Force and motion

Force and Motion

Lesson 1

Focus Question

What causes a change in motion?

New Vocabulary

force

system

free-body diagram

net force

Newton’s second law

Newton’s first law

inertia

equilibrium

Review Vocabulary

acceleration: the rate at which the velocity of an
object changes

• In physics, a force is a push or a pull.

• If you push harder on an object, you have a greater

effect on its motion.

• All accelerations are the result of an unbalanced

force acting on an object.

Force

• The direction in which force is exerted matters. The

acceleration is in the same direction as the
unbalanced force.

size and direction of a force, while F represents only
the magnitude.

Force

• System: objects of interest

• External world: Everything

around the object that exerts
forces

• Forces result from interactions:

Force is the agent of interaction

Force

• A contact force exists when an

object from the external world
touches a system and thereby
exerts a force on it.

• Field forces are exerted without

contact.

Force

• A physical representation that shows the forces

acting on a system is called a free-body diagram.

Force

• Vector sum of all the forces on an object is net force.

• When forces are in the same direction

• When forces are in opposite

directions

Combining Forces

F1 = 20 N

F2 = 30 N

F net = 50 N

F net = 10 N

F1 = 20 N

F2 = 30 N

Combining Forces

F1

F2

F1

F2

F

𝐹 =

𝐹1

2 + 𝐹2

2 + 2. 𝐹1𝐹2𝑐𝑜𝑠𝜃



• Newton’s second law states that the acceleration of

an object is proportional to the net force acting on
the object and inversely proportional to the mass of
the object.

Newton’s Second Law

• To use Newton’s second law to solve a problem,

follow these steps.

•

Draw a free-body diagram showing the direction
and relative strength of each force acting on the
system.

Then add the force vectors to find the net force.

Next, use Newton’s second law to calculate the
acceleration.

Use what you know about accelerated motion to
find the velocity or position of the object.

Newton’s Second Law

Use with Example Problem 1.
Problem
Kwaku pushes one end of a table with a force of
15.0 N. Salali pushes on the other end of the table
with a force of 20.0 N. What is the net force on
the table?

Response

●

20.0 N

15.0 N

net

Kwaku

Salali

15.0 N

20.0 N

5.0 N

=

+

=

−

= −

F

F

F

SKETCH AND ANALYZE THE PROBLEM
•Draw the situation and a free-body
diagram.

•Identify the knowns and unknowns.

SOLVE FOR THE UNKNOWN
•Determine the net force.

That is, Fnet = 5.0 in the direction Salali is
pushing.

KNOWN

UNKNOWN

FKwaku = 15.0 N

Fnet = ?

FSalali = −20.0 N

EVALUATE THE ANSWER
•The net force is in the direction of the
force with the large magnitude.

• In the absence of a net force, the motion (or lack of

motion) of both the moving object and the stationary object continues as it was.

• Newton’s first law is that an object that is at rest will

remain at rest, and an object that is moving will continue to move in a straight line with constant speed, if and only if the net force acting on that object is zero.

• Newton’s first law is sometimes called the law of

inertia.

Newton’s First Law

• Inertia is the tendency of an object to resist changes

in velocity.

• If the net force on an object is zero, then the object

is in equilibrium.

• If there is no net force acting on the object, then the

object does not experience a change in speed or
direction and is in equilibrium.

Newton’s First Law

• When you exert a force on your friend to push him

forward (FA on B), he exerts an equal and opposite force
on you (FB on A), which causes you to move backward.

• The forces FA on B and FB on A are an interaction pair.

• An interaction pair is two forces that are in opposite

directions, have equal magnitudes, and act on
different objects.

Interaction Pairs

• Newton’s third law states that all forces come in pairs

and that the two forces are equal in strength, opposite
in direction, and act on different objects.

Interaction Pairs

• When identifying interaction pairs, remember that they

always occur in two different free-body diagrams and
they always have symmetry in the subscripts.

Interaction Pairs

Use with Example Problem 4.
Problem
You are walking along when you slip on some ice
and fall. For a moment you are in free fall. During
this time, what force do you exert on Earth if your
mass is 55 kg?

(

)(

)

you on Earth

Earth on you

you

55 kg

9.8 N/kg

540 N

m

= −

= −

= −

−

=

F

F

g

Response
ANALYZE AND SKETCH THE PROBLEM
•Draw the situation and a free-body diagram.

•Identify the knowns and unknowns.

SOLVE FOR THE UNKNOWN
•Use Newton’s third law and the
definition of weight to determine the
force you exert on Earth.

EVALUATE THE ANSWER
•The force you exert on Earth is equal in
magnitude and opposite in direction to the
force Earth exerts on you.

KNOWN

UNKNOWN

myou = 55 kg

Fyou on Earth = ?

g = −9.8 N/kg

FEarth on you

●

Fyou on Earth

●

Interaction pair

Quiz

v

D

F

C

a

B

F

A

Which symbol represents the size and magnitude
of a force?

1.

CORRECT

Quiz

the unknown

D

the system

C

Which is the term for the object or objects of
interest in a free-body diagram?

2.

CORRECT
the external world

A

the vector

B

Quiz

the system

D

the net force

C

the acceleration

B

the external world

A

Which is the vector sum of all the forces on an
object?

3.

CORRECT

Quiz

Newton’s second law

D

law of inertia

C

law of equilibrium

B

Newton’s first law

A

Which law is represented by the formula?

4.

CORRECT

net
m
= F
a

Quiz

mass

D

acceleration

C

force

B

inertia

A

Which is the tendency of an object to resist
changes in velocity?

5.

CORRECT