Physics final

What is the net force when a pair of 5-N forces simultaneously act in the same direction on an object?

10 N

Which of the following quantities would be acceptable representations of weight?

12.0 lbs 0.34 g 120 kg 1600 kN 0.34 m 411 cm 899 MN Using the universal law of gravity, we can find the weight of an object feeling the gravitational pull of a nearby planet. We can write an expression w=GmM/r2, where w is the weight of the object, G is the gravitational constant, m is the mass of that object, M is mass of the planet, and r is the distance from the center of the planet to the object. If the object is on the surface of the planet, r is simply the radius of the planet.

Which of the following quantities represent mass?

12.0 lbs 0.34 g 120 kg 1600 kN 0.34 m 411 cm 899 MN

If you pull horizontally on a crate with a force of 200 N, it slides across the floor in dynamic equilibrium. How much friction is acting on the crate?

200 N

A ball freely falling at 20 m/s will in the next second have a speed of _______.

30m/s

What beat frequency will occur when a 370-Hz and a 374-Hz sound source are sounded together?

4 Hz

A 400 kg bear grasping a vertical tree slides down at constant velocity. What is the friction force that acts on the bear?

400 N

In its first second of free fall, a dropped softball will fall a vertical distance of

5m

Consider the video demonstration that you just watched. Which of the following changes would make it more likely for the ball to hit both the white can and the green can?

By Newton's first law, after it has left the circular track, the ball will travel in a straight line until it is subject to a nonzero net force. Thus, the ball can only hit the white can, because that is the only can in the ball's straight-line path.

Imagine that the Sun could be turned into a black hole without changing its mass. How would Earth's orbit change?

Earth's orbit would not change. Notice that as you reduce the radius of the Sun (on the "main screen" in the interactive figure), the curvature of spacetime changes only in the region that was inside the Sun. At greater distances, such as the distance of Earth's orbit (or the distance of any of the other planet orbits), there is no change to the structure of spacetime and hence there would be no change to the orbit.

If Dr. Hewitt applies 3 N of force to keep the block moving at a steady rate, what must be the force of friction, and why?

Equal to 3 N, because the force of friction should exactly balance out the force Dr. Hewitt applies, so that there is no net force acting on the block.

You are dragging a block on a surface with friction at a steady speed of 2 m/s and exert a force of 5 N to do so. What is the force of friction? Why?

Equal to 5 N, because the force of friction should exactly balance out the force Dr. Hewitt applies, so that there is no net force acting on the block.

Why does the distance traveled by the ball in each half second after it is released change in the way that it does?

Gravity is pulling down on the ball, so it accelerates down at a steady rate.

How does Dr. Hewitt break the piece of wood?

He causes a change in momentum of his hand over a short period of time.

After he gets the block to move, what does Dr. Hewitt do to drag the block across the table at a steady rate?

He continues to apply a steady force.

How is Dr. Hewitt able to break a piece of wood in his demonstration?

He moves his hand very quickly.

What would Dr. Hewitt need to have done to exert an even greater force than he did in his karate demonstration?

Increase the change in momentum, and decrease the time duration.

What factors does the speed of sound depend upon? What are some factors that it does not depend upon?

It depends on temperature and humidity, not on loudness and frequency.

How does the distance traveled by the ball in the second half second compare with the distance traveled by the ball in the first half second?

It increases.

For what kind of gravitational field is Newton's law of gravity valid?

Newton's law is a good approximation in weak gravity fields.

Imagine that the Sun gained mass without changing its radius. How would the structure of spacetime change at the distance of Earth's orbit?

Spacetime would become more curved at Earth's orbit. notice that as you increase the central mass (with constant radius) spacetime becomes more highly curved at all distances from the object.

Why does the gravity in the Space Shuttle compare with the gravity on Earth the way it does?

The Space Shuttle is about the same distance from the center of the Earth than from the surface of the Earth.

Why do the astronauts in the Space Shuttle float around?

The Space Shuttle is in free fall, so the shuttle and the astronauts inside it are continuously falling toward the Earth. They thus experience apparent weightlessness.

A block of mass 2kg is acted upon by two forces: 3N (directed to the left) and 4N (directed to the right). What can you say about the block's motion?

The acceleration of an object tells you nothing about its velocity--the direction and speed at which it is moving. In this case, the net force on (and therefore the acceleration of) the block is to the right, but the block could be moving left, right, or in any other direction

Object A has three times the mass of object B. Identical forces are exerted on the two objects. Which statement is true?

The acceleration of object B is three times that of object A. so the object with the lower mass has a higher acceleration

How does the magnitude of acceleration of the rifle compare with the magnitude of acceleration of the bullet, and why?

The acceleration of the rifle is smaller than the acceleration of the bullet. They experience the same magnitude of force, but the mass of the rifle is greater, and so the rifle will experience a smaller acceleration than the bullet.

Why does the final speed attained by the ball at the bottom of the inclined plane change in the way that it does when the angle of the incline plane increases?

The ball accelerates at a greater rate for the same duration of time.

What happens the second time Dr. Hewitt lifts the bowling ball near his teeth and gives it a push?

The ball leaves Dr. Hewitt and returns to him, going past the point where it was released. The extra energy from the push is converted into kinetic energy, which is then converted into more potential energy at the end of the motion than the ball had when it was released.

What happens the first time Dr. Hewitt lifts the bowling ball near his teeth and lets go? Why does the bowling ball behave the way it does?

The ball returns to Dr. Hewitt, stopping almost exactly at the point where it was released. All of the initial energy of the ball was converted completely back to potential energy when the ball returned.

Dr. Hewitt has clay "blobs" hanging on either side of his head in this video. When he turns around, what happens to the blobs?

The blobs tend to stay where they were.

How does the distance traveled by the bullet compare with the distance traveled by the rifle in the same time, and why?

The bullet will travel farther than the rifle because it has a greater magnitude of acceleration.

What general rule can you conclude about the force needed to keep an object in motion at a steady rate?

The force that you need to apply to keep an object moving at a steady rate is equal to the force resisting the motion of the object.

How does the gravity in the Space Shuttle compare with the gravity on Earth's surface?

The gravity in the Space Shuttle is approximately equal to the gravity on the surface of the Earth.

How does the force on the rifle compare with the force on the bullet, and why?

The magnitude of the force of the rifle is equal to the magnitude of force on the bullet and the forces are in the opposite direction, because for every action there is an equal and opposite reaction.

An object cannot remain at rest unless which of the following holds?

The net force acting on it is zero.

If a block is moving to the left at a constant velocity, what can one conclude?

The net force applied to the block is zero.

A bowling ball and a small marble will fall downward to the surface of the Moon at the same rate. Why?

The ratio of the force of gravity exerted on an object to the object's mass is the same.

How does the speed of radio waves compare with the speed of gamma rays?

The speed of radio waves is equal to the speed of gamma rays.

What relationship did Galileo discover about a ball's acceleration and the steepness of an incline? What acceleration occurs when the plane is vertical?

The steeper the incline, the greater the acceleration. On a vertical incline, the ball is in free fall.

Predict what will happen if you reduce the magnitude of the velocity of the green truck to a magnitude smaller than that of the red truck.

The trucks will stick together after the collision and move to the left.

predict what will happen if you change the velocity of the red truck so that it has a magnitude smaller than that of the green truck but is positive.

The trucks will stick together after the collision and move to the right.

predict what will happen if you increase the mass of the green truck so that it is greater than the mass of the red truck.

The trucks will stick together after the collision and move to the right.

what will happen if you change the velocity of the red truck so that it has a magnitude smaller than that of the green truck but is negative.

The trucks will stick together after the collision and move to the right.

what will happen if the red and green truck with equal mass and equal magnitude of velocity collide

The trucks will stick together after the collision and not move.

A parachutist is falling toward the ground. The downward force of gravity is exactly equal to the upward force of air resistance. Which statement is true?

The velocity of the parachutist is not changing with time.

How do the wavelength and frequency of infrared light compare to the wavelength and frequency of ultraviolet light?

The wavelength of infrared is larger than that of ultraviolet, and the frequency is smaller.

Three pucks, A, B, and C, are shown sliding across ice at the noted speeds. Air and ice friction forces are negligible. Rank them, from greatest to least, by the force needed to keep them moving.

They all need the same force to keep moving even if they are moving at different speeds.

Why do the clay blobs do what they do when Dr. Hewitt turns around?

They have inertia−the tendency of an object at rest to stay at rest

When Dr. Hewitt pulls the tablecloth, what happens to the items on the tablecloth?

They tend to stay where they were. Inertia makes them stay at rest.

Some alphabet letters are floating in a bowl of soup. You want to move a letter that is far away from you closer to you. How could you rotate the bowl to do that?

Unfortunately, neither rotating the bowl clockwise nor rotating it counterclockwise will work. They have inertia−the tendency of an object at rest to stay at rest.

Why must the Sun be eclipsed to measure the deflection of starlight passing near the Sun?

Without an eclipse, the Sun's glare makes it impossible to see stars near the Sun.

How can you double the frequency of a wave if you have control over both the wavelength and the wave velocity?

You can either halve the wavelength, keeping the wave speed constant; or double the wave speed, keeping the wavelength constant.

What is the net force on a crate sliding at an unchanging speed when pushed with a steady force of 75 N?

Zero

Interference is a phenomenon that occurs primarily in _______.

all kinds of waves

If the path-length difference between two identical and coherent beams is two wavelengths when they arrive on a screen, will they produce a dark or a bright spot?

bright spot

When traveling twice as fast your kinetic energy is increased _______.

by 4

_______ travels in a wave from one location to another.

energy

A boxer rides with the punch so as to reduce _______.

force

What happens to the frequency of the wave if you increase the wave velocity, keeping the wavelength of the wave constant?

frequency increases

Follow through is important in golf because it results in a greater _______.

impulse on the ball

The iridescent colors seen in thin films are the result of _______.

interference

As the angle of the inclined plane increases, how does the final speed attained by the ball at the bottom of the inclined plane change?

it increases

A kilogram is a measure of an object's __________.

mass

An object is lifted from the surface of a spherical planet to an altitude equal to the radius of the planet. As a result, which of the following changes in the properties of the object take place?

mass remains the same; weight decreases

Resonance occurs when forced vibration _________.

matches natural frequency

A massive block is being pulled along a horizontal frictionless surface by a constant horizontal force. The block must be __________.

moving with a constant nonzero acceleration

A rifle recoils while firing a bullet. The speed of the rifle's recoil is small because the

rifle has much more mass than the bullet.

The unit of mass is the kilogram, and the unit of weight is _______.

the newton

A lunar vehicle is tested on Earth at a speed of 10 km/h. When it travels as fast on the Moon, is its momentum more, less, or the same?

the same

Each second, a ball rolling down an inclined plane picks up _______.

the same amount of speed

The gravitational field on the surface of the earth is stronger than that on the surface of the moon. If a rock is transported from the moon to the earth, which properties of the rock change?

weight only

Where does an object on an elliptical orbit experience the greatest acceleration?

where spacetime has the most curvature The object experiences the greatest acceleration when it is closer to the central mass, where spacetime has more curvature. You can understand this by remembering that gravity arises from curvature of spacetime, so the greatest curvature also means the strongest force of gravity — and Newton's second law of motion tells us that a stronger force will cause the object to accelerate more.

You tell your friend about the neat tablecloth trick that Dr. Hewitt demonstrated. Your friend tries to repeat this trick at home and later complains to you that it failed. Which of the following is most likely the reason that it failed?

your friend pulled the table cloth too slowly