Physics 10 Answers to All Chapter Exercises

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 of friction which cancels out the pushing force +200N

A child learns in school that Earth is travelling at 100,000 kph around the sun, "how do we no fly off?" he asks. Explain to him so he doesn't become a flat earther!

100,000 kph refers to linear tangential velocity, when actually determining the rotational spin of earth, it turns out to be an extremely small number. Also, we and all objects are relative to the earth's motion, and we retain its momentum.

If the strong man exerts a downward force of 800N on the rope, how much upward force is exerted on the block?

800 Newtons of upwards force is exerted

How does a headrest counter whiplash?

A headrest prevents your head from jerking back too far, causing whiplash. It effectively stops any further motion.

In tearing a paper towel or plastic bag from a roll, why is a sharp jerk more effective than a slow pull?

A sharp jerk is more effective than a slow pull since it counters inertia faster than if you were to slowly rip it out.

For straight-line how does a speedometer indicate whether or not acceleration is occurring.

A speedometer can indicate acceleration by detecting an evident change in speed over time.

Asteroids have been moving through space for billions of years. What keeps them moving?

Asteroids have been moving billions of years through space due to no other force counteracting them. Since there is no medium for air to travel, there is no air res in space, and thus no fluid friction force to counteract the asteroid's movement.

If you toss a coin straight upward while in a train moving at constant velocity, the coin will land

Back in your hand. Again, the coin presents an inertia reference frame which means it retains the momentum of the moving train.

One airplane travels due north at 300km/h while another travels due south at 300 km/h. Are theirs speeds the same? Are their velocities the same?

Both airplane's speeds are the same; however, their respective velocities are different since they are going in different directions.

Was it Galileo or Newton who first came up with the concept of inertia?

Contrary to popular conscience, Galileo first studied the effects of inertia and defended the idea. However, Newton officially developed the law of inertia and circulated it throughout science.

Copernicus postulated that Earth moves around the sun, but he was troubled about the idea. What concepts of mechanics was he missing that would have eased his doubts?

Copernicus was one of the first astronomers to postulate a proper solar system model; however, he was very skeptical due to a lack of understanding inertia and gravity (later modeled by Newton and Galileo). So in short, the mechanics of the Sun and Earth gravitationally pulling on each other (keeping them in orbit) and the force of inertia resists motion providing the tendency to remain in a stable speed; were not fully understood by Copernicus so he remained skeptical.

How many significant forces act on a book at rest on a table.

Downwards vector: Gravity Upwards vector: Supportive, normal force

An empty jug of weight W is at rest on a table. When water of weight w is poured into it, the amount of support force supplied by the table is

Exactly opposite to the weight, cancelling it out (think of the direction of vectors, gravity is the negative, supportive is the positive).

Why is it important that Tim pulls slightly downward when he attempts to whip the cloth from beneath the dishes? What occurs when he pulls upwards?

For Tim to effectively keep the objects in rest, he must pull slightly downward to prevent the dishes from countering the static friction of the tablecloth. If he pulled upwards, the dishes would most likely be flung back.

What Aristolean idea did Galileo demolish with his experiments with inclined planes?

Galileo demolished the arbitrary Aristolean idea of "objects find their nature" with the inclined plane and instead proved objects are influenced by force and not a pseudoscientific "destiny".

What Aristolean idea did Galileo discredit in his fabled leaning tower experiment?

Galileo discredited the fact that lighter objects were assumed to fall faster than heavier objects, which was false. Both objects (without factoring air resistance) would fall at the same speed due to a constant 9.8m/s^2, which is acceleration due to gravity.

In the answer to the question "what keeps the earth moving around the sun?" A friend asserts that it is inertia that keeps the Earth moving. Correct his ridiculous claim!

His friend is mixing up inertia with the pull of gravity. Yes, an object's resistance to motion allows for earth to remain in constant velocity around the sun, however, it is gravity that keeps the planet itself in orbit. Remember, inertia is simply resistance of motion!

The smokestack of a stationary toy train consists of a vertical spring gun that shoots a steel ball a meter or so straight into the air--so straight that the ball always falls back into the smokestack. Suppose the train moves at a constant speed along the straight track. Do you think the ball will still return to the smokestack? What if the train gains speed along the straight track? What if it moves at a constant speed in a circular track?

If travelling at constant speed, the ball will land in the smokestack, regardless of how fast the train is moving. If there is any change in speed or direction, the ball will not exhibit an inertial reference frame and thus not fall back into the smokestack. Even if it is travelling in a circle at constant speed, it is still accelerating due to change in direction. *Any change in velocity changes acceleration*.

Jacob says acceleration is how fast you go. Emily says it is how fast you get fast. Who is correct?

In truth, they are both wrong. To be specific, acceleration is the change in velocity over time not "how fast you go" or "how fast you get fast". Their answers are extremely ambiguous and do not properly define acceleration.

Your friend states that inertia is a force that keeps things in their place, either at rest or in motion. Do you agree?

Inertia doesn't necessarily KEEP objects in their place or in motion, it simply resists the motion imparted on said object.

Is a fine for speeding based on one's average speed or instantaneous speed?

Instantaneous speed, the radars that are used to track the speed of one individual's vehicle determine its instantaneous speed (or speed at that one moment).

Suppose you're in a moving car and the motor stops running. You step on the brakes and slow the car to half speed. If you release your foot, will the car speed up a bit or will it continue at half speed and slow due to friction.

It will most definitely slow due to friction since there is no more force accentuating any kind of acceleration.

Light travels in a straight line at a constant speed of 300,000km/s. What is the acceleration of light.

Light doesn't accelerate, its speed is a constant 3*10^5 km/s. If it were accelerating, the unit would be km/s^2.

Chapter 3

Linear Motion

Consider a pair of forces, one having a magnitude of 20 N and the other a magnitude of 12 N. What maximum net force is possible for these two forces? What is the minimum net force possible?

Max magnitude: 32 N Minimum magnitude: 2 N All depends on direction of the vectors

Chapter 2

Newton's First Law: Inertia

Can an automobile with a velocity toward the north simultaneously have an acceleration toward the south?

No since acceleration is dependent on what direction you are moving in, which is also part of your velocity. You can't be going in two directions at once.

Can an object be in mechanical equilibrium when only a single force is acting upon it?

No, an object is only in equilibrium when two forces are cancelling out, like normal force and gravitational force; or momentum and sliding friction. If one force is acting upon it, the objects net will be >0

Can an object reverse its direction of travel while maintaining a constant acceleration?

No, if an object changes its direction, it will change its acceleration, and thus, it will not remain constant.

Can you say no force acts on an object in rest? Is it correct to say that no net force influences it?

No, supportive force and gravitational force are acting upon an object at rest. It is proper to say no net force is acted upon it, however. Again, since Σf=0.

Start a ball rolling down a bowling alley and you'll find that it moves slightly slower with time. Does this violates the law of inertia?

No, this does no violate Newton's first law of motion since the friction of the floor serves as a counteracting force to the motion of the ball.

When you push a cart, it moves. When you stop pushing, it comes to rest. Does this violate Newton's law of inertia?

No, this does not violate the law of inertia. When you stop pushing, there is no more force imparted on the cart and thus, rolling friction will cause it to loose momentum.

Suzie Surefoot can paddle a canoe in still water at 8 km/h. How successful will she be canoeing upstream in a river that flows 8 km/h?

She would be extremely unsuccessful since both vectors would cancel out and she would be unable to advance.

A space probe may be carried by a rocket into outer space. What keeps the probe moving after the rocket no longer pushes it?

Similar to the previous question, the space probe will retain its momentum from the rocket so it will move indefinitely unless it encounters an obstacle. The main purpose of the rocket was to reach escape velocity and counteract the gravitational attraction of earth.

Place a heavy book on a table and the table pushes up on the book. Why doesn't this upward push cause the book to rise from the table?

Since it is in static equilibrium with gravity, which keeps it on the table.

A force of gravity pulls downward on a book on a table. What force prevents the book from accelerating downward?

Supportive or normal force. The base result of Newton's Third Law which states all actions have an equal and opposite reaction. This force is fundamental caused by atomic propulsion (unbinding valence orbitals)

Consider a ball resting in the middle of a toy wagon. When the wagon is pulled forward, the ball rolls against the back of the wagon. Interpret this observation in terms of Newton's First Law.

The ball resting in the middle of the toy wagon is resisting the motion imparted on it by being relative to the wagon and thus will roll in the opposite direction compared to what direction in was pulled.

You're in a car travelling at some specified speed limit. You see a car moving at the same speed coming toward you. How fast is the car approaching you, compared with the speed limit?

The car is approaching you at the same speed so compared to the speed limit it is going the same speed as you.

Correct your friend who says, "The dragster rounded the curve at a constant velocity of 100 km/h."

The dragster would not be experiencing constant velocity since its direction is changing and thus its relative velocity is different.

What is the impact speed when a car moving at 100km/h bumps into the rear of another vehicle traveling in the same direction at 98km/h?

The impact speed would be 2km/h

Why do you lurch forward in a bus that suddenly slows? Why do you lurch backward when it picks up speed? What law applies here?

The law of inertia is applies here. Go forward when you stop, back when it accelerates.

Nellie Newton hangs at rest from the ends of the rope as shown. How does the reading on the scale compare with her weight.

The reading on the scale is the the upward, cancellation force so it is exactly opposite of the gravitational, downwards force.

Harry the painter swings year after year from his bosun's chair. His weight is 500N and the rope, unknown to him, has a breaking point of 300N. Why doesn't the rope break when he is supported as shown at the left below? One day Harry is painting near a flagpole, and, for a change, he ties the free end of the rope to the flagpole instead of to his chair, as shown at the right. Why did Harry end up raking his vacation early?

The rope doesn't break since the supporting force of the chair is in equilibrium with the painter. If he attached his free line to the pole, there is no equilibrium so he will fall and get vacation early.

Before the time of Galileo and Newton, some learned scholars thought that a stone dropped off of the mast would fall behind the mast and an equal distance to how much the ship had traveled. With your understanding of inertia, what do you think about this.

The stone and the mast exhibits an inertial reference frame with the moving ship, so its momentum is retained and it will fall straight down.

Consider a crate at rest on a factory floor. As a pair of workmen begin to lift it, does the support force on the crate increase, decrease, or remain he same?

The supportive force decreases to 0 since the box is no longer stationary on the ground., and thus, no inter molecular forces are exerted on it.

A monkey hangs stationary at the end of a vertical vine. What two forces act on the monkey? Which, if either, is greater?

The two forces acting upon the monkey are gravitational pull and muscle tension from the arm. Since the monkey is stationary: Σf=0

For the pulley system shown, what is the upper limit of weight the strong man can lift?

The upper limit is how much weight he can lift, generally his own weight.

Because Earth rotates every 24 hours, the west wall in your room moves in a direction towards you at a linear speed that is 1000+kph. Why doesn't the wall slam into you?

The wall doesn't slam into you since you and everything else is relative to the earth so you do not fly in a line thousands of miles.

In order to slide a heavy cabinet across the floor at consant speed, you exert a horizontal force of +600N, how much frictional force is there?

There is a -600N frictional vector which counteracts the 600N of pushing force, leaving it in constant speed

A ball rolling along a floor doesn't continue rolling indefinitely. Is it because it is seeking a place of rest or because some force is acting upon it?

To understand this question we must first identify, clearly, Newton's First Law of Motion. His first law simply states: an object remains in rest unless and acted upon by an outside force and then that object will continue in motion until acted on by another force. This ball, thus, will be acted on by another force (friction μ) so the answer is the latter: "some force is acting upon it".

Consider an airplane that flies due east on a trip, then returns flying due west. Flying in one direction, the plane flies with Earth's rotation, and in the opposite direction, against Earth's rotation. But, in the absence of winds, the times of the flight are equal either way. Why is this so?

We have already established that objects on earth retain the momentum imparted on them by the rotation. Planes, although not on the ground, are still relative to the atmosphere, which is relative to the Earth. So in short, the flight times are equal due to relative motion.

When any object is in mechanical equilibrium, what can be correctly said about all the forces that act on it? Must the net force necessarily be zero?

When in mechanical equilibrium: Σf=0 must be true. Meaning, all forces acting upon the object must equate to 0.

Each bone in the chain of bones forming your spine is separated from its neighbors by disks of elastic tissue. What happens, then, when you jump heavily onto your feet from an elevated position? Can you think of a reason you might be taller in the morning?

When you jump from an elevated position, your spine will bend down due to inertia. You are taller in the morning since activities during the day cause the spinal section to compress, making you slightly shorter.

If you're in a car at rest that gets hit from behind, you can suffer a serious neck injury called whiplash. What does whiplash have to do with Newton's first law?

Whiplash has to do with the fact that objects resist motion and when a heavy jerk is imparted on an object, it will be flung backwards or forwards depending on the direction of impact. It really describes inertia.

Consider the normal force on a book at rest on a tabletop. If the table is tilted so that the surface is at an incline, will the magnitude of normal force change?

Yes, if the table was at an incline, the normal force would change if the angle increases. The book would fall since the normal force is decreased which would decrease the total static friction causing it to fall.

Two forces act on a parachutist falling in air: weight and air drag. If the fall is steady, with no gain or loss of speed, then it is in equilibrium.

Yes, the parachutist is in dynamic equlibrium since speed is constant and Σf=0

When you push downward on a book at rest on a table, you feel an upward force. Does this force depend on friction?

Yes, this upwards force is dependent on friction since the friction is proportional to the normal force.

When a ball is tossed straight up, it momentarily comes to a stop at the top of its path. Is it in equilibrium during this brief moment?

Yes, Σf=0 is true since both the gravitational force and momentum have cancelled each other out. This is also known as the equilibrium point.

A hockey puck slides across the ice at constant speed. Is Σf=0?

Yes, Σf=0 since the hockey puck is in constant velocity, in other words, it is in dynamic equilibrium.

As you stand on a floor, does the floor exert an upward force against your feet? How much force does it exert? Why are you not moved upward by this force?

You are not moved upwards by this force since gravity counteracts the supportive force via static equilibrium.

You are driving north on a highway. Then, without changing speed, you round a curve at a constant velocity (100 km/h). a) Does the velocity change? b) Do you accelerate?

a) Your velocity evidently changes due to a change in direction b) Due to a change in velocity, acceleration does occur

Two people each pull with 300 N on a rope in a tug of war. What is the net force on the rope?

Σf=0, no net force