RECAP

Inertia: Mass and Weight

Think!Does a 2-kilogram bunch of bananas

have twice as much inertia as a 1-kilogram loaf of bread? Twice as much mass? Twice as much volume? Twice

as much weight, when weighed in the same location?

Mass—A Measure of Inertia

Mass and weight are proportional to each other in a given place:–In the same location, twice

the mass weighs twice as much.

What is the relationship betweenmass and weight?

WeightMass

Newton’s 1st Law of Motion

An object at rest will stay at rest or an object in motion

will stay in motion and travel in a straight line, as

long as no external net force acts on it.

Quick Question!

What path will the planets take when the

Sun suddenly disappears?

FORCE AND ACCELERATION• After this activity, you

should be able to describe how the net force acting on an object affects its acceleration.

Activity

FORCE AND ACCELERATION

Activity

F = 1 F=2 F=3 F=4

Activity

1

2

3

4

5

Label strips 1-5

Activity5

Compute for average

velocity per strip

22 cm

velocity = 22 cm / 0.10 s

velocity = 220 cm/s

Strip Distance (cm) Velocity (m/s)

Change in V (m/s)

Acceleration (m/s2)

Activity – per strip (F = 4)

Strip Distance (cm) Velocity (cm/s)

Change in V (cm/s)

Acceleration (cm/s2)

1 12 14516 160

2 14 161

27 2703 16 188

22 2204 18 210

20 2005 20 230

AVERAGE21.3 cm/s

AVERAGE213 cm/s2

Activity – per strip

Tape Chart No. Rubber BandsAverage

Acceleration (cm/s2)

F = 1 1 163F = 2 2 182F = 3 3 200F = 4 4 213

Activity – per group

Activity – Line Graph

F = 1 F = 2 F = 3 F = 40

50

100

150

200

250

Acceleration

Acceleration

FORCE AND ACCELERATION

• What corresponds to the amount of force in the activity?

Activity

Instantaneous vs. Average

Recap!

VelocityAcceleration

VelocityAcceleration

• Velocity

Formula

• Acceleration

Formula

FORCE AND ACCELERATION• What caused the

acceleration to change?• What caused the velocity

to change?

Activity

How is the acceleration of the cart related

to the amount of force acting on

it?

Activity

Acceleration

Movement

• When we see something move we see– Start– Slow– Curve– Stop

• All these things represent a CHANGE in motion

What is the cause of acceleration?

• FORCE causes ACCELERATION• Example – ball rolling on the floor

– Still until force is placed on it– Stays moving in a straight path until another force

causes it to accelerate • Change direction• Speed up• Slow down

– Change in velocity acceleration

Net Force causes acceleration

• Combination of force yields acceleration

Force

Acceleration

Direct Proportionality

How about Mass?

• Mass resists acceleration!

Inverse Proportionality

Acceleration

Mass

Mass resists acceleration

• Example– Full shopping cart vs. empty shopping cart

The greater the mass the more force it takes to accelerate the

object

Mass resists Acceleration

The amount of acceleration

depends not only on the force, but also on the mass being

pushed.

F

F

Force of hand accelerates the brick

The same force accelerates 2 bricks ½ as much.

Newton’s Second Law

• If you combine these two relationships, you would come up with this relationship:

The acceleration of an object is directly proportional to the magnitude of the net force acting on it and is inversely

proportional to its mass.

Newton’s Second Law

F = ma

Applications of 2nd Law

Using consistent units

• a = F m• a =acceleration

(m/s^2 )• F = force

(newtons)• m = mass (kg)

Problem 1

• How much force, or thrust, must a 30,000-kg jet plane develop to achieve an acceleration of 1.5 m/sec^2

• F = ma

= (30,000 kg)(1.5 m/sec^2) = 45,000 kg m/sec^2

= 45,000 N

Problem 2

• What acceleration is produced by a force of 2000 N applied to a 1000-kg car?

• a = F/m = 2000 N/ 1000 kg = 2000 kg m/sec^2/1000 kg = 2 m/sec^2

• If the force is 4000 N, the acceleration doubles – 4000N/1000 kg = 4 m/sec^2

Think!

Why do objects of various masses fall withequal accelerations?

Question #1

• What do you call an influence on a body producing a change in its movement?

• What is the general equation for that?

Newton’s Second Law

F = ma

Question #2

Superman and Aquaman joined a competition called, “Around the World in a Day”. Both heroes had different modes of transport – Superman flies in the sky while Aquaman swims in the ocean. If

they have the same mass, acceleration, and route, who would win the

competition? Why?

Superman would likely win!

Less force from air molecules!

Question #3

How much force is needed to triple the acceleration of an

object with mass M?

Force

Three times the force!

3

Question #4

•Can you walk on a frictionless floor? Why?

1st Law and 2nd Law of Motion

No. There would be no opposing force. You will slide forever and ever and ever.

Recall Formulas!

•Velocity•Acceleration•Force

Kinematic Equations

Please remember!

Kinematic Equations

Please remember!

Problem 1

• What is the force needed to accelerate an object to 2.92 m/s^2 if the object weighs 411kg?

Problem 2

• Suppose we have two different masses. One has thrice the mass of the other. If you apply the same amount of force, describe the acceleration of both masses.

Problem 3

• A toy car having a mass of 0.2kg is accelerated so that its velocity changes from 1.5m/s to 4m/s in 4 seconds. What amount of force caused this change in motion?

Problem 4

• A delivery van (8000kg) changes its velocity from 20m/s to 80m/s while covering a distance of 300m. What is the van’s acceleration? What is the force on the van?

Problem 5

• A 910-kg car is speeding at 97 km/h on a level road. Determine the horizontal braking force applied to stop it in 67m.

Problem 6

• A 1,500-kg car moving at a velocity of 25 km/h along EDSA is accelerated by a force of 1,800 N. What will be its velocity after 11 seconds? (Neglect friction.)