Forces Year 11 GCSE Physics Module 11

Starter  What is the unit of measurement of a force?  How fast is a cat travelling who covers 30m in 5s?  What 2 things does friction do?  How many days in 1 Earth year?  How many stars in the milky way?  How many galaxies in the universe?

Objectives  Recall and use the calculation for speed (KS 3)  Draw and interpret a distance/time graph – calculate gradient  Draw and interpret a velocity/time graph – calculate distance travelled

Speed  This is the time taken for a moving object to cover a specific distance.  Speed is measured in Metres per second (m/s)  Speed = distance/time

Distance/time graph  READ THE AXES FIRST  Gradient = steady speed  Steeper gradient = faster speed  Horizontal line = stationary  To calculate speed, calculate the gradient of the graph

Velocity/time graph  READ THE AXES FIRST  Gradient = constant acceleration  Steeper the gradient = faster acceleration  Horizontal line = constant velocity

Velocity/time graph  To calculate acceleration, calculate the gradient of the graph.  To calculate the total distance travelled – calculate the area beneath the graph  VELOCITY = SPEED IN A GIVEN DIRECTION  It is possible for speed to remain the same but the velocity to change (when an object changes direction)

objectives  Use calculations to work out acceleration  Use knowledge of acceleration to answer GCSE questions

Acceleration  To calculate acceleration you need to know the change in speed (m/s) and the time taken for the change (s).  Acceleration (m/s 2 ) = change in speed (m/s) /time taken (s)

Acceleration (higher)  There is another relationship which affects acceleration.  It takes 1 newton to accelerate 1kg, 1 m/s 2  Therefore: force = mass x acc.

Starter  Write down the two equations used to calculate acceleration  How do you calculate distance on a velocity/time graph?  On a distance/time graph, what does a steep gradient mean?  What is the unit of measurement for acceleration?

Objectives  Answer higher level questions using knowledge of forces and motion  Put Newton’s 3 laws of motion into ‘kidspeak’

Newton’s first law of motion  Unless acted on by a net force, a body at rest stays at rest, and a moving body continues moving at the same speed in the same straight line

Newton’s second law of motion  A net force applied to a body gives it an acceleration proportional to the force and in the direction of the force.

Newton’s third law of motion  When a body A exerts a force on a body B, B exerts an equal and opposite force on A; that is, to every action there is an equal and opposite reaction

Newton’s first law of motion  If forces acting on an object are balanced an object stays still, or if it is moving, continues to move at a constant speed.

Newton’s second law of motion  Force = mass X acceleration!

Newton’s third law of motion  Forces always act in pairs. Action forces and reaction forces are equal and opposite

Story so far  Produce a two page revision aid to summarise forces and motion

Starter  What is the unit of measurement for MASS?  What does a horizontal line mean on a d/t graph?  How much force is needed to accelerate an object of 500g at 3m/s 2 ?  Describe the forces acting on a car travelling at 30 m/s

Objectives  Summarise balanced and unbalanced forces  Describe the forces acting on a falling object, and how its fall is affected  Calculate the weight of objects

Balanced and unbalanced forces - summary  If forces are balanced a stationary object stays still – a moving object carries on moving at a constant speed.  If forces are unbalanced a stationary object starts to move, or changes shape. A moving object changes speed, direction or shape

Falling objects  All falling objects immediately accelerate because gravity is acting on them, and is greater than air resistance.  Gradually air resistance increases so acceleration slows down until the two forces are balanced.  The object now falls at a constant speed. This is called its terminal velocity  On Earth, all objects fall with a terminal velocity of approx. 10 m/s

Gravity  Gravity is a force that pulls all objects to the centre of a mass. Everything which has mass has gravity – the larger the mass the larger the force of gravity.  On Earth, gravity pulls every kg of mass with a force of 10 newtons (10 N/kg)

Calculating gravity  There is a relationship between weight, gravity and mass.To calculate either of these from given information we must understand the relationship.  Weight = Mass X Gravity (N) (kg) (N/kg)

Starter  What is the weight of a cat with a mass of 5kg?  What would its weight be on the moon? (gravity = 0.6 N/kg)  What happens to a falling object when there is no resultant force?  What do all falling objects do initially, and why?

Objectives  To explain the role of friction in stopping moving vehicles  To identify and explain factors affecting braking and stopping  To be able to calculate Kinetic energy and relate this to road accidents

Braking and stopping  To stop a moving vehicle the friction forces must be greater than the forward force. (unbalanced)  The greater the speed of the vehicle;  the greater the braking force needed to stop it in a certain time  The greater the distance needed for it to stop with a certain braking force

Braking and stopping (higher tier)  Kinetic energy is the energy an object has because of its movement.  We calculate it using;  K.E. = ½ x mass x speed 2 (J) (kg) (m/s) 2

Braking and stopping  The object has more K.E.  The greater its speed  The greater its mass

Braking and stopping  If too great a braking force is applied, friction between the tyres and the road may not be great enough to hold the car and it will skid.  Pressing the brakes causes metal pads with a high friction coating to push against a disc on the wheel. This opposes the forward force and the car slows down. The friction forces in the brakes produce a lot of heat.  Work done against friction is mainly transferred as heat energy.

Thinking distance  This is the distance a car travels during the driver’s reaction time.  The distance travelled between the driver seeing the need to brake and actually pressing the pedal  Reaction time can be increased by the driver’s reactions being impaired due to drugs, alcohol, illness, tiredness, distractions in the car, poor visibility.

Stopping distance  This is the distance a vehicle travels under braking force (from the moment the brakes are applied until it stops)  This is increased by:  Worn brakes/tyres, icy/wet roads, worn road surface, travelling fast.

Overall stopping distance  This = thinking distance + stopping distance

Summary task  Design an information poster to sum up the factors affecting the stopping of a moving vehicle.  Include images where possible.  Include the K.E. calculation to show affect of increased speed on movement