Physics Chapter 5

In the International Space Station which orbits Earth, astronauts experience apparent weightlessness because a. the station is so far away from the center of the Earth. b. the station is kept in orbit by a centrifugal force that counteracts the Earth's gravity. c. the astronauts and the station are in free fall towards the center of the Earth. d. there is no gravity in space. e. the station's high speed nullifies the effects of gravity.

the astronauts and the station are in free fall towards the center of the Earth.

In a rotating vertical cylinder (Rotor-ride) a rider finds herself pressed with her back to the rotating wall. (Figure 1) Which is the correct free body diagram?

w, Fn up and down. acceleration to the middle of the circle. (a)

A car rounds a curve at a steady 50 km/h. If it rounds the same curve at a steady 70 km/h, will it's acceleration be any different?

yes (think of the equation ac= v^2/r ; as v increases, r stays the same)

A car maintains a constant speed v as it traverses the hill and valley shown in Fig. 5-31. Both the hill and valley have a radius of curvature R. At which point, A, B, or C, is the normal force acting on the car (a) the largest, (b) the smallest? Explain, (c) Where would the driver feel heaviest and (d) lightest? Explain, (e) How fast can the car go without losing contact with the road at A?

(a) The normal force on the car is largest at point C. In this case, the centripetal force keeping the car in a circular path of radius R is directed upward, so the normal force must be greater than the weight to provide this net upward force. (b) The normal force is smallest at point A, the crest of the hill. At this point the centripetal force must be downward (towards the center of the circle) so the normal force must be less than the weight. (Notice that the normal force is equal to the weight at point B.) (c) The driver will feel heaviest where the normal force is greatest, or at point C. (d) The driver will feel lightest at point A, where the normal force is the least. (e) At point A, the centripetal force is weight minus normal force, or mg - N = mv2/r. The point at which the car just loses contact with the road corresponds to a normal force of zero. Setting N=0 gives mg=mv2/r or v= sqrt(gr).

Which of the following point towards the center of the circle in uniform circular motion? a.Velocity, net force. b. Acceleration. c.Velocity, acceleration, net force. d.Acceleration, net force. e. Velocity, acceleration.

Acceleration, net force.

A child whirls a ball in a vertical circle. Assuming the speed of the ball is constant (an approximation) , when would the tension in the cord connected to the ball be greatest? a. At the bottom of the circle. b.At the top of the circle. c.Nowhere; the cord is stretched the same amount at all points. d.A little before the bottom of the circle when the ball is descending quickly. e.A little after the bottom of the circle when the ball is climbing.

At the bottom of the circle.

A car drives at steady speed around a perfectly circular track. a. Both the acceleration and net force on the car point outward. b. Both the acceleration and net force on the car point inward. c. The net force on the car is zero. d. If there is no friction, the acceleration is outward. e.The car's acceleration is zero.

Both the acceleration and net force on the car point inward.

Which pulls harder gravitationally, the Earth on the Moon, or the Moon on the Earth? Which accelerates more? a.Both the same; the Earth. b.The Moon on the Earth; the Moon. c.The Earth on the Moon; the Moon. d.Both the same; the Moon. e.The Moon on the Earth; the Earth. f.The Earth on the Moon; the Earth.

Both the same; the Moon.

A bucket of water can be whirled in a vertical circle without the water spilling out, even at the top of the circle when the bucket is upside down. Explain and draw an FBD also.

For water to remain in the bucket, there must be a centripetal force forcing the water to move in a circle with the bucket. Centripetal force gets larger with greater velocity. The FC comes from tension in the persons arm and the weight of the water. If the velocity isn't great enough the water will spill out.

Would it require less speed to launch a satellite a) toward the east or b) toward the west? Why?

In order to orbit, a satellite must reach an orbital speed relative to the center of the earth. Since the satellite is already moving eastward when launched (due to the rotation speed at the surface of the earth) it requires less additional speed (therefore less energy) to launch it east to get to final orbital speed

A space shuttle in orbit around the Earth carries its payload with its mechanical arm. Suddenly, the arm malfunctions and releases the payload. What will happen to the payload? a.It will fall straight down and hit the Earth. b. It will follow a curved path and eventually hit the Earth. c. It will remain in the same orbit with the shuttle. d. It will drift out into deep space.

It will remain in the same orbit with the shuttle

A child on a sled comes flying over the crest of a small hill. his sled does not leave the ground (he does not catch "air") but he feels the normal force between his chest and the sled decrease as he goes over the hill. Explain this decrease using Newton's second law.

Let Fn = normal force When the sled is moving on level ground then Fn = mg (on level ground)------(1) The crest of a hill is rounded. Therefore the child experiences downward centripetal acceleration. Let centripetal acceleration = a The two forces acting on the child are 1. Weight mg downward 2. Fn upward Acceleration = a (downward) Therefore mg - Fn = ma Fn = mg - ma (on crest)-----------(2) From (1) and (2), Fn on crest is less than that on level ground.

Will the acceleration of a car be the same when a car travels around a sharp curve at a constant 60 km/h as when it travels around a gentle curve at the same speed?

No (ac= v^2/r; v is constant or staying the same while r increases)

A girl is whirling a ball on a string around her head in a horizontal plane. She wants to let go at precisely the right time so that the ball will hit a target on the other side of the yard. When should she let go?

She should let go of the string when the ball is at a position where the tangent line to the circle at the ball's location, when extended, passes through the target's position. That tangent line indicates the direction of the velocity at that instant, and if the centripetal force is removed, then the ball will follow that line horizontally.

Sometimes it is said that water is removed from clothes in a spin dryer by centrifugal force throwing water outwards. Is this correct?

Spinning gets your clothes dry using the same idea as a centrifuge. A centrifuge is simply a machine that spins around to make a large and useful force - the force that makes things shoot outward when they go round in a circle.

A Ping-Pong ball is shot into a circular tube that is lying flat (horizontal) on a table-top. When the Ping-Pong ball exits the tube, which path will it follow in the figure?

Straight out, follows velocity vector

While driving fast around a sharp right turn, you find yourself pressing against the car door. What is happening? a.The door is exerting a rightward force on you. b.Centrifugal force is pushing you into the door. c.Both of the above. d. Neither of the above.

The door is exerting a rightward force on you.

Two satellites orbit the Earth in circular orbits of the same radius. One satellite is twice as massive as the other. a.The lighter satellite moves twice as fast as the heavier one. b.The two satellites have the same speed. c.The heavier satellite moves twice as fast as the lighter one. d. The ratio of their speeds depends on the orbital radius.

The two satellites have the same speed. a=v^2/r Does not depend on mass.

Does an apple exert a gravitational force on the earth? If so, how large a force? Consider an apple (a) attached to a tree, and (b) falling.

Yes an apple exerts a force on the earth, because as Newton's 3rd law states, for every force, there is an equal but opposite reaction force. The earth pulls on the apple with a gravitational force, so the apple must pull with the same strength force, but pulls up on the earth. a) If the apple is attached to a tree, it has two forces acting on it, the force of the earth (gravity) on it, and the force of the tree holding it up. The NET force is 0N. So the force of the apple on the earth is the same as the force of the earth on the apple, its weight. b) When the apple is falling, the only force acting on it is the force of the earth which is the same as before, so the apple still pulls up on the earth with the same force as before, its weight.

Can a particle with constant speed be accelerating? What if it has constant velocity?

Yes. A particle traveling around a curve while maintaining a constant speed is accelerating because its direction is changing. A particle with a constant velocity cannot be accelerating, since the velocity is not changing in magnitude or direction.

A penny is placed on a turntable which is spinning clockwise as shown in Fig. 5-37. If the power to the turntable is turned off, which arrow best represents the direction of the acceleration of the penny at point P while the turntable is still spinning but slowing down?

down to the right

The moon does not crash into the earth because a.the net force on it is zero b.it is beyond the main pull of the earth's gravity c.it is being pulled by the sun as well as by the earth d.it is freely falling but it has a high tangential velocity

it is freely falling but it has a high tangential velocity