheel.
The pavement exerts forward frictional forces on the bottoms of the front and rear wheels.
The pavement exerts forward support forces on the bottoms of the front and rear wheels.

9. If you throw a ball directly upward at 10 meters-per-second (22 mph), it will rise upward about 5 meters before coming momentarily to a stop. If you throw that ball directly upward at 20 meters-per-second (44 mph), how far will it rise upward before coming momentarily to a stop?

About 10 meters.
About 5 meters.
About 20 meters.
About 40 meters.

10. Earth’s oceans exhibit tides, so that the average water level near most shores rises and falls at roughly six-hour intervals. A somewhat simplistic explanation for the tides is that the moon’s and sun’s gravity slightly distort Earth’s oceans, creating high and low regions, and the Earth’s rotation causes those high and low regions to shift on the Earth’s surface. There are a few tidal generating plants on Earth that use energy in the tides to generate electricity. Where does nearly all of the energy in the tides originally come from?