Physics - Chapter 4: Forces & the Laws of Motion

State Newton’s 1st law

an object at rest will remain at rest, an object in motion continues in motion with constant velocity unless acted upon by a net external force

State Newton’s 2nd Law

an acceleration of an object is directly proportional to the net external force acting on the object and inversely proportional to the object’s mass

State Newton’s 3rd law

for every action, there is an equal and opposite reaction

What is a force?

any push or pull on an object

How does a seat belt use inertia to work?

due to inertia, the large mass under the seat continues to slide forward along the tracks causing the pin connection between the mass and the rod to make the rod pivot and lock the ratchet wheel in place so the seat belt can’t extend or retract

What is an action-reaction pair?

a pair of simultaneous equal but opposite forces resulting from the interaction of two objects.

Give an example of an action-reaction pair.

a hammer hitting a nail

What is the normal force?

a force exerted by one object on another in a direction perpendicular to the surface of contact

In terms of normal force, explain why it is easier to push a chair across the floor at a constant speed than to push a heavy desk across the floor at the same speed.

because the chair has a lesser mass than the desk and since mass and acceleration are inversely proportional as stated in Newton’s 2nd law, the chair is going to have a greater acceleration than the desk because a higher mass equals a lower acceleration and a lower mass equals a higher acceleration

Is it possible for an object to move if no net force is acting on it? Explain.

Yes, for an object to have a net force, it needs to have an acceleration, for an acceleration to occur, an object must increase or decrease its velocity (or be acted on by a net external force) therefore if an object is moving with a constant velocity there is no acceleration meaning no net force, yet the object is still moving

If an object is at rest, can we conclude that no external forces are acting on it?

No, gravity is always acting upon all objects even when they are at rest, When an object is at rest, there isn’t a net force acting on the object meaning the net force is equal to zero therefore, the object is in equilibrium

What physical quantity is a measure of the amount of inertia an object has?

Mass, the more mass an object has, the greater its inertia

A large crate is placed on the bed of a truck but is not tied down. a.) As the truck accelerates forward, the crate slides across the bed until it hits the tailgate. Explain what causes this.

A large crate is placed on the bed of a truck but is not tied down. b.) If the driver slammed on the brakes, what could happen to the crate?

Earth is attracted to an object with a force equal and opposite the force Earth exerts on the object. Explain why Earth’s acceleration is not equal to and opposite that of the object

the Earth has a greater mass than the object and mass and acceleration are inversely proportional so because the Earth’s mass is large, it has a smaller acceleration and because the object has a small mass, it has a larger acceleration

Explain the relationship between mass and weight.

Weight changes depending on gravity while mass always stays the same.

Which is usually greater, the maximum force of static friction or the force of kinetic friction? Why?

Static friction, because of adhesion, the force required to cause a stationary object to begin moving is usually greater than the force necessary to keep it moving at a constant speed

What is a contact force?

a force that arises from the physical contact of two objects

What is a field force?

a force that can exist between objects, even in the absence of physical contact between the objects

What is equilibrium?

the state in which there is no change in motion

What is the reason that one object accelerates more than another when equal forces are applied to them?

different masses

What is the SI unit for force?

newton (N)

What does weight measure?

the gravitational force between an object and the Earth

What do we notice between electrical charges?

attraction or repulsion

The effect of a force depends in its magnitude and direction so therefore?

force is a vector quantity

What are force diagrams?

diagrams that show vectors as arrows

What is a free-body diagram used for?

to analyze only the forces affecting the motion of a single object

What can you use to find the net external force of an object?

a free-body diagram

What is inertia?

an objects tendency to resist a change in its motion

Why is Newton’s 1st law often referred to as the law of inertia?

because it states that in the absence of forces, a body will preserve its state of motion

What is an external force?

a singular force that acts on an object as a result of the interaction between the object and its environment

What is a NET external force?

the total force resulting from a combination of external forces on an object

Objects can be in equilibrium even when what?

many forces are acting on it

What is the force of attraction?

the weight of an object

What is the force that is perpendicular to the surface of the inclined plane?

the normal force

What is the force that is parallel to the inclined plane?

F_^P

What is the inclined plane assumed to be?

frictionless

In a collision, what does the car undergo?

a large acceleration and rapidly comes to rest

What is quantified in Newton’s 2nd Law?

the relationship between mass, force, and acceleration

What does mass × acceleration equal?

the net external force

What does ΣF represent?

the vector sum of all external forces acting on an object

What do forces always exist in?

pairs

Why don’t action-reaction pairs result in equilibrium?

because they act on different objects

Does Newton’s 3rd law apply to field forces?

yes

What is always perpendicular to the contact surface but is not always opposite in direction to the force of gravity?

the normal force

What does friction oppose?

the applied force

What is static friction?

the resistive force that opposes the relative motion of two contacting surfaces that are at rest with respect to one another

Under what condition is the force of static friction always equal to and opposite in direction to the component of the applied force that is parallel to the surface?

as long as the object does not move

When does the force of static friction reach its max value?

when the applied force is as great as it can be without causing the object to move

What is the force of kinetic friction?

the frictional force on an object in motion

What is the net external force acting on an object equal to?

the difference between the applied force and the force of kinetic friction

What is the magnitude of the force of friction proportional to?

the magnitude of the normal force exerted on an object by a surface

The quantity that expresses the dependence of frictional forces on the particular surfaces in contact is called…

the coefficient of friction

What is the symbol μ?

the Greek letter mu

What is the coefficient of friction defined as?

the ratio between the force of friction and the normal force between two surfaces

What is the coefficient of KINETIC friction?

the ratio of the force of kinetic friction and the normal force

What is the coefficient of STATIC friction?

the ratio of the maximum value of the force of static friction to the normal force