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### Chapter 5

Newton’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

Newton’s 2nd Law of Motion

Force, mass and acceleration

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 2nd Law

If you double the force on an object, the acceleration will:

A) ½ B) ¼ C) not change D) Double

Force = Mass x Acceleration

Mass = Force / Acceleration

Acceleration = Force / Mass

Acceleration is measured in m/s²

Force is measured in Newtons

Mass is measured in kg

If the man pulls on the box with 400 N,

And the box accelerates at .5 m/s², what

Is the mass of the box?

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

Newton’s 2nd Law

More practice

- If a truck can accelerate at 2 m/s²,
- What acceleration can it attain of it is
- Towing another truck of equal mass?
- .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 2nd Law

Friction

Irregularities of surfaces when objects are in contact with each other. This is sliding or rolling friction

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

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

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

B

A

C

200N

500 N

500 N

400 N

300 N

While your driving 55 mi/hr in your car, you need to apply a constant

Force (gas pedal) to maintain a constant speed - you are overcoming the

Frictional force that opposes motion.

Friction

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

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

A gas pedal on a car is pressed and the car accelerates to 50 mi/hr at

Which point it maintains a constant speed. If the car has to maintain

A constant force of 900 N (by pressing the gas), what is the force

Applied by friction?

A) 0 N B) less than 900 N C) 900 N D) more than 900 N

Friction

A skydiver jumps out of an airplane and accelerates to 120 mi/hr. Air resistance (Friction) is equal to the force falling (gravity).

This point at which forces are balanced in freefall is known as Terminal 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 Resistance

Which sky diver will achieve a greater Terminal Velocity?

A) B)

If person A has 2 times the mass of person B and they both pull their identical parachutes out

At the same time, who will reach the ground first?

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

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

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

Falling and Air Resistance

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

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

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

The cannonball has more force towards the ground….

By dropping the

Objects off a tower,

The air friction is

Minimal because the

Objects can’t reach

Fast enough speeds

For air friction to

Make a noticeable

Difference.

But remember, F =ma rearranged is acceleration = Force / Mass

for the cannonball a = 98 N / 10 kg = 10 m/s²

for the stone a = 9.8 N / 1 kg = 10 m/s²

10 m/s² is gravity’s acceleration

If 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?

Applying Force - Pressure

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

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

Metric Unit of pressure is the PASCAL

Newtons of force per square meter

N/m²

Pressure and Force

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

For example, if you stand on one foot, you are exerting a force in less area 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 Area

The force of the hammer is spread out to hundreds of nails

500 N of force from the hammer on 500 nails = 1 N of force on every nail

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