Chapter 4 Flashcards by Kimberly Hodnett

Force

Any push or pull exerted on an object, measured in newtons (or pounds in the British system).

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Friction

The resistive force that opposes the motion or attempted motion of an object either past another object with which it is in contact or through a fluid.

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Mass

The quantity of matter in an object. More specifically, it is the measure of the inertia or sluggishness that an object exhibits in response to any effort made to start it, stop it, deflect it, or change in any way its state of motion.

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Weight

The force upon an object due to gravity, mg. (More generally, the force that an object exerts on a means of support.)

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Kilogram

The fundamental SI unit of mass. One kilogram (symbol kg) is the mass of 1 liter (1 L) of water at 4°C.

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Newton

The SI unit of force. One newton (symbol N) is the force that will give an object of mass 1 kg an acceleration of 1 m/s2.

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Volume

The quantity of space an object occupies.

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Newton’s second law

The acceleration of an object is directly proportional to the net force acting on the object, is in the direction of the net force, and is inversely proportional to the mass of the object.

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Free fall

Motion under the influence of gravitational pull only.

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Terminal speed

The speed at which a falling object stops accelerating. Air resistance becomes gravitational force.

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Terminal velocity

Terminal speed with direction specified.

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Is acceleration proportional to net force, or does acceleration equal net force?

Acceleration and net force are directly proportional to each other.

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When you push horizontally on a crate on a level floor that doesn’t slide, how great is the force of friction on the crate?

It takes more force to get it going to keep it sliding. The force of the floor’s friction is opposite and equal to your push.

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As you increase your push, will friction on the crate increase also?

Yes. As you increase your push, friction also increases just as much (Ex: if you push with 70 N, the friction builds up to become 70 N)

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Once the crate is sliding, how hard do you push to keep it moving at constant velocity?

Push with force equal to and opposite the friction force

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Which is normally greater: static friction or sliding friction on the same object?

Static friction.

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How does the force of friction for a sliding object vary with speed?

The friction force remains approximately the same whether it is high speed or low speed, which means it is not dependent on speed.

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Does fluid friction vary with speed?

Yes.

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Which is more fundamental: mass or weight? Which varies with location?

Mass is more fundamental than weight. Weight varies with location.

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Fill in the blanks: Shake something to and fro and you’re measuring its . Lift it against gravity and you’re measuring its .

Mass

Weight

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Fill in the blanks: The Standard International unit for mass is the . The Standard International unit for force is the .

Kilogram (kg)

Pounds (lb)

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What is the approximate weight of a quarter-pound hamburger after it is cooked?

About 1N.

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What is the weight of a 1-kilogram brick resting on a table?

About 10N.

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In the string-pull illustration in Figure 4.8, a gradual pull of the lower string results in the top string breaking. Does this occur because of the ball’s weight or its mass?

The string breaks because of the weight of the ball.

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In the string-pull illustration in Figure 4.8, a sharp jerk on the bottom string results in the bottom string breaking. Does this occur because of the ball’s weight or its mass?

It occurs because of the mass of the ball.

Is acceleration directly proportional to mass, or is it inversely proportional to mass?

Acceleration is inversely proportional to mass.

State Newton’s second law of motion.

The acceleration of an object is directly proportional to the net force acting on the object, is in the direction of the net force and is inversely proportional to the mass of the object.

If we say that one quantity is directly proportional to another quantity, does this mean they are equal to each other? Explain briefly, using mass and weight as an example.

No. Weight is proportional to mass, but not equal to mass.

If the net force acting on a sliding block is somehow tripled, what happens to the acceleration?

The acceleration would also be tripled.

If the mass of a sliding block is tripled while a constant net force is applied, by how much does the acceleration change?

The acceleration decreases to one-third.

If the mass of a sliding block is somehow tripled at the same time the net force on it is tripled, how does the resulting acceleration compare with the original acceleration?

The acceleration remains the same.

How does the direction of acceleration compare with the direction of the net force that produces it?

Acceleration and net force are in the same direction.

What is the condition for an object experiencing free fall?

Gravity is the only force acting on the object.

The ratio circumference/diameter for all circles is π. What is the ratio force/mass for freely falling bodies?

The ratio of mass is g.

Why doesn’t a heavy object accelerate more than a light object when both are freely falling?

Acceleration of free fall is independent of an object's mass because in free fall they have the same acceleration.

What is the net force that acts on a 10-N freely falling object?

The net force is 10N.

What is the net force that acts on a 10-N falling object when it encounters 4 N of air resistance? 10 N of air resistance?

6 N | 10 N

What two principal factors affect the force of air resistance on a falling object?

Speed and frontal area affect the force of the air resistance.

What is the acceleration of a falling object that has reached its terminal velocity?

Why does a heavy parachutist fall faster than a lighter parachutist who wears a parachute of the same size?

A heavier parachutist must fall faster for air resistance to balance weight.

If two objects of the same size fall through the air at different speeds, which encounters the greater air resistance?

The faster object.

When you push horizontally on a crate that doesn't slide on a level floor, how great is the force of friction on the crate?

The friction force is equal and opposite to your push.

What is the weight of a 1-kilogram brick resting on a table?

10 N.

State Newton's second law of motion.

Acceleration is proportional to net force and inversely proportional to mass.

How does the direction of acceleration compare with the direction of the net force that produces it?

The acceleration is in the direction of the net force.

What is the condition for an object experiencing free fall?

When gravity is the only force acting on an object, it is in free fall.

As a skydiver gains speed in falling through the air, air resistance _______.

increases.

Air resistance on a parachutist at terminal speed _______.

is greater for a heavier person.

Once the crate is sliding, how hard do you push to keep it moving at constant velocity?

You push with a force equal to and opposite the dynamic friction force.

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

Newton.

Whenever a net force acts on an object, there is a change in the object's _______.

velocity.

A crate sits at rest on a factory floor. Friction between the crate and floor occurs _______.

when the crate is pushed horizontally, whether sliding or not.

As you increase your push on a stationary crate, will friction on the crate increase also?

The friction force increases and is opposite in direction to your push.

Which varies with location, mass or weight?

Weight varies with location, but mass does not.

If the mass of a sliding block is somehow tripled at the same time the net force on it is tripled, how does the resulting acceleration compare with the original acceleration?

The acceleration remains the same.

Why doesn't a heavy object accelerate more than a light object when both are freely falling?

The ratio of the weight to mass is the same for all objects in the same locality.

What is the acceleration of a falling object that has reached its terminal velocity?

The acceleration is 0.

If two objects of the same size move through the air at different speeds, which encounters the greater air resistance?

The faster object encounters more air resistance.

An object at rest 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 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?

It could be moving to the left, moving to the right, or be instantaneously at rest.

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.

Two forces, of magnitude 4N and 10N, are applied to an object. The relative direction of the forces is unknown. The net force acting on the object __________.

cannot have a magnitude equal to 5N.