Forces (Acceleration/Newtons Law/Stopping Distance)

Acceleration

Rate of change in velocity

• change in velocity = change in speed/direction or both

What is negative acceleration

Deceleration

How to calculate average acceleration (if it is not constant)

• a = delta V / t

• a = accel m/s²

• delta v = change in velocity = m/s

• t = time taken (s)

Units for acceleration

• m/s²

• MUST BE SQUARED

Estimating Accelerations

• Might have to estimate acceleration of the object

• Use the typical speeds in order to estimate

• eg. car will have typically speed of 25m/s

• car comes to a stop in one second

• so deceleration is -25m/s²

Uniform Acceleration (Constant Acceleration)

v² (m/s) - u² (m/s) =2 a (m/s) s(m)

Distance

How far an object has moved (Scalar Quantity)

Displacement

Measures distance and direction of an object (vector quantity)

Speed

How fast something is going (scalar quantity)

Velocity

How fast something is going and in which direction (vector quantity)

Constant Speed not true

Constant Speed not true

• Objects can have a constant speed but changing velocity

• Circular motion

• Happens when an object is changing direction whilst at the same speed

Important Speeds

• Walk - 1.5m/s

• Run - 3m/s

• Cycle - 6m/s

• Sound 330m/s

• Car - 25m/s

• Train - 55m/s

• Plane -250m/

Factors that affect speed in every day life

Fitness,age,terrain,effort

• What sound waves are travelling through

• Wind speed is affected by pressure,and objects

Distance formula

D=ST or S=VT

Important Stuff When Calculating-

• When object is stationary its (INITIAL) velocity is 0m/s

• When object is dropped normally its 9.8m/s

When to choose which acceleration equation?

• if given distance use constant

• if give time use average

Friction

• acts in opposite direction to the movement

• to travel at a steady speed driving force need to balance the frictional forces

• friction occurs when two surfaces are in contact or when object passes through a fluid (usually caslled drag)

Drag

• air resistance eg. drag

• Frictional forces from fluids always increase with speed

• Car has much more friction to work against

Ways on icnreasing the top speed of a vehicle

• reducing drag (change the shape of vehicle)

• increasing power of vehicles engine

Terminal Velocity

• Maximum velocity attanable by an object as it falls through a fluid (liquid or gas)

• RESULTANT FORCE = ZERO

Air resistance

Force Caused by moving object colliding with particles in the air

What two forces are in balance at terminal velocity?

• weight and air resistance (so will cancel each other out)

Terminal velocity graph

Parachute and Plane Example

• As a person jumps out of a plane they are statinoary and their velocity is zero

• As they have a mass weight starts to bring them down and because the velocity is low there is little air resistance

• As they are falling air resistance starts to increase and eventually the magnitude of the weight and resistance area equal

• Therefore the resultant force is zero this is terminal velocity

• Once a parachute is open the surface area increases by a lot meaning that resistant force will now be upwards because of more air resistance

• This means that the person is decelerating (accel upwards)

• Eventually they will reach a new terminal velocity

Reasons for larger air resistance

• More collisions with air particles

• Velocity - Faster the object is moving the more particles it collides with

• Surface Area - Large SA means there is a larger area over which collisions can take place

What happens to an object that is dropped in a fluid?

• Initially accelerates due to force of gravity

Stopping Distance

sum of the distance the vehicle travels during the driver’s reaction time (thinking distance) and the distance it travels under the braking force (braking distance).

Stopping Distance equals

Stopping Distance = Thinking + Braking Distance

Thinking Distance

Distance a car travels between the point a driver spots a danger and the point they apply the brakes

Braking Distance

Distance a car travels between the point a driver applies the brakes and the point the car stops moving

Factors that affect thinking distance

• how fast you are going

• reaction time

5 Factors that affect Braking Distance

• Quality of breaks

• Quality of tyres (how much tread they have)

• The road surface (ice/wet)

• Speed of Vehicle

• Mass of Vehicle

• speed limits affect stopping distance

• as car speeds up thinking distance increases at the same rate as the speed (directly proportional)

  • as thinking time stays constant (higher speed more distance covered)

Braking distance and Speed

• increases faster as you speed up

• speed doubles braking distance increases x4 (2²)