Forces and Newton's 1st Law

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A 6-kg object is moving to the right at a constant velocity of 2 m/s. Which of the following horizontal forces is required to maintain this state of motion?

The ball is experiencing an unbalanced force; it is directed downwards

A ball is thrown vertically upwards. After the ball is released and is moving upwards towards its peak, it slows down. During this time, ______.

Aristotle would say that the ball comes to rest because the ball seeks its natural state of rest. Galileo would likely have said it comes to rest because of some forces acting on it; likely friction between the ball and table surface and with the air.

A ball rolls across the top of a billiard table and slowly comes to a stop. How would Aristotle interpret this observation? How would Galileo interpret it? 1. Aristotle would say that the ball comes to rest because the ball seeks its natural state of rest. Galileo would likely have said it comes to rest because of some forces acting on it; likely friction between the ball and table surface and with the air. 2. All are wrong. 3. They both would say that the ball comes to rest because the ball seeks its natural state of rest. 4. Galileo would say that the ball comes to rest because the ball seeks its natural state of rest. Aristotle would likely have said it comes to rest because of some forces acting on it; likely friction between the ball and table surface and with the air. 5. They both would say that it comes to rest because of some forces acting on it; likely friction between the ball and table surface and with the air.

Gravity, normal

A book is at rest upon a table.

friction, normal, gravity, tension

A dog is being slowly pulled by his dog chain across the ground at a constant speed.

It is in equilibrium

A hockey puck slides across the ice at a constant speed. Which of the following is true? 1. It is in equilibrium. 2. The puck is at rest. 3. The puck is moving and thus not in equilibrium. 4. The puck can be considered neither at rest nor in equilibrium. 5. None of these

friction, gravity, normal, applied

A large crate is being pushed leftward across the floor at a constant velocity. Neglect air resistance

Directed rightward

A leftward moving object is coasting to a stop along a straight, level surface. The net force acting upon the object must be _____.

gravity, friction, normal

A physics book is sliding across a level table

a force

A push or pull that gives energy to an object, causing that object to start moving, stop moving, or change its motion is 1. a force. 2. inertia. 3. pressure. 4. friction.

air resistance, normal, gravity, friction

A rightward moving truck skids to a stop from a very high speed with its wheels locked

Gravity, tension

A sign is suspended by two cables and hangs at rest

air resistance, gravity

A skydiver is falling at a constant velocity with a not yet opened parachute

The skydiver will continue to fall, but now will slow down; the skydiver will have an acceleration that is directed upwards

A skydiver pulls the chord of his parachute. The forces acting upon the skydiver and his parachute are shown in the diagram at the right. From this diagram, one might infer that ______.

the presence of an unbalanced force (e.g. friction) can cause a moving object to stop

A sled slides down a hill, reaches the level surface and eventually comes to a stop. The fact that the sled ultimately comes to a stop can best be explained by _______ .

air resistance, friction, gravity, normal

A sledder slows down as it glides into the wind across some unpacked snow.

Nothing specific; in the absence of forces it would continue moving in a straight line.

A space probe is carried by a rocket into outer space where it continues to move on its own in a straight line. What keeps the probe moving? 1. The gravitation forces from different stars and planets 2. a propeller 3. None of these 4. Nothing; the probe will eventually stop. 5. Nothing specific; in the absence of forces it would continue moving in a straight line.

must either be directed eastward or westward

A westward-moving object is changing its speed. The net force on the object ______.

Inertia

All objects have what?

air resistance, gravity

An eagle glides through the air along a horizontal path

Gravity, tension

An elevator is suspended by cables and slowly descends through the elevator shaft. There is no contact with the walls of the elevator shaft. Neglect air resistance

Zero

An object is moving along a level surface with a constant velocity. The net force acting upon the object is _____.

be experiencing a balance of forces

An object moving at a constant velocity must ______.

Nonsense! All objects have inertia

An object will not have any inertia if it is _____ .

No; at least one other force is needed to cancel the action of the first force

Can an object be in mechanical equilibrium when only a single force acts on it? 1. Yes; the object will act back with an equal and opposite force. 2. None of these 3. Yes; a single force is necessary to keep the object in mechanical equilibrium. 4. No; at least one other force is needed to cancel the action of the first force. 5. No; even one force is too much. There should be no forces acting on an object.

Zero, since no force is necessary to keep an object moving

If you were in a spaceship and fired a cannonball into frictionless space, the amount of force needed to keep the cannonball in motion would be _____.

Speed/velocity

Inertia does not depend on what?

Force

Inertia is not what?

the tendency of moving objects to maintain their state of motion the tendency of stationary objects to remain at rest

Inertia refers to _____.

Jack

Jack and Jill are arguing in the cafeteria. Jill says that if she flings the Jello with greater speed it will have greater inertia. Jack argues that inertia does not depend upon speed, but that inertia depends upon mass. Who is correct?

An object at rest stays at rest; an object in motion stays in motion with the same speed and direction unless acted upon by an unbalanced force

Newton's 1st Law

keep an object moving the way it is already moving

No force is necessary to 1. cause a change in the motion of an object. 2. start an object moving. 3. stop an object from moving. 4. keep an object moving the way it is already moving.

No; air resistance and friction act upon the ball

Start a ball rolling down a bowling alley and you'll find it moves slightly slower with time. Does this violate Newton's law of inertia? 1. Yes; no force acts upon it. 2. No; air resistance and friction act upon the ball. 3. No; the law of inertia can also be applied to moving objects. 4. Yes; the air resistance cancels the friction and the total force on the ball is zero. 5. None of these

continue in motion with the same speed and direction

Suppose that an astronaut throws a rock in outer space at a location far from significant influences of gravity and air resistance. One would expect that the rock would ____.

Is not accelerating

The forces acting upon an object are balanced. Therefore, one can be absolutely certain that the object _______.

is accelerating

The forces acting upon an object are not balanced. Therefore, one can be absolutely certain that the object _______.

to both moving and nonmoving objects

The law of inertia (Newton's first law) applies _____.

False

True or false: inertia is a force that keeps stationary objects at rest and moving objects in motion at a constant velocity

Balanced forces

Two forces equal in magnitude, but opposite in direction; object's motion does not change (at rest, stays at rest; in motion, continues at a constant speed along a straight line); no acceleration

Forces

What are NOT needed to keep objects moving?

inertia

What is the tendency of matter to resist any change in motion? 1. pressure 2. friction 3. momentum 4. inertia

Unbalanced forces

When a force is not cancelled out by one or more opposing forces; object's motion will change (speed up, slow down, change directions); acceleration will be in the same direction as unbalanced force

maintain its state of motion; keeps its velocity constant

When all individual forces acting on an object are balanced, it is the natural tendency of an object to ____.

maintain its state of motion, keep its velocity constant (either at zero or non-zero)

When all individual forces acting upon an object are balanced, it is the natural tendency of the object to _______.

a mosquito

Which object has the smallest inertia? 1. An ocean liner 2. A car 3. A VW bug 4. A mosquito

The billions of force pairs are internal to the book and exert no net force on the book

Within a book on a table there are billions of forces pushing and pulling on all of the molecules. Why is it that these forces never by chance add up to a net force in one direction, causing the book to accelerate "spontaneously" across the table? 1. The forces cause the book to move across the table spontaneously all the time, but the movement is too weak to observe. 2. These forces are counteracted by grav- ity. 3. These forces between molecules are much smaller than the friction between the book and the table. 4. The billions of force pairs are internal to the book and exert no net force on the book.

Disagree; inertia is a property of matter to behave this way, not some kind of force.

Your friend says that inertia is a force that keeps things in their places, either at rest or motion. Do you agree? Why or why not? 1. Agree; inertia is not a force that keeps things moving. 2. Disagree; inertia is a force that keeps things moving. 3. Agree; only forces can keep things in their places. 4. All are wrong. 5. Disagree; inertia is a property of matter to behave this way, not some kind of force.

gravity

a football is moving upward and rightward toward the peak of its trajectory. Neglect air resistance

applied

a force that is applied to an object by a person or another object; reserved for situations where other specific force types do not suitably describe the force

gravity, normal

a person is standing upon the ground

Force

a push or pull exerted upon an object as a result of its interaction with another object; vector quantity

mechanical equilibrium

a state in which there is no change in an object's motion; no net external force (no external forces at all, or all external forces cancel each other out)

field forces

force resulting when two interacting objects are not in physical distance; gravitational, electrical, magnetic

contact forces

force resulting when two interacting objects physically contact each other; friction, tension, normal, air resistance, applied

less inertia

less mass =

More inertia

more mass =

dynamic equilibrium

object moves with constant velocity (no acceleration)

static equilibrium

object remains at rest

Inertia

tendency of an object to resist changes in its state of motion (velocity, acceleration)

spring

the force exerted by a compressed or stretched spring upon any object that is attached to it

friction

the force exerted by a surface as an object moves across it or makes an effort to move across it; opposes motion or attempted motion—directed opposite velocity

air resistance

the force exerted on moving object by the surrounding air

tension

the force transmitted through a string, rope, cable, or wire when it is pulled tightly by forces acting from opposite ends

gravity

the force with which the earth, moon, or other massively large object attracts another object towards itself

Dynamics

the study of how forces affect the motion of objects

Kinematics

the study of motion without considering its causes

normal

the support force exerted upon an object that is in contact with another stable object; anything on a supported surface

net force

the vector sum of all the forces that act upon an object

false

true or false: if a rightward moving object experiences an unbalanced force, then the object will have an acceleration which is directed to the right