FORCES AND MOTION

Physics

Chapter 14.

(Week 1)

What is a force?

■ Forces are what makes things move, like a push or a pull.

Forces can:

1) Make

things

move

2) Make a

moving

object stop.

3) Change

the speed

of a moving

object.

4) Change

the direction

of a moving

object.

5) Change

the shape

of an

object.

Force is measured in Newton (N). The more Newton, the bigger the force.

How do we measure force?

Force can be measured using a Newton meter (Force meter/spring balance).

Different types of forces

Contact forcesNon-Contact

forcesOccurs when the material or object

exerting the force touches the object

or material on which the force acts.

Occurs when the materials or objects

do not touch each other.

Examples:

- Impact force

- Strain force

- Friction

- Air resistance

- Water resistance

Examples:

- Magnetic force

- Gravitational force

- Weight

Contact forces

1) Impact force- When a moving object

collides with a stationary object.

- Impact force can be large ex. Hammer striking nail.

- OR small ex. Air particles striking the skin.

2) Strain force- When some materials are

squashed, stretched, twisted or bent, they exert a force opposite the direction of the force acting on them.

- These materials are called elastic materials.

- The force they undergo when deformed is called the strain force.

Look at the racket and the tennis ball.

Draw arrows to represent the forces.

- Tension is a strain force.

- Exerted by a stretched rope, spring or string.

Pulling

force

Tension

force

3) Friction

- Occurs when an object (being pushed or pulled) moves over the surface of another object.

- As the push or pull on the object increases, frictionincreases.

- The friction that exits between two objects when there is no movement is called Static friction.

- Push or pull = static friction (no movement)

- Push or pull , than friction the object starts to move.

- When the object moves there is still friction, it is called sliding friction.

- The force of the sliding friction is than the maximum strength of the static force.

A closer look at friction

- The surfaces of objects are not completely smooth.

- Under the microscope it can be seen that they have tiny projections with hollows between them.

Metal objects appear smooth to our eyes.Have a look what they look like under the microscope.

- When projections from one surface meet the projections from another surface they stick.

- These connections between the surface produce the force of friction between the objects.

Reducing friction

- If a liquid is placed between the two surfaces, the projections are forced apart a little and the number of connections is reduced.

- Reducing connections means reducing friction.

- Reducing friction can be helpfulor cause problems.

- Example: Water running between car tyres and the road reduces friction and increases the chance of skidding.

- But, oil between moving metal parts of an engine reduces friction and reduces wear on the metal parts.

Increasing friction

- To increase friction between 2 surfaces we should press the surfaces together more strongly.

- This makes the projections press against each other more and increases the connections between the surfaces.

Friction AND ROAD SAFETY

Driver in a

moving car

sees a hazard

Driver reacts

and applies

Brakes .Car stops

Thinking

distance

Braking

distance

Speed Thinking distance /m Braking distance /m Total stopping

distance /m

48 km /h

(30 mph)

9 14 23

80 km/h

(50 mph)

15 38 53

112 km /h

(70 mph)

21 75 96

Thinking and braking distance

■ Thinking distance

■ The distance travelled by the car before the driver reacts and applies the brakes.

■ Braking distance

■ The distance covered by the car after the brakes are applied and before the car stops.

Other forces affecting speed

Friction Air resistance Water resistance

Air resistance

When an object moves through air it pushes air out of the

way.

Air moves over the object sides and pushes back on the

object.

This push on the object is called air resistance or DRAG.

The value of air resistance depends on the size and shape of an

object .

Many cars are designed to move with low resistance.

The cars body is designed like a wedge to cut it’s way through the

air . And the surfaces are curved to allow the air to flow over the

sides with minimum drag .

Shapes that are designed to reduce air resistance are called

streamlined shapes.

■ A dragster is a vehicle that accelerates very quickly.

■ At the end of the race they are slowed down by brakes and a parachute.

■ The parachute has a large surface area that offers high air resistance ( drag ) that slows down the dragster and stops it in a short distance.’

The air resistance

produced by a parachute

is also used to bring sky

divers safely to the

ground.

The resistance of the gases

in the atmosphere of other

planets in the solar system is

used to land space probes

safely.

Water resistance

■ When an object moves through water it pushes water out of the way.

■ Water moves over the object sides and pushes back on the object .

■ This push on the object is called water resistance or DRAG.

■ Object that move through water quickly have a streamlined shapes .

■ A barracuda which moves quickly through water has a much more streamlined shape than the slow-moving sun fish.

Fast-moving

Barracuda fish

Slow-moving

Sun fish

■ Water resistance affects the movement of boats and ships on the water surface .

■ Boats designed for high speed are hull shaped to reduce water resistance .

Hydrofoil boat

Some boats are equipped with

a device called a hydrofoil

which reduces the area of

contact between the boat and

the water to the minimum

making the boat move quickly

over the water surface .