Physics 8463 - 4.5 Forces

4.5.6 Forces and motion

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Distance is how far an object moves.

Distance does not involve direction.

Distance is a scalar quantity.

Displacement includes both the distance an object moves, measured in a straight line from the start point to the finish point and the direction of that straight line.

Displacement is a vector quantity. Students should be able to express a displacement in terms of both the magnitude and direction.

Speed does not involve direction.

Speed is a scalar quantity.

The speed of a moving object is rarely constant.

When people walk, run or travel in a car their speed is constantly changing. Speed quoted is therefore usually an average speed.

The speed at which a person can walk, run or cycle depends on many factors including: age, terrain, fitness and distance travelled.

It is not only moving objects that have varying speed.

The speed of sound and the speed of the wind also vary.

A typical value for the speed of sound in air is 330 m/s.

You should be able to make measurements of distance and time and then calculate speeds of objects.

For an object moving at constant speed the distance travelled in a specific time can be calculated using the equation:

distance travelled = speed × time

s = vt

This equation is NOT given on the Physics equation sheet - you need to know it, what the letters mean and are measured in!

s = distance, in metres, m

v = speed, in metres per second, m/s

t = time, in seconds, s

You should be able to calculate average speed for non-uniform motion.

The velocity of an object is its speed in a given direction.

Velocity is a vector quantity.

You should be able to explain the vector–scalar distinction as it applies to displacement, distance, velocity and speed.

You should be able to explain qualitatively, with examples, that motion in a circle involves constant speed but changing velocity.

The average acceleration of an object can be calculated using the equation:

acceleration = change in velocity/time taken

a = Δv/t

This equation is NOT given on the Physics equation sheet - you need to know it, what the letters mean and are measured in!

a = acceleration, in metres per second squared, m/s²

Δv = change in velocity, in metres per second, m/s

t = time, in seconds, s

An object that slows down is decelerating.

You should be able to estimate the magnitude of everyday accelerations.

The acceleration of an object can be calculated from the gradient of a velocity–time graph.

The distance travelled by an object (or displacement of an object) can be calculated from the area under a velocity–time graph.

Students should be able to:

draw velocity–time graphs from measurements and interpret lines and slopes to determine acceleration

interpret enclosed areas in velocity–time graphs to determine distance travelled (or displacement)

measure, when appropriate, the area under a velocity–time graph by counting squares.

The following equation applies to uniform acceleration:

final velocity² − initial velocity² = 2 × acceleration × distance

v² − u² = 2as

This equation is given on the Physics equation sheet - but you need to know what the letters mean and are measured in.

v = final velocity, in metres per second, m/s

u = initial velocity, in metres per second, m/s

a = acceleration, in metres per second squared, m/s²

s = distance, in metres, m

Near the Earth's surface any object falling freely under gravity has an acceleration of about 9.8 m/s².

An object falling through a fluid initially accelerates due to the force of gravity.

Eventually the resultant force will be zero and the object will move at its terminal velocity.

You should be able to:

draw and interpret velocity–time graphs for objects that reach terminal velocity
interpret the changing motion in terms of the forces acting.

Newton's First Law

If the resultant force acting on an object is zero and:

the object is stationary, the object remains stationary

the object is moving, the object continues to move at the same speed and in the same direction.

So the object continues to move at the same velocity.

This means that, when a vehicle travels at a steady speed the resistive forces balance the driving force.

The velocity (speed and/or direction) of an object will only change if a resultant force is acting on the object.

You should be able to apply Newton's First Law to explain the motion of objects moving with a uniform velocity and objects where the speed and/or direction changes.

The tendency of objects to continue in their state of rest or of uniform motion is called inertia.

The acceleration of an object is proportional to the resultant force acting on the object, and inversely proportional to the mass of the object.

As an equation:

resultant force = mass × acceleration

F = ma

F = force, in newtons, N

m = mass, in kilograms, kg

a = acceleration, in metres per second squared, m/s²

You should be able to estimate the speed, accelerations and forces involved in large accelerations for everyday road transport.

You should recognise and be able to use the symbol for proportionality,

∝

You should recognise and be able to use the symbol that indicates an approximate value or approximate answer

≈

Whenever two objects interact, the forces they exert on each other are equal and opposite.

You should be able to apply Newton's Third Law to examples of equilibrium situations.

The stopping distance of a vehicle is the 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).

For a given braking force the greater the speed of the vehicle, the greater the stopping distance.

You should be able to estimate how the distance for a vehicle to make an emergency stop varies over a range of speeds typical for that vehicle.

You will be required to interpret graphs relating speed to stopping distance for a range of vehicles.

Reaction times vary from person to person. Typical values range from 0.2 s to 0.9 s.

A driver's reaction time can be affected by tiredness, drugs and alcohol.

Distractions may also affect a driver's ability to react.

You should be able to:

explain methods used to measure human reaction times and recall typical results

interpret and evaluate measurements from simple methods to measure the different reaction times of students

evaluate the effect of various factors on thinking distance based on given data.

The braking distance of a vehicle can be affected by adverse road and weather conditions and poor condition of the vehicle.

- Adverse road conditions include wet or icy conditions.

- poor condition of the vehicle is limited to the vehicle's brakes or tyres.

You should be able to:

explain the factors which affect the distance required for road transport vehicles to come to rest in emergencies, and the implications for safety

estimate how the distance required for road vehicles to stop in an emergency varies over a range of typical speeds.

When a force is applied to the brakes of a vehicle, work done by the friction force between the brakes and the wheel reduces the kinetic energy of the vehicle and the temperature of the brakes increases.

The greater the speed of a vehicle the greater the braking force needed to stop the vehicle in a certain distance.

The greater the braking force the greater the deceleration of the vehicle.

Large decelerations may lead to brakes overheating and/or loss of control.

You should be able to:

explain the dangers caused by large decelerations

estimate the forces involved in the deceleration of road vehicles in typical situations on a public road.