Forces

What is a Force?

A push or a pull. ◦When one object pushes or pulls another object,

the first object is exerting a force on the second object.

◦Like velocity and acceleration, force is also described by the strength and direction in which it acts.

◦The strength of a force is measured in an SI unit called Newton (N). (named after Isaac Newton)

Combining Forces

In any situation the overall force acting on an object is the net force.

The net force determines whether an object moves and also in which direction it moves.

◦ Force in the same direction add forces.

◦ Force in opposite direction is like adding positive and negative numbers. They cancel each other out.

◦ Size of the arrows indicates the strength and direction of the force.

Unbalanced vs. Balanced

Unbalanced force acting on the object results in a net force which will change the objects motion.

Balanced forces equal forces acting on one object in opposite directions. Balanced forces acting on an object will NOT change the objects motion.

Unbalanced vs. Balanced

Friction

What is Friction

The force that two surfaces exert on each other when they rub against each other.◦Acts in the direction opposite to the objects

motion. Friction opposes motion.◦Eventually friction will cause an object to come

to a stop.◦Without friction the object would continue to

move at a constant speed until acted upon by another force.

Determining the Amount of Friction

The amount of friction depends on two factors1. Surface Type

Rougher surfaces have greater friction2. The amount of force pushing surfaces together.

Less massive objects exert less force.

Changing the amount of the surface (surface area) that touches does not change the amount of friction.

When is Friction Helpful?

Salt, sand or gravel on icy surfaces Tires on a car Boots or shoes with traction Other examples?

Types of Friction

Static friction-the friction that acts on objects that are not moving◦ Ex: Moving a desk. It takes more force to get it moving

(overcome static friction)

Sliding friction-when solid surfaces slide over each other◦ Ex: Sliding a box across the floor, the breaks on your bike

tire.

Rolling friction-when an object rolls over a surface◦ Ex: Skateboards, bikes. Anything with wheels!

Fluid friction-when an object moves through a fluid◦ Ex: Sailing, surfing, swimming.

Ways to Reduce/ Increase Friction

ReduceLubricants

◦Ex: motor oil, wax, grease.

Switching to rolling◦Ball bearings.

Smoother surface◦Plastic as opposed to

wood.

IncreaseMake surfaces

rougher.◦Sand on icy roads,

batting gloves.

Increase amount of force pushing surfaces together.◦Add weights.

Friction Video Clips

Physics of The Olympics◦Competition Suits◦Curling

Gravity

What is Gravity?

The force that pulls objects toward each other.

The Law of Universal Gravitation

“All objects in the universe attract each other through gravitational force . The size of the force depends on the masses of the objects and the distance between them.”◦Sir Isaac Newton (1642-1727)

Factors that Affect Gravity

Mass- The more mass an object has the greater it’s gravitational pull.◦ Ex: The sun has a large

mass which keeps the planets in orbit

Distance- The farther apart two objects are, the lower the gravitational force between them.

Free Fall

When the only force acting on a falling object is gravity, the object is said to be in free fall.

The acceleration of any object due to gravity is 9.8 m/s/s

(ignoring air resistance).

Mass vs. Weight

Mass is the amount of matter in an object.◦Measured in grams or kilograms

Weight is the measure of the gravitational force exerted on an object.◦Measured in Newtons

Why do we weigh less on the moon?◦Because the moon has less mass than the earth.

Moon Weight

Try it out…

Weight = mass X acceleration due to gravity (9.8 m/s/s)

How much does a 50 kg person weigh on earth? ◦Weight = mass X 9.8 m/s/s ◦Weight = 50 kg X 9.8 m/s/s ◦Weight = 490 Newtons ◦Where force (weight) is measured in Newtons,

acceleration is measured in m/s/s and mass is measured in kg.

How many Newtons do you weigh?

1. Convert lbs. to kg (Weight in pounds X 0.4536)

2. Convert kg to Newtons(Weight in kg X 9.8)

Ex. 190 lbs X 0.4536 = 86.18 kg 86.18 kg X 9.8 = 844.60 N

To get weight on moon just divide by 6.

Projectile Motion

An object that is thrown is called a projectile.

Which Will Hit The Ground First?

Air Resistance

A type of fluid friction.◦The greater the surface area the greater the air

resistance. ◦The greater the velocity the greater the air

resistance.

Terminal Velocity

As velocity increases air resistance increases. When the force of the air resistance equals the force of the gravity on an object, the object will no longer accelerate. It will continue to fall at a constant rate.

So, terminal velocity is the point at which a falling object stops accelerating and begins to fall at a constant speed.

Exploring Gravity