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Forces and Motion

Chapter 3


Forces, Motion, and Gravity

Skill WarmUp

Page 55, Activity: Comparing

" Know Your Strength "

Forces Around You

Force: is a push or pull that starts, stops, or changes the direction of an object.

Transfers energy to an object

Lifting a wheel-barrow

Figure 3.1 answer the question

Balancing the wheel-barrow

What has to happen?

How does the load change if it is heavier?

Force and Gravity

Activity page 56: " Calculating "

w = mg

Use this equation: m = w / g

SI unit of force: Newton ( N )

One newton is the amount of force needed to cause 1-kg mass to accelerate at a rate of 1 m/s2 of motion.

N = 1 kg x 1m/s2

Spring scale measure weight or force

Figure 3.2, page 56

Gravity – weight of an object depends on the force that pulls the object toward the Earth.

Force of gravity on the Earth is 9.8 m/s2, the weight of a 1 – kg object is 9.8 N.

Falling Objects

Feather and a coin

Acceleration due to gravity ? ______

Figure 3.3, page 57

Motion opposed by friction

Friction: occurs when the surfaces of any kind of matter move past each other.

Terminal velocity: definition?

Shape of object affect

Projectile Motion

A curved path

Figure 3.4, page 58

Horizontal motion

Vertical motion

The Curve Ball

Science and You: The Curve Ball

Figure 3.5, page 59


Unbalanced air forces


Video analysis

Wrap Up

Reinforcement page 59 of teachers text.

Take notes in class.


The First Law of Motion


Fourth century China. They knew, but stayed in China.

Newton was first person to state laws of motion and forces.

An Object at rest will remain at rest and an object in motion will remain in motion unless acted upon by an outside force.

Inertia: comes from Latin iners = idle

The tendency of an object to resist any change in motion.

Bike rider, figure 3.7, page 61.


An outside force that resist motion when two surfaces come in contact.

Types of Friction

Sliding – Which kind of surface causes less sliding friction, a smooth surface or a rough surface?

Rolling – which tire in the picture will create more rolling friction? ( page 62 )

Fluid – air, water, oil

Friction Control

Ball bearings

Engine oil


All above keep surfaces from direct contact and reduces friction.


Second Law of Motion

Accelerated Motion

The net force on an object equals its mass times its acceleration.

F = ma

Figure 3.9, page 64

Force, Mass, and Acceleration

Figure 3.10, page 65

If both carts have the same acceleration how much force must be added to the cart on the right?

Cart A: a = 2.5 m/s2, m = 30 kg

Cart B: a = 1.5 m/s2, m = 30 kg + 10 kg + 10 kg

Next page for problems

Graphing Acceleration

Figure 3.11, page 65

3. How would each dog affect the acceleration of the sled? Answer:

Each dog would increase the acceleration of the sled because each dog would increase the force applied to the sled.

4. How does the acceleration of the 200 kg sled compare to the acceleration of the 100 kg sled? Answer:

The 200 kg sled accelerates half as fast as the 100 kg sled.

5. How does mass effect the acceleration of each sled? Answer:

The greater the mass, the slower the acceleration of the sled. The sled with a 100 kg mass is accelerated at twice the rate of the sled with the 200 kg mass.

Using Newton’s Second Law

Terms: mechanics, dynamics, kinematics


f = ma

a = f / m

m = f / a

Let’s do the practice problems on page 66.

Practice problem # 1, page 66

F = m x a

F = 40 kg x 2.5 m/s2

F = 100 kg x 1 m/s2

F = 100 N

Practice problem # 2, page 66

a = F / m

a = 90 N / 60 kg

a = 1.5 N / kg

Cancel units

kg x m / s2


Answer is in units: m / s2

Practice problem # 3, page 66

F = m x a

F = 60 kg x 2.5 m/s2

F = 150 kg x m/s2

To find the additional force, the initial force must be subtracted from the final force:

150 N – 90 N = 60 N


momentum = m x v

Momentum doesn’t change unless the velocity or mass changes.

Momentum can transfer from one object to another.

CONSIDER THIS: Should seat belts and air bags be required? Page 67. Research this and find out more about crash cars involving dummy drivers.

Forces in Circular Motion

Any force that causes and object to follow a circular path.

How does centripetal force apply to the cars to keep them moving around a curved track? Figure 3.12, page 68

More Miles for The Size

Do the activity, Skills Workout, on page 69 of the textbook, collecting data: Gas Usage

Read page 69 of textbook.

Question: How does this compare with the force needed for an 800 kg car? answer

The force needed for an 800 kg car is 2400 N. Show the work below.

800 kg x m/s2 = 2400 kg x m/s2, or 2400 N


Third Law of Motion

Equal and Opposite Forces

Balanced and Unbalanced Forces: look at figure 3.14 on page 71

How will the force of the diving board affect the diver’s performance? Answer

The greater the force exerted upon the diving board, the higher the dive will be.

Observing Newton’s Third Law: look at figure 3.15 on page 71

Why do you think it’s important for all crew members to pull their oars at the same time? Answer:

If the crew members do not work together, their own forces will balance each other, decreasing the overall unbalanced force they are trying to achieve.

From Fireworks to Outer Space, page 72
( fill in the blanks )

1. 500 years ago the Chinese _____________.

2. Chinese invented ____________________.

3. The ancient Chinese developed fireworks for _________.

4. All rockets require __________________.

5. What kind of fuel is used for space rockets and why?

6. Investigate rocket-propulsion systems. Use the internet for your research. Include labeled drawings and pictures to explain your research.


Universal Forces

Gravitational Force

Let’s take a look at table 3.1 on page 73, " Universal Forces ".

Answer this question: What would happen to objects on the earth if the force of gravity was as strong as the electromagnetic force?


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