# Forces and Pressure

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Forces and Pressure

## 1. Types of Forces

### 1.1. Magnetic force

1.1.1. Force exerted by a magnet on a magnetic material or another magnet

1.1.1.1. Poles of a bar magnet

1.1.1.1.1. North-Seeking pole (N)

1.1.1.1.2. South-Seeking pole (S)

1.1.1.2. Force between magnets

1.1.1.2.1. Like poles repel

1.1.1.2.2. Unlike poles attract

### 1.2. Frictional Force

1.2.1. Occurs when two moving surfaces are in contact

1.2.2. Effects

1.2.2.1. Slows down or stop a moving object

1.2.2.2. Produces Heat

1.2.2.3. Positive effects of friction

1.2.2.3.1. Eg. Holds a nail to the wall

1.2.2.3.2. Eg. Bicycle Brakes

1.2.2.3.3. Eg. enables us to walk

1.2.2.4. Negative effects of friction

1.2.2.4.1. Eg. makes movement more difficult

1.2.2.4.2. Eg. wears away materials

1.2.2.4.3. Eg. it produces heat

1.2.3. Ways of reducing friction

1.2.3.1. Using smooth surfaces

1.2.3.2. Using Ball bearings

1.2.3.3. Lubrication

1.2.3.4. Streamlining

### 1.3. Gravitational Force

1.3.1. Also called Gravity

1.3.2. Exists between any two objects

1.3.3. Causes all objects to be atrracted to the Earth

1.3.3.1. Eg. A coconut falling from a tree

1.3.3.2. Eg. A ball coming down after being thrown skywards

1.3.4. Weight

1.3.4.1. Gravitational force acting on an object

1.3.4.2. Measured using a spring balance in Newtons.

1.3.4.3. The greater the force of gravity pulling an object, the greater the weight.

1.3.4.4. The force of gravity on Earth is greater than that on Mars or moon.

1.3.4.4.1. Therefore, we weigh more on Earth than on Mars or moon.

## 2. Effects of Forces

### 2.1. change in the shape or size

2.1.1. Eg. Stretch a spring

2.1.2. Eg.Sit on a cushion

## 3. What is a Force?

### 3.1. Push or Pull

3.1.1. Eg. Lifting

3.1.2. Eg. Bending

3.1.3. Eg. Stretching

3.1.4. Eg. Twisting

3.1.5. Eg. Squeezing

## 4. Measuring a Force

### 4.1. Force meters or Newton meters

4.1.1. Spring balances

4.1.1.1. Extension Spring Balance

4.1.1.2. Compression Spring Balance

## 6. Pressure

### 6.2. Thus,pressure increases when

6.2.1. The force increases

6.2.2. The area decreases

### 6.3. Increasing pressure by reducing area

6.3.1. Example: cutting an object using a sharp knife

6.3.1.1. A sharp knife has a very small area in contact with an object

6.3.1.2. As the force acts on this small area, the pressure is high

### 6.4. Reducing pressure by increasing area

6.4.1. Eg. When people or machines move on soft ground or snow

6.4.2. Eg.When a larger surface is in contact with the ground, the pressure exerted on the ground is lower, so people or machines can move without sinking.

### 6.5. Calculating pressure

6.5.1. Pressure = Force / Area

6.5.2. SI unit: N/m2 or Pascal (Pa)

6.5.3. Other units: N/cm2

6.5.4. Example: An elephant weighs 40 000 N. It stands on one foot with an area of 1000 cm2. What is the pressure on the ground? Pressure = 40 000 / 1000 = 40 N/cm2