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Units• Time (s)

• Distance (m)

• Speed: average speed and Instantaneous speed (m/s)

• Velocity (m/s) +direction

• Acceleration (m/s2)

• Force (N) or lbs

• Inertia (Kg)

• Density (g/mL)

Example Problem 1

Which of the following is NOT an appropriate unit of measurement for speed?

A. cm/s

B. km/hr

C. ft/year

D. kg/s

E. miles/month

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Example Problem 2A car accelerates along a straight road, which

is 15 km long. The car reaches a speed of 75 km/h in 10 s. The average acceleration of the car isA. 2.1 m/s2.

B. 7.5 m/s2.

C. 1.5 m/s2.

D. 5.0 m/s2.

Example Problem 3The speed of light in space is 3.00 x 108 m/s.

The time for light from the Sun to reach Earth is 8 minutes 20 seconds. Therefore, the distance from the Sun to Earth isA. 2.40 x 109 m.

B. 8.64 x 1012 m.

C. 1.50 x 1011 m.

D. 2.25 x 109 m.

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Net ForceForce

simply a push or a pull

Net force• combination of all

forces that act on an object

• changes an object’s motion

Motion: free fall

d = Δ𝑣

2t

Where v = gt

d= ½ g t2

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Example Problem 5

A bowling ball of mass 7 kg is dropped from the top of a tall building. It safely lands on the ground 3.0 seconds later. Neglecting air friction, the height of the building is

A. 44 m.

B. 24 m.

C. 15 m.

D. 29 m.

Example Problem 6

A bowling ball of mass 7 kg is dropped from the top of a tall building. Neglecting air drag, the speed of the ball when it strikes the ground is

A. 29 m/s.

B. 10 m/s.

C. 44 m/s.

D. 88 m/s.

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Galileo’s Concept of Inertia

The tendency of a moving body to keep moving or to remain at rest is natural—every material object resists change in its state of motion. This property of things to resist change is called inertia.

Galileo’s Concept of Inertia

Experiment:

Balls rolling down inclined planes and then up others tend to roll back up to their original heights.

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Galileo’s Concept of Inertia

Italian scientist Galileo demolished Aristotle’s assertions in early 1500s.

In the absence of a force, objects once set in motion tend to continue moving indefinitely.

Inertia of moving objects

Inertia

Inertia of non-moving objects

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Inertia—How do we measure inertia?

All things are made of atomsAnd all atoms have Inertia

Ex: Temperature Degrees (ᵒC, K, ᵒF)

The units of Inertia are: Kilograms (Kg)

Mass—A Measure of Inertia

Mass• Quantity of matter in an object

• Measure of inertia or sluggishness that an object exhibits in response to any effort made to start it, stop it, or change its state of motion in any way

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The amount of inertia possessed by an object depends on the amount of matter—the amount of material that composes it—its mass:

greater mass greater inertiasmaller mass smaller inertia

Mass—A Measure of Inertia Mass—A Measure of Inertia

Mass versus volume:

• Mass involves how much matteran object contains

• Volume involves how much spacean object occupies

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The concept of inertia mostly involves

A. mass.

B. weight.

C. volume.

D. density.

Mass—A Measure of Inertia

CHECK YOUR NEIGHBOR

Explanation :

Anybody get this wrong? Check the title of this slide! :-)

Chapter 2

Newton’s laws

Physical

Science, 6e

1642-1727

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Newton’s Laws of Motion

The law of acceleration:

The acceleration produced by a net force on an object is directly proportional to the net force, is in the same direction as the net force, and is inversely proportional to the mass of the object. Fnet ≠ 0 mass

forcenetacc =

The law of inertia: (originating with Galileo)Every object continues in a state of rest or of uniform speed in a straight line unless acted on by a nonzero force. Fnet = 0

Newton’s First Law (law of inertia)

An object at rest tends to stay at rest

and an object in motion tends to stay

in motion unless acted upon by an

unbalanced force.

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Newton’s First Law (law of inertia)

• INERTIA is a property of an object that describes how ______________________ the motion of the object

• more _____ means more ____

much it will resist change to

mass inertia

1st Law (Inertia)

• Unless acted upon by an unbalanced force, this golf ball would sit on the tee forever.

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Examples of Inertia

Inertia

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Newton’s

2nd Law

Newton’s Second Law

Force equals mass times acceleration.

F = ma

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Balanced Force

Equal forces in opposite directions produce no motion

Unbalanced Forces

Unequal opposing forces produce an unbalanced force

causing motion

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Newton: 2nd law

F (N) = m (kg) a (m/s2)

Example 1: What resultant force will give a

3 kg mass an acceleration of 4 m/s2?

2(3 kg)(4 m/s )F =

F = 12 N

Remember F = m aF = ?

a = 4 m/s2

3 kg

Example 2: A cart and driver have a mass of 120 kg. What force F is required to give an acceleration of 6 m/s2 with no friction?

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Example 3: What is the mass of a block that has an acceleration of 2 m/s2 and a resulting force of 12 N ?

F = 12N? kg

a = 2 m/s2

Remember consistent units for force, mass, and acceleration in all problems.

Newton's Second Law of Motion

• When acceleration is g—free fall.

– Twice the forceon twice the mass same acceleration as half the force on half the mass.

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Weight and mass are not the same

• WEIGHT is a measure of the force of ________ on the mass of an object

• measured in __________

gravity

Newtons or lbs

Mass vs Weight• Mass is not the same as Weight

Inertia

Atoms/matter

Force of gravity on an object

Gravity: Earth Moon

9.8m/s2 1.6 m/s2

Weight= _Kg(9.8m/s2) _Kg(1.6 m/s2)

Weight = mass (kg) • gravity (m/s2)

The units of weight are Newtown (N) or pounds (lbs)

__ Newtons /4.4 lbsEx: 70 kg what my weight in N, lbs.?

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Kg N lbsMass and weight are proportional, as one increases the other increases too or vice versa

Weight = mass (kg) • gravity (m/s2)

The units of weight are Newtowns (N) or pounds (lbs)

4.4N= 1 lbs

Ex: 70 kg what my weight in N, lbs.?

150lbs → how much Inertia?

x9.8 /4.4 Example Problem 7

An astronaut (with equipment) has a mass of 125 kg on Earth. On the moon (g = 1.67 m/s2), her mass and weight would be

A. 125 kg and 1225 N.

B. 125 kg and 209 N.

C. 21 kg and 209 N.

D. 125 kg and 125 N.

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Weight and Mass: Examples

Example 4: What is the weight of a 10-kg

block?

9.8 m/s2W

m10 kg

W = mg = (10 kg)(9.8 m/s2)

Newton's Second Law of MotionCHECK YOUR NEIGHBOR

A 5-kg iron ball and a 10-kg iron ball are dropped from rest. For negligible air resistance, the acceleration of the heavier ball will be

A. less.

B. the same.

C. more.

D. undetermined.

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Always Remember!!

In Physics, the use of Newton’s second law and many other applications makes it absolutely necessary to distinguish between mass and weight. Use the correct units!

Metric SI units: Mass is in kg; weight is in N.

Summary

Newton’s Second Law: A resultant force produces an acceleration in the direction of the force that is directly proportional to the force and inversely proportional to the mass.

Newton’s First Law: An object at rest or an object

in motion at constant speed will remain at rest or

at constant speed in the absence of a resultant

force.

N = (kg)(m/s2)

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