Forces and Motion

Unit F

Ch. 1

What are forces?

• A force is a push or a pull.

• It causes objects to accelerate which means to speed up, slow down, stop, or change direction

• A force is equal to the mass of a moving object times its acceleration.

• For every force there is an equal and opposite reaction force.

Newton’s Second Law of Motion

• Force = mass X acceleration• The greater the force, the greater the

acceleration.• The greater the mass of an object the less

the acceleration if the same force is applied.

What is friction?

• Friction is a force which opposes motion.• Friction is an important force in our lives.

We rely on friction in many ways. An athlete usually wears shoes which provide him or her with a greater friction between the shoe and the surface. We rely on friction as an important aspect of our motion. In what other ways does friction play a role in our everyday lives?

From Glencoe Physical Science

What causes friction?• There are two factors which affect

friction between two surfaces:– Kind of surfaces in contact (rough or

smooth)– Amount of force pressing the surfaces

together.

The rougher the surface and the stronger the force between the surfaces, the greater the amount of friction.

Magnetism

• Magnetism is the force of repulsion or attraction between poles of a magnet

• Opposite poles attract

• Same poles repel

• Gravity is a force of attraction between two objects.

• Law of Gravitation – any two masses exert an attractive force on each other. The amount of attraction depends upon two things: the mass of the objects and the distance between the objects.

What is Gravity?

Gravity causes weight

• The weight of an object is caused by the force of attraction between the earth and objects on the surface of the earth.

Gravitation force is equal to weight, therefore…

What is weight?• When an astronaut moves away from

the earth they “lose” weight. Weight is caused by the force of attraction between the earth and objects on its surface. The greater the distance an object has from the earth, the less the force of attraction which would exist.

What is projectile motion?

Balanced Forces

• Forces always work in pairs

• Balanced forces are equal size and opposite in direction

• They cancel each other out

Unbalanced Forces

• Occur when one force is greater than its opposite force

• Motion CHANGES: speeds up, slows down, stops, or changes direction

• This is called Acceleration

Net Force

• Adding or subtracting the individual forces gives a value, called a Net Force, of the combined forces

• ONLY for unbalanced forces

• Same direction: add

• Opposite direction: subtract

• Direction of the net force acts in the same as the larger force

Work and Effort

• Work is the use of force to MOVE an object through distance

• If effort is applied but nothing moves there is no work being done

• Work is measured in Joules

• Work= force (N) x distance (m)

Power

• Power is the amount of work done for each unit of time

• It is a measure of how quickly someone or something does work

• Watt is the unit of power

• Power(W)= Work(J)/Time(sec)

Machines

• Simples machines make work easier

• Pulley, wheel and axle, screw, inclined plane, lever, wedge

• Compound machines like a shovel or faucet are made of 2 or more simple machines