31force and motion (part 1)

FORCES AND MOTIONPhysics

Chapter 14.(Part 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.

HOW DO WE MEASURE FORCE? Force is measured in

Newton (N). The more Newton, the bigger the force.

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

DIFFERENT TYPES OF FORCES

Contact forces Non-Contact forces

Occurs 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 FORCES1) 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, friction increases.

- 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 helpful or 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 distanc

e

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-movingSun 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 .