Physics 101 Chapter 4 New Sec Law Term Vel

ReviewAcceleration: the rate of change of velocity.Acceleration means a an object will start to move if at rest, and if moving, it will speed up, slow down, or change direction.

The relationship between force, mass, and acceleration.

Concept questionA rocket taking off straight upward develops a thrust of 10,000 N. The rocket weighs 2,000 N. The net force on the rocket, neglecting air resistance, isa.zerob.2,000 Nc.8,000 Nd.10,000 Ne.12,000 N

Force causes accelerationExamples: kick a ball, drop a ball, push down on the gas pedal or brake, turn the wheel.Acceleration requires a net force. (Recall the 1st Law and the equilibrium rule.)Newtons First Law says that it requires a net force to accelerate an object. Newtons Second Law tells how much acceleration the object will have when a net force is applied.Acceleration Net force

Fig. 4.2

Friction forcesAlways oppose the direction of motion (or the direction of impending motion.)It depends on the two surfaces in contact and how much they are being pushed together.Static friction and sliding friction. Static is always greater than sliding.

Fig. 4.4

Fig. 3-12, p.49

StaticKineticPush ForceCan there be motion (constant velocity? acceleration?)Text p 52?

KineticPush ForceStaticCan there be motion (constant velocity? acceleration?)?

Friction factsABS brakes prevent skids and thus insure your tires can use static friction to stop.Friction forces do not depend on speed or surface area!So why the big tires, dude?

Mass vs. WeightMass

The amount of matter (stuff) in an object.

It is the same no matter (no pun intended) where the object is.

Weight

It is the force of gravity on an object.

It is directly proportional to mass (weight = mg) where m is in kilograms. (g = 9.8m/s2)

Fig 4.6Weightless in space?Massless in space?

The astronaut and the anvilIs the anvil easier to accelerate from rest in space or on earth?What if the anvil has much more mass?Mass resists acceleration. Fig. 4.7

Fig. 4.11

XXXNewtons 2nd Law of Motion (text p 56-57)

Acceleration net force, But, acceleration 1/massSo acceleration net force/mass

If the force is in newtons (N) and the mass is in kilograms (kg) then or or . . .

Newtons 2nd Law of Motion (text p 56-57)

but

SoOr

Free Fall: Why is g the same regardless of the mass of the falling object??The force of gravity is the

acting on an objectin free fall, and is equalto the objects weight (x g).Only force Fig. 4.12

In the presence of air resistance Net force is less than weightFnet = weight air resistanceFnet = mg - RAcceleration less than gAir resistance depends uponFrontal (cross-sectional) area of falling objectSpeed of falling object

Mr. Green has fallen farther than Mr. Blue (before air resistance = weight).

Why?

Mr. Greens mass = 100 kg

Mr. Blues mass = 50 kg

Mr. Greens weight = ?

Mr Blues weight = ? Fig. 4:15

Mr. Green has fallen farther than Mr. Blue (before air resistance = weight). Why?

Mr. Greens mass = 100 kg

Mr. Blues mass = 50 kg

Mr. Greens weight = 1000 N

Mr Blues weight = 500 N

Starting Air Resistance = 200 N Fig. 4:15

Dont confuse the force with the effect of the force !

Speed is increasing at the same time acceleration is decreasing

As the speed of a falling object increases the force of air resistance increasesThe net force (weight air resistance) decreases so the acceleration decreasesThe force of air resistance increases until it equals the value of the weightAt this timethe net force (F) is 0the object is in equilibriumthe acceleration is 0the object moves with a constant (terminal) velocity

Fig. 4.14

speed increases

air resistance increases

net force decreases (weight air resistance)

acceleration decreases (can this happen while speed increases?)

air resistance increases

until air resistance force = weight force . . . then . . .

Fig. 4.14

When air resistance force = weight force

WHAT DO WE HAVE? !!!

Hint . . . the net force is ?

When air resistance force = weight force

EQUILIBRIUM !!!

Net Force = 0

Acceleration = 0

Velocity = constant . . . and this is . . .

Terminal velocity !!!

Equalibrium of ForcesSum of forces = 0Net force = 0Acceleration = 0Velocity = constant

Why do Sky Divers flatten out?to increase air resistance

to decrease net force (F)

to reach terminal velocity quicker

so terminal velocity is smaller

so . . . when at terminal velocity are we also at equilibrium?!!!

Skydiving Terminal Velocity (text p 59)Normal position closed parachute120 mi/h50% - 3 sec; 90% - 8 sec; 99% - 15 secPull in arms and legs200 mphHead down position (speed skydiving)Current world record: 382 mi/hOpen parachute 11.5 mi/h

Terminal Velocities of Some Objects m/s mi/hrBaseball 33 74Golf ball3272Hail Stone1431Raindrop 920

*CO-3: The descent of the U.S. Army Parachute Team, the Golden Knights, is slowed by air resistance. (Courtesy of U.S. Army Parachute Team, Golden Knights)

*Figure 3-12The static frictional force is equal and opposite to the applied force if the crate does not accelerate. The applied force can be small (a) or large (b) as long as it doesnt cause the box to move. (c) The kinetic frictional force has a constant value independent of the speed.

*

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Physics 101 Chapter 4 New Sec Law Term Vel