PH 110

Exam 2

In the absence of an external force, the momentum of a system remains unchanged.

True

A bug collides with an elephant. What's true about this situation?

 

The bug exerts a force on the elphant and the elephant exerts a force on the bug.

 

This is likely an elastic collision.

 

The net momentum before the collision equals the net momentum after the collision.

 

Assuming that the bug has the same velocity as the elephant before the collision their momenta before the collision were equal.

The bug exerts a force on the elphant and the elephant exerts a force on the bug. 

The net momentum before the collision equals the net momentum after the collision.

A shark is planning to eat a small fish who is swimming in front of it. Both are swimming in the same direction.

The shark's mass is 400 kg, it moves with 0.5 m/s to the right. The small fish has a mass of 1 kg, it moves with 0.1 m/s to the right. 

Let's do some calculations.

The momentum of the small fish before getting eaten is

The momentum of the shark before eating is

 

The velocity of the shark after eating is

 

The momenum of the shark after eating is

 

The momentum of the small fish before getting eaten is

 

0.1 kg m/s.

The momentum of the shark before eating is

 

200 kg m/s.

The velocity of the shark after eating is

 

nearly 0.5 m/s but a bit less.

The momenum of the shark after eating is

205kg m/s

Assume another fishy situation:

A larger fish (10 kg) swims with 1 m/s to the left, a smaller fish (5 kg) swims with 2 m/s to the right. The smaller fish will eat the larger fish. 

What is true for the velocity of the smaller fish after eating the bigger fish? 

It will stop

How fast is an 85 kg receiver traveling 6 m/s to the right going after catching a 0.43 kg football traveling at 30 m/s right?

6.12 m/s

Work is —— times ——- (write out the answers, no abbreviations).

Force x distance

If you push against a stationary brick wall for several minutes, you do no work

on the wall

Units of work are N m (Newton-meter) or (write out the unit).

Joule

How much work is needed to lift something twice as high?

2x work

A weightlifter wants to lift a barbell of 200 kg mass above his head at a height of 2 m. The barbell is currently resting on a table at 0.5 m height. How much work does he have to do? Give the numerical answer in units of Nm. 

Hint: First calculate the force acting on the resting barbell. Where does this force come from?

3000Nm

Power is related to work as

power=work done/answer

Time interval

A unit of power is

Joule per second

Two engines can do twice the work of one engine in half the amount of time as the power has doubled.

False

A job can be done slowly or quickly. Both may require the same amount of work, but different amounts of

Power

What is true about potential energy?

 

Gravitational potential energy depends on the mass and the height above the ground of an object.

 

The gravitational potential energy of the object of the same mass at the same height is different on the moon and on the Earth.

The moon and the Earth have different accelerations due to gravity.

 

It is stored energy with the potential to do work.

 

Moving objects have potential energy.

 

Potential energy always decreases over time.

Gravitational potential energy depends on the mass and the height above the ground of an object.

 

The gravitational potential energy of the object of the same mass at the same height is different on the moon and on the Earth.

The moon and the Earth have different accelerations due to gravity.

 

It is stored energy with the potential to do work

We saw a video of Sheldon and Leonard pushing a packet of furniture up the stairs, like a ramp. Assume they repeat this situation but now they carry the packet up the stairs. Another repetition sees them just using the elevator. In all cases, the packet arrives at Penny's door on the fourth floor.

What is true about the potential energy of the packet?

Same potential energy

The work-energy theorem states that a change in kinetic energy is the result of work being done. 

True

What will happen to the kinetic energy of a body when its momentum is doubled? Assume the mass of the body is constant.

Kinetic energy will become four times its original value.

A simple machine cannot multiply

 

Energy

Assume a car manufacturer has developed a new car with a 100%-efficient engine. The engine burns fuel that has an energy content of 90 MJ per liter (that's mega Joule). The total air drag and friction force on the car traveling on the highway is 900 N, how far can the car travel per liter fuel on the highway?

First use the definition of work, then assume that all work is used to overcome the air drag and friction force. 

100km/liter

Major sources of energy are 

 

nuclear fuels.

 

fossil fuels.

 

solar energy.

 

wind.

 

geothermal energy.

 

nuclear fuels.

 

fossil fuels.

 

solar energy.

 

wind.

 

geothermal energy.

____ speed is radial distance times rotational distance

Tangential

A uniform meterstick supported at the 25-cm mark balances when a 1-kg rock is suspended at the 0-cm end. What is the mass of the meterstick?  Give the numerical answer in units of kg.

1

Rotational inertia depends on

 

the length of the object.

 

the volume of the object.

 

the weight of the object.

 

distribution of mass around axis of rotation.

 

the mass of the object.

 

distribution of mass around axis of rotation.

 

the mass of the object.

In class we saw the physicists trying to rotate a stubborn bolt on a car tire. Howard found a post of a stop sign to use as handle for a wrench. Compared to the handle of the wrench in their toolbox, the post acts as a handle which is three times as long. How much does the torque change?

 

The torque is now three times bigger.

 

The torque is three times smaller.

 

The torque is now 27-times bigger.

 

The torque is twice as big.

 

The torque doesn't change.

 

The torque is now nine times bigger.

 

The torque is now three times bigger.

Question 9

1 / 1 pts

Consider a stone of mass 150 g which is rotated in a horizontal circle at a tangential speed of 10 m/s. The stone is attached to the end of a 1 m long string. What is its centripetal force? The centripetal force is N. 

What is the acceleration of the stone? The acceleration is m/s^2.

Give the numerical answers. 

15N

100 m/s

inertia of rotation

An object's resistance to changes in its rotational motion. It depends on the object's mass and how far that mass is from the axis of rotation.

angular velocity

The rate at which an object rotates, measured in radians per second (rad/s).

angular momentum

The amount of rotational motion an object has, depending on its rotational inertia and angular velocity.

How many high tides are there every day on Earth?

2

If you double the distance between two objects, the gravitational force drops to (answer) of its original strength

1/4

The gravitational force on an apple at Earth's surface is 1 N. What would the force be if the apple was raised to an altitude that matches twice the the Earth's radius? Think about what the altitude is before and after the raising in units of the Earth's radius.

1/9 N

Before raising, the apple was at the Earth's surface which is at an altitude of one Earth radius from the Earth's center. After raising it, it is 3 Earth's radii from the Earth's center. By the inverse square law, three times the distance means 1/9 of the force.

What is the speed of a projectile that covers a distance of 8000 meters in the time it takes for it to fall vertically 5 meters. Give the numerical answer in units of m/s.

8000

The moon doesn't crash into the Earth because

the moon has a sufficient tangential speed.

A car is moving at a speed of 60 km/h to the east, and a bus is moving at a speed of 40 km/h to the west. What is the relative velocity of the car with respect to the bus? Give the numerical answer in units of km/h.

100

The ether model does not work. 

true

Taking general relativity into account, what is older, your head or your feet?

Your head

What is the difference between special relativity and general relativity?

Special relativity treats physics in flat spacetime whereas general relativity concerns the warping of space and time by mass and energy.

Two events that are simultaneous in one frame of reference need not be simultaneous in a frame moving relative to the first frame.

True

Clocks will tick faster and faster as a spaceship approaches the speed of light.

 

True

 

False

False

Length contraction takes place only in the direction of travel.

 

True

 

False

True

A spaceship, 200 m long as seen on board, moves by the Earth at 0.97 c . What is its length as measured by an Earth-bound observer?

Give the answer in units of meter.

48.6

Tangential speed:

linear speed of something moving along a circular path

Rotational (angular) speed:

number of rotations or revolutions per unit of time.