Chapter 5Chapter 5Newton’s 2Newton’s 2ndnd Law of Motion Law of Motion

Force, mass and accelerationForce, mass and accelerationNewton’s 2nd Law – Force is directly related to the mass and acceleration of an object (F=ma)

Acceleration = change in velocity / time

Force causes acceleration – Force ~ acceleration

Mass resists acceleration - acceleration is inversely proportional to the mass:acceleration ~ 1/mass

Keeping the force the constant, if you triple the acceleration, the mass is:A) Tripled B) 1/3 C) 1/9 D) 1/2

Newton’s 2Newton’s 2ndnd Law Law

Force = Mass x AccelerationForce = Mass x Acceleration

Mass = Force / AccelerationMass = Force / Acceleration

Acceleration = Force / MassAcceleration = Force / Mass

If you double the force on an object, the acceleration will:A) ½ B) ¼ C) not change D) Double

Acceleration is measured in m/s²Force is measured in NewtonsMass is measured in kg

If the man pulls on the box with 400 N,And the box accelerates at .5 m/s², whatIs the mass of the box?

A) 200 kg B) 400 kg C) 800 kg

Newton’s 2Newton’s 2ndnd Law Law

More practiceMore practiceIf a truck can accelerate at 2 m/s², What acceleration can it attain of it is Towing another truck of equal mass?A).5 m/s² B) 1 m/s²

C) 2 m/s² D) 4 m/s²

How much force is required to move a 4000 kg mass 2 m/s²?A) 2000 N B) 4000 N C) 8000 N D) 16,000 N

Newton’s 2Newton’s 2ndnd Law Law

FrictionFrictionIrregularities of surfaces when objects Irregularities of surfaces when objects

are in contact with each other. This are in contact with each other. This is sliding or rolling frictionis sliding or rolling friction

Fluid friction –resistance that objects Fluid friction –resistance that objects undergo as they flow through a fluid undergo as they flow through a fluid (not to be confused with a liquid)(not to be confused with a liquid)

What type of friction is air resistance? A) Sliding B) Rolling C) Fluid

FrictionFriction

When friction is present, an object may When friction is present, an object may move at a constant velocity even move at a constant velocity even when an outside force is applied to it. when an outside force is applied to it. The frictional force balances out the The frictional force balances out the force applied.force applied.

While your driving 55 mi/hr in your car, you need to apply a constantForce (gas pedal) to maintain a constant speed - you are overcoming the Frictional force that opposes motion.

500 N200N

A BC

500 N 400 N300 N

FrictionFriction

If the forces on an object are balanced – IT If the forces on an object are balanced – IT DOES NOT MEAN IT IS ALWAYS DOES NOT MEAN IT IS ALWAYS STOPPED!!!!!!STOPPED!!!!!!

If forces are balanced – it means there is If forces are balanced – it means there is no acceleration!no acceleration!

A gas pedal on a car is pressed and the car accelerates to 50 mi/hr atWhich point it maintains a constant speed. If the car has to maintainA constant force of 900 N (by pressing the gas), what is the forceApplied by friction?A) 0 N B) less than 900 N C) 900 N D) more than 900 N

FrictionFrictionA skydiver jumps out of an airplane and A skydiver jumps out of an airplane and

accelerates to 120 mi/hr. Air resistance accelerates to 120 mi/hr. Air resistance (Friction) is equal to the force falling (Friction) is equal to the force falling (gravity).(gravity).

This point at which forces are balanced This point at which forces are balanced in freefall is known as in freefall is known as Terminal VelocityTerminal Velocity..

Is the sky diver accelerating when at Terminal Velocity?

If a 70 kg person jumps out of an airplane, how much FORCE is gravity pulling them down?A) 70 kg B) 70 N C) 700 N D) 7000 N

When the sky diver reaches terminal velocity, how much force is friction providing?A) Less than 700 N B) 700 N C) more than 700 N

Air ResistanceAir ResistanceWhich sky diver will achieve a greater Which sky diver will achieve a greater

Terminal Velocity?Terminal Velocity?

A)A) B)B)

A) 8 kg bowling ball B) .002 kg penny C) they will be the same

Think about the amount of frictional force needed to balance out the forces.

If person A has 2 times the mass of person B and they both pull their identical parachutes outAt the same time, who will reach the ground first?

Which will reach the ground first if is thrown from a high flying airplane?

Falling and Air Falling and Air ResistanceResistance

So if there is a cannonball that has a So if there is a cannonball that has a 10 kg mass and a stone that has 1 kg 10 kg mass and a stone that has 1 kg mass and you drop them off a tower mass and you drop them off a tower – why do they hit the ground at the – why do they hit the ground at the same time?same time?

Cannon ball’s weight = m x a = 10kg x 9.8m/s² = 98 N of Cannon ball’s weight = m x a = 10kg x 9.8m/s² = 98 N of forceforce

Stone’s weight = m x a = 1 kg x 9.8 m/s² = 9.8 N of forceStone’s weight = m x a = 1 kg x 9.8 m/s² = 9.8 N of force

The cannonball has more force towards the The cannonball has more force towards the ground….ground….

But remember, F =ma rearranged is acceleration = Force / Massfor the cannonball a = 98 N / 10 kg = 10 m/s²m/s² for the stone a = 9.8 N / 1 kg = 10 m/s²m/s²

10 m/s²m/s² is gravity’s accelerationIf you add in the effects of friction, the rate of falling changes – the cannonball needs to apply 98 N of force to make balanced forces whereas the stone needs only 9.8 N to balance forces. If you were a friction devil – what would be easier to slow down, 98 N or 9.8 N?

By dropping the Objects off a tower,The air friction isMinimal because the Objects can’t reach Fast enough speeds For air friction to Make a noticeable Difference.

Applying Force - Applying Force - PressurePressure

Pressure = Force / area of application Pressure = Force / area of application P=F/AP=F/A

Think of the air in your car tire, it’s Think of the air in your car tire, it’s measured in lbs per in². lbs is the force, measured in lbs per in². lbs is the force, in² is the area.in² is the area.

Metric Unit of pressure is the PASCALMetric Unit of pressure is the PASCAL

Newtons of force per square meterNewtons of force per square meter

N/m²N/m²

Pressure and ForcePressure and Force

The more surface area a force is The more surface area a force is applied to the more the force is applied to the more the force is spread out.spread out.

For example, if you stand on one foot, For example, if you stand on one foot, you are exerting a force in less area you are exerting a force in less area than if you are standing on 2 feet. than if you are standing on 2 feet.

Spreading out a person’s weight (Mass x gravity) is a lot easier when there is more surface area

Importance of Surface Importance of Surface AreaArea

The force of the hammer is spread out to hundreds of nails500 N of force from the hammer on 500 nails = 1 N of force on

every nail