Conceptual Physics--Chapter 4: Newton's Second Law of Motion

The briefest form of Newton's 2nd Law of Motion

*a = Fnet / m* also known as Acceleration=(net force)/ (mass)

Mass -mass doubled, then weight... -the same force applied to twice the mass produces... -three times the 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. (Acceleration depends on this as well.) -doubled -1/2 acceleration -1/3 acceleration Acceleration= (1/mass)

-Friction does not depend on speed... A car skidding at high or low speed gives -Friction also does not depend on... -Fluid friction depends on... -Air resistance increases with

-the same friction -area of contact -speed -increasing speeds

-Friction between tire and ground is nearly the same whether the tire is wide or narrow. Purpose of greater contact area is... -Anti lock break of a car...

-to reduce heating and wear -keeps tires from sliding.

A race car travels along a raceway at a constant velocity of 200 km/h. What horizontal net force acts on the car?

0 because its going a constant velocity.

If your friend Katelyn weighs 500 N, what is her weight in pounds?

500/4.45 =

Summarized form of Newton's 2nd Law of Motion

Acceleration ~ net force/ mass

acceleration ~ net force -acceleration of whatever weight is the... -acceleration due to gravity is

An object's acceleration is directly proportional to the net force acting on it. -same -symbolized by g

Force

Any influence that can cause an object to be accelerated, measured in newtons (or in pounds, in the British system).

fluid

Anything that flows; in particular, any liquid or gas.

How does the force of gravity on a raindrop compare with the air drag the drop encounters when it falls at constant velocity?

Because the drop is moving at constant velocity the net force acting on the raindrop is 0N. Therefore the force of gravity, say XN, pulling the drop downward is canceled out by the force of the air pressing onto the raindrop. So this force will be equal and opposite to the force of gravity, -XN.

sliding friction

Contact force produced by the rubbing together of the surface of a moving object with the material over which it slides.

Acceleration ~ 1/ mass

For a given force, the acceleration produced is inversely proportional to the mass.

static friction

Force between two objects at relative rest by virtue of contact that tends to oppose sliding.

Consider a business jet of mass 30,000 kg in takeoff when the thrust for each of two engines is 30,000 N. Show that its acceleration is 2 m/s2.

Force(N)=mass(kg or N) x acceleration(m/s^2) As there are two engines, 30000 x 2 = 30000 x a therefore 2 = a acceleration is therefore 2 m/s^2

air resistance (a.k.a. air drag)

Friction, or drag, that acts on something moving through air.

Static friction is somewhat _____ than sliding friction.

Greater

If you double the force on an object...

Its acceleration doubles as well. (triple,triple)

Free Fall

Motion under the influence of gravitational pull only.

Free fall is motion in which gravity is the only force acting. a) Is a skydiver who has reached terminal speed in free fall? b) is a satellite above the atmosphere that circles Earth in free fall?

Remember that free fall means only gravity is acting the body: (a) The skydiver is not in free fall since they are experiencing air resistance (terminal speed); (b) the satellite is in free fall since it is falling toward Earth - as it is orbiting.

Terminal velocity

Terminal speed together with the direction of motion (down falling objects).

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 1m/s².

Newton's second law -formula -If mass increases

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. -Acceleration ~ net force/ mass -acceleration decreases

If a ball is thrown vertically into the air in the presence of air resistance, would you expect the time during which it rises to be longer or shorter than the time during which it falls? (principle of exaggeration)

The ball rises in less time than it falls. If we exaggerate the circumstance and consider the feather example in exercise 98, the time for the feather to flutter from its maximum altitude is clearly longer than the time it took to attain that altitude. The same is true for the not-so-obvious case of the ball.

Explain how Newton's first law of motion can be considered to be a consequence of Newton's second law?

The first law states that an object will stay at rest or at constant velocity until a force acts on it. The second law states that the acceleration of a body is equal to the force acting on the body divided by its mass. If no force acts on a body it will have no acceleration and hence either remain at rest or remain at a constant velocity. This shows that for the case of zero force, the 1st and 2nd laws are equivalent, or that the first law is a consequence of the 2nd law

Weight

The force due to gravity on an object.

Kilogram

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

If a motorcycle moves with a constant velocity, can you conclude thta there is no ne force acting on it? How about if it's moving with constant acceleration?

The net for is 0 when it is at constant velocity. The accleration is proportional to the net force. Because F=ma

Volume

The quantity of space an object occupies.

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.

Terminal speed

The speed at which the acceleration of a falling object terminates because air resistance balances its weight. When direction is specified, then we speak of *terminal velocity*.

Inversely

The two values change in opposite directions. (As the denominator increases, the whole quantity decreases. For example, the quantity 1/100 is less than 1/10.

Which contains more apples; a 1lb bag of apples on Earth or 1 lb bag of apples on the moon? WHich contains more a 1kg bag of apples on the earth or a 1kg bag of apples on the moon?

a) 1lb bag of apples on the moon contains more apples because gravity on earth is much more than the gravitational pull on the moon. b) They both have the same number of apples because kg-mass so they have the same mass and number of apples

Acceleration and Net Forces are...

directly proportional

An object sliding down an incline experiences... An object sliding right..,

friction directed up friction toward left

The direction of the friction is always...

in the direction that opposes motion.

When you apply a force to an object, friction usually...

reduces the net force and the resulting acceleration.