Guided Discussion

Student notes are shown in blue.

» Any influence that can cause a change in an object’s motion.

A bat strikes

the ball with a

force that

causes the ball

to stop and

then move in

the opposite

direction.

» A push or pull that one object exerts on another

» Has a size and direction

» Measured in newtons (N)

You have to apply a force of about 3 N

to lift a full can of soda.

» The combination of all forces on an object

When two or more forces act on an object at

the same time, the forces combine to form the

net force.

In a tug of war, each side

exerts a force on the ground.

If the opposing forces on the

ground are equal, they are

balanced and the people do

not move.

If one of the forces on the

ground is greater than the

other, the forces are

unbalanced and the people

move in the direction of the

greater force.

Balanced forces: no acceleration

Unbalanced forces: acceleration

» Do not change motion

» Cancel each other

» Combine to produce a net force of zero

» Do not have the same strength (size)

» Cause acceleration

» A force that opposes the sliding motion of two surfaces

» Depends upon the materials the surfaces are made from and the roughness of the surfaces

» Rule of Friction – Friction always acts in a direction to oppose motion.

Examples:

If you push a solid block along the floor to the right, the force of

friction on the block will be to the left.

A boat propelled to the east by its motor experiencing water

friction to the west.

When an object falls downward through the air, the force of

friction (air drag or air resistance) acts upward.

PUSH

FRICTION

AIR RESISTANCE

WEIGHT

Unbalanced forces: acceleration Balanced forces: constant speed Balanced forces: no motion

A When a car is

accelerating, the forces

are unbalanced. The

force moving the car

forward is greater than

the opposing force of

motion.

B When a car is cruising

at constant speed, the

forces moving the car

forward is balanced by

the force of friction.

C This car does not

roll because the

force of friction

between the brakes

and the wheels

balances the force of

gravity.

» Static Friction – The frictional force that prevents two surfaces in contact from sliding past each other.

» Sliding Friction – The force that acts in the opposite direction to the motion of a surface sliding on another surface.

» A type of frictional force that opposes the motion of objects that move through the air

» Causes objects to fall with different accelerations and different speeds

» Acts in the direction opposite to the velocity of an object moving in air

» Size of the force depends on the size and shape of the object.

The flying squirrel increases its

area by spreading out. This

increases air resistance and

decreases the speed of its fall.

Air resistance, not an object’s mass, is why

feathers, leaves, and sheets of paper fall more

slowly than pennies, acorns, and apples.

As an object falls, the downward force of gravity causes

the object to accelerate. As the speed of the object

increases, the upward force of air resistance also

increases.

» When the upward air resistance force equals the downward force of gravity, terminal velocity is reached – the velocity becomes constant.

» Depends on the size, shape, and mass of the object

Terminal velocity is reached for the sky divers when air

resistance balances the downward force of gravity.