Download Chapter 2: Forces


A force is a push or a pull.

There are three main types of force that
we are going to focus on.

A contact force is when one object
pushes or pulls another object by
touching it.

Gravity is the force of attraction
between two masses.

the strength of the gravitational force
between two objects depends on their
masses.
› The greater their masses, the greater the
force.

Friction is a force that resists motion
between two surfaces that are pressed
together.

For example, carpet has greater friction
that wood floors.

Force is a vector because it has both size
and direction.

The overall force acting on an object
when all of the forces are combined is
called the net force.

If the net force on an object is zero, then
it will not move. The forces are said to be
balanced forces.

When the net force is not zero, then the
object will move. The forces in this case
are unbalanced forces.

If two forces are pushing on an object
from the same side, then you add the
forces together.

If forces are acting from opposing
directions, you would subtract the lower
force from the higher force.
› Which direction will the object go?

Came up with three laws of motion.

Worked closely with other scientists,
including Galileo.

Influenced several other physicists and
mathematicians.

Objects at rest will remain at rest, and
objects in motion will remain in motion
with the same velocity, unless acted
upon by an unbalanced force.

A soccer ball will stay at rest until you kick
it. Once you kick it, it will stay in motion
until an outside force stops it (friction, air
resistance, another player, etc.).

Inertia is the resistance of an object to a
change in velocity (speed or direction).

Closely related to mass – the more mass
an object has, the higher resistance to
change it will have.

The acceleration of an object increases
with increased force and decreases with
increased mass.

An object moves in the direction the
force is acting on it.

Example:
If you are pushing a full grocery cart and
a friend is pushing an empty grocery
cart, you would have to apply more
force to your cart to have the same
acceleration.
 Both of your carts would move in the
direction of the force.


Formula:

Force = mass x acceleration

If mass OR acceleration increases, so will
the force needed to act on the object. If
either of them decreases, so will the
force.

Changing the formula:
Force = mass x acceleration
 Mass = force / acceleration
 Acceleration = force / mass


Measuring Force: newtons (N)
Mass = kg; acceleration = m/s2
 So 1 N = (1 kg)(1 m/s2)


If a 5 kg ball is accelerating 1.2 m/s2,
what is the force acting on it?

A person on a scooter is acceleration 2
m/s2. If the person has a mass of 50 kg,
how much force is acting on that
person?

A girl pulls a wheeled backpack with a
force of 3 N. If the backpack has a mass
of 6 kg, what is its acceleration?

A boy pushes a shopping cart with a
force of 10 N, and the cart accelerates 1
m/s2. What is the mass of the cart?

Centripetal force: any force that keeps
an object moving in a circular path.

Force points to the center of the circle; if
it is “let go” from its path, it will fly off in a
straight line.

You can use F = (m)(a) with centripetal
force.

Every time one object exerts a force on
another object, the second object exerts
a force that is equal in size and opposite
in direction back on the first object.

A jellyfish moving through water; rowing
a boat

The first force in the pair is the action
force, and the second force is the
reaction force.

See examples on page 58 in your
textbook.

Not the same as balanced forces;
balanced forces act on the same object
but action/reaction pairs act on different
objects.

Momentum is a measure of mass and its
velocity.

Similar to inertia because they both
depend on mass.

Momentum = mass x acceleration

Or, p = (m)(v)

Unit: (kg)(m/s)
*kilogram-meter/second

A 3 kg ball is moving with a velocity of 1
m/s. What is the ball’s momentum?

What is the momentum of 0.5 kg ball
moving 0.5 m/s.