A Level Physics: Mechanics and Materials

What is a scalar quantity?

Something with only magnitude

What is a vector quantity?

Something with magnitude and direction

Give 6 examples of scalar quantities.

Mass, temperature, time, distance, speed and energy

Give 5 examples of vector quantities.

Displacement, velocity, force, acceleration and momentum

When resolving, the horizontal component is always ...

... xcosθ

When resolving, the vertical component is always ...

... xsinθ

A body in equilibrium can be ...

... at rest or moving with a constant velocity

If a closed loop triangle can be drawn with 3 coplanar forces, what does it say about the body?

That it is in equilibrium

Why can the vertical component here be ignored?

P has no vertical component so the other forces must cancel the vertical component out because the body is in equilibrium

What is a moment?

The product of the force and the perpendicular distance from the point to the line of action of the force

M \=

F x d

What is the principle of moments?

For the body to be in equilibrium, the sum of the clockwise moments about any point equals the sum of the anticlockwise moments

What is a couple?

A pair of forces of equal size which act parallel to each other, but in opposite directions

How do you calculate the moment of a couple?

The magnitude of one force x the perpendicular distance between the lines of action of the forces

What are the units for a moment?

Nm

What is inertia?

An object's resistance to a change in velocity

What is weight?

The force experienced by a mass due to a gravitational field

w \=

m x g

What are the units for gravity?

Nkg^-1

What is the centre of mass of an object?

The point through which a single force on the body has no turning effect and where its whole weight can be considered to act through

How can you fin the centre of mass via experiment?

1. Hang an object freely from a point 2. Draw a vertical line downwards from the point of suspension, using a plumb bob 3. Hang the object from another point and repeat 4. Where these two lines cross is the centre of mass

An object will topple over if a vertical line drawn downwards from its centre of mass ...

... falls outside its base area, due to a resultant moment providing a turning force

An object will be more stable if it has ...

... a low centre of mass and wide base area

What is displacement?

How far an object has travelled from its starting point in a given direction

What is velocity?

The rate of change of an object's displacement

What is acceleration?

THe rate of change of an object's velocity

velocity \=

s/t

acceleration \=

v/t

What is the name of the speed of an object at a given point in time?

Instantaneous speed

How do you find the average speed or velocity?

Divide the total displacement by the total time

For a displacement-time graph, how would an accelerating object be shown?

Curved

If the object is accelerating at a uniform rate, the rate of change of the gradient will be ...

... constant

What kind of acceleration does the displacement-time graph show?

Big acceleration

What kind of acceleration does the displacement-time graph show?

Small acceleration

What kind of acceleration does the displacement-time graph show?

Deceleration

How can you tell if the velocity is constant on a displacement-time graph?

The graph is a straight line

What is the gradient of a displacement-time graph?

The velocity

What does it mean if, in a velocity-time graph, the graph becomes negative?

The object is travelling in the opposite direction

What is the gradient of a velocity-time graph?

The acceleration

What kind of acceleration does the velocity-time graph show?

Large acceleration

What kind of acceleration does the velocity-time graph show?

Decreasing accleration

What kind of acceleration does the velocity-time graph show?

Increasing acceleration

What kind of acceleration does the velocity-time graph show?

Small acceleration

What is the are under a velocity-time graph?

The displacement

The velocity-time graph shows a ball being dropped and bouncing twice. What is happening here?

Ball hits the floor

The velocity-time graph shows a ball being dropped and bouncing twice. What is happening here?

Ball rebounds

The velocity-time graph shows a ball being dropped and bouncing twice. What is happening here?

Top of bounce

What is the area under an acceleration-time graph?

The change in velocity

On an acceleration-time graph, if a \= 0, what does it mean?

The object is moving at constant velocity

On an acceleration-time graph, if the acceleration becomes negative, what does it mean?

The object is decelerating

Give 3 advantages of using a data-logger to track motion compared to traditional methods like a stopwatch.

1. Data is more accurate as you don't have to account for human reaction times 2. Automatic systems have a much higher sampling rate than humans 3. You can see data displayed in real time

u +at \=

v

(u+v)/2 x t \=

s

ut + at^2/2 \=

s

u^2 + 2as \=

v^2

What is free fall?

The motion of an object undergoing an acceleration of 'g'

What is the only force acting on an object in free fall?

It's weight

All objects in free fall ...

... accelerate at the same rate

Describe a method to calcutlate g.

1. Set up a vertical electric circuit with a switch, timer, electromagnet and trapdoor and attach a ball bearing to the electromagnet 2. Measure the h from the ball bearing to the trapdoor 3. Flick the switch to start the timer and disconnect the electromagnet 4. The ball will fall and knock the trapdoor down, breaking the circuit which stops the timer- record this time 5. Repeat 3 times and find an average, then repeat at different h 6. Plot a graph of h against t^2 to find 1/2g as the gradient

In an experiment to calculate g, why is it best to use a small and heavy ball bearing?

Air resistance will be so small, it can be ignored

In an experiment to calculate g, why is it best to have a computer automatically release and time the ball bearing's fall?

There will be a smaller uncertainty than with human reaction time

In an experiment to calculate g, where will the largest source of error come from?

Measuring h as the ruler will have an uncertainty of +/- 1 mm which is much larger than any error from switch delay or air resistance

How would you find g on a displacement-time graph?

By finding the change in gradient between two points because a \= v / t

Which parts of SUVAT can be predicted if an object is falling?

u \= 0 a \= -9.81

Which parts of SUVAT can be predicted if an object is thrown upwards?

u is not 0 a \= -9.81

Which parts of SUVAT can be predicted if an object is thrown downwards?

u is not 0 a \= -9.81

Which parts of SUVAT can be predicted if an object is thrown horizontally?

Vertical u \= 0 a \= -9.81 Horizontal a \= 0 u \= constant

Which parts of SUVAT can be predicted if an object is projected at an angle?

Vertical a \= -9.81 Half way through v \= 0

What is Newton's 1st law?

The velocity of an object will not change unless a resultant force acts on it

What is Newton's 2st law in words?

Acceleration is proportional to force

What is Newton's 2nd law as an equation?

F \= ma

What is the resultant force?

The vector sum of all the forces

In which direction is acceleration always measured in?

The same as the resultant force

Prove that all objects fall at the same rate, regardless of mass using Newton's 2nd Law.

F \= ma. Ignoring air resistance, weight is the only force acting on the object so W \= F \= mg \= ma. The two masses cancel out so g \= a and g is a constant for a uniform gravitational field

What is Newton's 3rd law?

When two objects interact, they exert equal and opposite forces on each other

In Newton's 3rd law, the two forces must be equal, opposite and ...

... the same type, like both gravitational or electrical

What are the two main types of friction?

Dry friction and fluid friction

What does the force of fluid friction depend on?

The viscosity of the fluid

As speed increases, fluid friction ...

... also increases

The larger the area pushing against the fluid ...

... the greater the resistance force

Frictional forces always act ...

... in the opposite direction to the motion of the object

Frictional forces can never ...

... speed things up or start something moving

Frictional forces convert kinetic energy into ...

... heat and sound

What is lift?

An upwards force on an object moving through a fluid

Lift happens when ...

... the shape of an object causes the fluid flowing over it to change direction

In which direction does lift act?

Perpendicular to the direction the fluid flows in

What is the terminal speed?

When the friction force equals the driving force

What conditions are required to reach terminal speed?

A driving force that stays the same and a frictional or drag force that increases with speed

Describe this picture which shows the first stage of reaching terminal speed.

The car accelerates from rest using a constant driving force

Describe this picture which shows the second stage of reaching terminal speed.

As speed increases, frictional forces increase, reducing the resultant force on the car and hence reducing acceleration

Describe this picture which shows the third stage of reaching terminal speed.

The frictional forces are now equal to the driving force and there is no resultant force and no acceleration, so the car carries on at constant speed

Give two ways you can increase a vehicle's maximum speed

Increasing the driving force by increasing the engines size and reducing the frictional force by making the body more streamlined

Describe a skydiver's fall before they open their parachute.

The skydiver's weight will initially be greater than the air resistance, but will soon speed up so the air resistance and weight are the same. The skydiver has now reached terminal speed

Describe a skydiver's fall after they open their parachute.

The parachute increases the air resistance so it is bigger than the skydiver's weight, slowing them down. Eventually the air resistance and weight will equal again, reaching a lower terminal speed

Momentum \=

mass x velocity

What units is momentum measured in?

kg ms^-1

Assuming no external forces act, momentum is ...

... always conserved

What is an elastic collision?

A collision where momentum and kinetic energy are conserved

What is an inelastic collision?

A collision where momentum is conserved, but kinetic energy isn't

The rate of change of momentum is directly proportional to ...

... the resultant force