# Forces

# Concept 5: Types of Forces

## Success Criteria

I can define ‘force’

I can name four forces that act on many everyday objects.

## Vocabulary

*Energy*

*Joule*

*Transformation*

A force is a push or a pull. A force has both size and direction. A force can change the speed, direction, or shape of an object.

1 newton is the force that causes a mass of 1 kg to accelerate at 1 m/s.

## Types of Forces

*Success Criteria:**I can define ‘force’I can name four forces that act on many everyday objects. *

### Contact Forces

Contact forces act at the point of contact between two objects.

Support is a force exerted by a surface that opposes the force of gravity.

Pushes or pulls are forces exerted by people, animals, or machines.

Friction is a force created when one surface slides, or attempts to slide over another surface.

Tension is a pulling force exerted by a string or cable, either downwards or side ways.

Contact forces also act between an object and a liquid or gas.

Buoyancy is a force, exerted by a liquid or gas, that opposes an object's weight. *(Note: buoyancy is different to lift).*

Thrust is when an object pushes gas or liquid to create this force in the opposite direction.

Drag (*also *air or water resistance) is a force that opposes an object moving through a gas or a liquid.

Lift is an upward force from a flow of air over an aerofoil (wing) or water over a hydrofoil (underwater fin).

### Non-Contact Forces

Non-contact forces act between objects that are not touching.

A gravitational force (weight) is the force of attraction between two objects due to their mass.

An electric force is the force created y electric charges. Like charges repel, opposite charges attract.

A magnetic force is the force created by magnets. It can attract or repel.

## Force Diagrams

A force diagram shows all the forces (as arrows) acting on an object. Arrows must:

Touch the object

Point out from the centre of the object in the direction the force is acting

Have a length that represents the size of the force.

Forces on a box being pushed along a floor.

Forces on a plane flying at a steady speed and height.

Forces on a sail boat moving through water.

## Task: Complete the following:

### SciPAD Workbook

Page 35 - Introduction to Forces

Page 36 - Force Problems

### sciPAD ONLINE

Go to Forces > Introduction to Forces

### Education Perfect

Complete the task called "1.1 Concept 5: Types of Forces"

# Concept 6: Net Force

## Success Criteria

I can describe the effect of balanced and un balanced forces on motion (Newton’s First Law)

I can calculate force using F = ma (Newton’s Second Law)

I can rearrange F = ma to solve an unknown variable (Newton’s Second Law).

## Vocabulary

*Energy*

*Joule*

*Transformation*

## Balanced & Unbalanced Forces

*Success Criteria: *

*I can describe the effect of balanced and un balanced forces on motion (Newton’s First Law)*

**Newton's first of law of motion states that an object at rest stays at rest and an object in motion stays in motion unless an unbalanced (net force) acts on the object. **

### Net force

The net force is the combined effect of all the forces acting on an object.

Forces acting in the same direction are added (Picture 1 and 2).

Forces acting in opposite directions are subtracted (Picture 3).

### Balanced forces

(net force is zero)

(net force is zero)

If the forces on an object are balanced, then there is no acceleration.

A stationary object will remain stationary.

A moving object will continue to move at the same speed and direction.

### Unbalanced forces

(net force is not zero)

(net force is not zero)

If the forces of an object are unbalanced, then the object will accelerate.

A stationary object will start to move.

A moving object will change its speed and/or direction (i.e. it accelerates).

## Acceleration Due to Net Force

*Success Criteria*

*I can calculate force using F = ma (Newton’s Second Law)*

I can rearrange F = ma to solve an unknown variable (Newton’s Second Law).

I can rearrange F = ma to solve an unknown variable (Newton’s Second Law).

**Newton's ****second ****of law of motion states that ****the size of the acceleration depends on the net force and the mass of the object. **

To double the acceleration of a mass, double the net force is required.

Wen the same force is applied to twice the mass, acceleration is halved.

## Task: Complete the following:

### SciPAD Workbook

Page 37 - Balanced and Unbalanced Forces

Page 38 - Sarah's Ride to School

Page 39 - Force, Mass and Acceleration

Page 40-41 - Force, Mass and Acceleration Problems

### sciPAD ONLINE

Go to Forces > Balanced and Unbalanced forces + Force, Mass and Acceleration

### Education Perfect

Complete the task called "1.1 Concept 6: Net Force"

# Concept 7: Mass, Weight & Drag

## Success Criteria

I can explain the difference between mass and weight.

I can define friction, and describe its effect on motion.

## Vocabulary

*Energy*

*Joule*

*Transformation*

## Mass, Weight & Gravity

The mass and weight of an object are two very different quantities:

The mass of an object is the amount of matter (in kg) in the object.

The weight of an object is the downward force in Newtons) on the object due to gravity.

The strength of gravity (g) on Earth's surface is 9.8 Newtons per kilogram of mass (approximately 10 N/kg.

So to find the approximate weight of an object on Earth, multiply the object's mass by 10.

An object's mass is constant, however the object's weight will change if the strength of gravity (g) changes. The value of g is also called acceleration due to gravity, as the strength of gravity causes a free-falling object to accelerate by the same amount.

## Free Falling Objects & Drag

The Earth's gravity causes a free-falling object to initially accelerate downwards at a rate of 9.8 m/s (approximately 10 m/s). On Earth, air resistance (drag) will reduce the object's acceleration.

In the absence of air resistance, two objects of different mass, dropped from the same height will hit the ground at the same time.

When astronaut, Commander David Scott, dropped a feather (0.03 kg) and a hammer (1.32 kg) from the same height on the Moon (no air), the two objects hit the ground at the same time.

Because they were essentially in a vacuum, there was no air resistance and the feather fell at the same rate as the hammer.

This supports Galileo's conclusion from hundreds of years ago, that all objects released together fall at the same rate regardless of mass.

A falling object on Earth experiences a constant force of gravity (weight) and an opposing force of air resistance (drag) that increases with speed. When These forces are balanced, the object reaches its terminal velocity.

## Task: Complete the following:

### SciPAD Workbook

Page 42-43 - mass and Weight

Page 44 - Friction

Page 45 - Air Resistance

### sciPAD ONLINE

Go to Forces > Mass vs. Weight + Friction + Air Resistance

### Education Perfect

Complete the task called "1.1 Concept 7: Mass, Weight & Drag"

# Concept 8: Pressure

## Success Criteria

I can define ‘pressure’

I can calculate pressure using P = F/A

I can rearrange P = F/A to solve an unknown variable.

## Vocabulary

*Energy*

*Joule*

*Transformation*

Pressure is the force per unit of area acting on a surface.

A force exerts a pressure on a surface, dependent on the size of the contact area.

The same force acting over a smaller area creates more pressure on the surface.

The pressure created by a force acting on an area, is given by the equation:

## Task: Complete the following:

### SciPAD Workbook

Page 46 - Pressure

Page 47 - Under Pressure

Page 48 - Pressure Problems

### sciPAD ONLINE

Go to Forces > Pressure

### Education Perfect

Complete the task called "1.1 Concept 8: Pressure"