Physics Unit 5 Forces

What is the primary difference between a scalar quantity and a vector quantity?

A vector quantity has both magnitude and direction, whereas a scalar quantity has magnitude only.

List three examples of scalar quantities found in physics.

Distance, speed, and mass.

List three examples of vector quantities found in physics.

Displacement, velocity, and acceleration.

How is the magnitude of a vector represented visually in a diagram?

By the length of the arrow.

Define 'Weight' in the context of gravitational forces.

The force acting on an object due to gravity.

What is the formula used to calculate the weight of an object?

$W = m \times g$

In the equation $W = m \times g$, what does the symbol $g$ represent?

Gravitational field strength (measured in $N/kg$).

What is the approximate value of the gravitational field strength on Earth?

$9.8\ N/kg$

Define the 'centre of mass' of an object.

The single point through which the weight of an object is considered to act.

What specific piece of equipment is used to measure the weight of an object?

A calibrated spring-balance (newtonmeter).

If the mass of an object doubles, what happens to its weight in a constant gravitational field?

The weight also doubles because weight and mass are directly proportional.

Define the term 'resultant force'.

A single force that has the same effect as all the original forces acting together.

Under what condition is the resultant force on an object considered to be zero?

When all forces acting on the object are balanced.

How is a single force 'resolved' in a vector diagram (Higher Tier)?

It is split into two components acting at right angles to each other (usually horizontal and vertical).

What must be done to calculate work done when a force causes displacement?

Multiply the force by the distance moved along the line of action of the force ($W = F \times s$).

What is the equivalent unit for one Joule ($1\ J$) in terms of Newtons and metres?

$1\ Newton-metre$ ($1\ Nm$).

What physical effect occurs when work is done against frictional forces on an object?

The temperature of the object rises.

Define 'elastic deformation'.

The object returns to its original shape and length when the force is removed.

What is meant by 'inelastic deformation'?

The object does not return to its original shape after the force is removed (it is permanently stretched).

State the relationship described by Hooke's Law.

The extension of an elastic object is directly proportional to the force applied, provided the limit of proportionality is not exceeded.

What is the formula for the force applied to a spring?

$F = k \times e$

In the equation $F = k \times e$, what does $k$ represent and what are its units?

Spring constant, measured in Newtons per metre ($N/m$).

How is work done on a spring related to elastic potential energy?

The work done and the elastic potential energy stored are equal, provided the spring is not inelastically deformed.

What is the formula for calculating elastic potential energy?

$E_{e} = \frac{1}{2} \times k \times e^{2}$

In a force-extension graph, what indicates that the limit of proportionality has been exceeded?

The line begins to curve and is no longer straight.

What does the gradient of a Force-Extension graph represent if Force is on the y-axis?

The spring constant ($k$).

What is a 'moment' in physics terms?

The turning effect of a force.

What is the formula for the moment of a force?

$M = F \times d$

In the moment equation ($M = F \times d$), how must distance ($d$) be measured relative to the force?

It must be the perpendicular distance from the pivot to the line of action of the force.

State the Principle of Moments for a balanced object.

The total clockwise moment about a pivot equals the total anticlockwise moment about that pivot.

What is the primary function of levers and gears?

They are used to transmit and multiply the rotational effects of forces.

What is the general formula for pressure on a surface?

$p = \frac{F}{A}$

In what direction does the pressure in a fluid exert a force on a surface?

Normal to (at right angles to) the surface.

What is the formula for pressure due to a column of liquid (Higher Tier)?

$p = h \times \rho \times g$

In the liquid pressure formula, what does the symbol $\rho$ represent?

Density of the liquid ($kg/m^{3}$).

Explain why liquid pressure increases with depth.

As depth increases, the height of the column of liquid above that point increases, increasing the weight of the liquid above.

What causes the 'upthrust' experienced by a submerged object?

The pressure on the bottom surface of the object is greater than the pressure on the top surface.

Under what condition will an object float in a fluid?

When the object's weight is equal to the upthrust.

Why does atmospheric pressure decrease as altitude increases?

The number of air molecules (and the weight of the air) above a surface decreases as height increases.

Define 'displacement' as a vector quantity.

Distance moved in a straight line from the start point to the finish point, including the direction.

What is the typical speed for a person walking?

$1.5\ m/s$

What is the typical speed for a person running?

$3\ m/s$

What is the typical speed for a person cycling?

$6\ m/s$

What is the typical value for the speed of sound in air?

$330\ m/s$

How is distance travelled calculated for an object moving at a constant speed?

$s = v \times t$

Define velocity in terms of speed.

Velocity is speed in a given direction.

Explain why an object moving in a circle has changing velocity but can have constant speed.

Because its direction is constantly changing, and velocity is a vector quantity dependent on direction.

What is the formula for average acceleration?

$a = \frac{\Delta v}{t}$

What does a negative value for acceleration typically represent?

Deceleration (the object is slowing down).

How is acceleration determined from a velocity-time graph?

From the gradient (slope) of the line.

How is distance travelled determined from a velocity-time graph (Higher Tier)?

By calculating the area under the graph.

State the equation for uniform acceleration involving initial and final velocity.

$v^{2} - u^{2} = 2 \times a \times s$

What is the acceleration of an object falling freely under gravity near the Earth's surface?

Approximately $9.8\ m/s^{2}$.

Define 'terminal velocity'.

The constant velocity reached when the resistive forces on a falling object balance the force of gravity.

State Newton's First Law regarding a stationary object with zero resultant force.

The object remains stationary.

State Newton's First Law regarding a moving object with zero resultant force.

The object continues to move at the same speed and in the same direction (constant velocity).

What is 'inertia' (Higher Tier)?

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

State Newton's Second Law as an equation.

$F = m \times a$

According to Newton's Second Law, how is acceleration related to mass for a constant force?

Acceleration is inversely proportional to mass.

Define 'inertial mass' (Higher Tier).

The ratio of force over acceleration ($m = F/a$); it measures how difficult it is to change an object's velocity.

State Newton's Third Law.

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

Define 'Stopping Distance'.

The sum of the thinking distance and the braking distance.

What is 'thinking distance'?

The distance a vehicle travels during the driver's reaction time.

List three factors that can increase a driver's reaction time.

Tiredness, drugs, and alcohol.

What is 'braking distance'?

The distance a vehicle travels under the braking force.

How do wet or icy roads affect the braking distance of a car?

They increase the braking distance by reducing friction between the tyres and the road.

What dangerous physical effect can occur due to large decelerations during braking?

Brakes can overheat and/or the driver may lose control of the vehicle.

What is the formula for momentum (Higher Tier)?

$p = m \times v$

What is the unit for momentum?

Kilogram metre per second ($kg\ m/s$).

State the Law of Conservation of Momentum.

In a closed system, the total momentum before an event is equal to the total momentum after the event.

What is the formula for force expressed as the rate of change of momentum (Higher Tier)?

$F = \frac{m \Delta v}{\Delta t}$

How do safety features like seatbelts or airbags reduce the force on a person during a collision?

They increase the time taken for the change in momentum to occur, which reduces the rate of change of momentum and thus the force.

In Required Practical 7, why must masses be moved from the trolley to the mass hanger to keep mass constant?

To ensure the total mass of the entire system (trolley + hanger + masses) remains constant while varying the pulling force.

In a distance-time graph, what does a flat horizontal line indicate?

The object is stationary.

In a velocity-time graph, what does a flat horizontal line indicate?

The object is moving at a constant speed.

What does a curve represent on a distance-time graph?

Acceleration or deceleration.

What is the difference between contact and non-contact forces?

Contact forces require objects to be physically touching, while non-contact forces act when objects are physically separated.

Give two examples of non-contact forces.

Gravitational force and magnetic force.

What name is given to the force that acts in the string during a trolley investigation?

Tension.

Why is the actual maximum height reached by a projectile usually lower than the calculated value?

Energy is transferred to the surroundings as thermal energy due to air resistance.

In the context of the Mariana Trench problem, why does pressure increase as a submarine dives deeper?

The height of the column of water above the submarine increases, which increases the weight of water acting on it.

When a force of $60\ N$ is applied to a piston with a pressure of $120,000\ Pa$, what is the area?

$0.0005\ m^{2}$ ($5.0 \times 10^{-4}\ m^{2}$).

What happens to the acceleration of a trolley if the runway is raised at one end while the pulling force remains the same?

The acceleration increases because a component of the trolley's weight now acts parallel to the runway, increasing the resultant force.

Define 'uncertainty' in the context of repeat measurements.

Half the range of the measurements ($\pm \frac{Max - Min}{2}$).

What is the advantage of repeating measurements and calculating a mean?

It reduces the effect of random errors and increases the reliability/accuracy of the result.

In the bicycle gear system, how is a moment produced about the rear axle?

The force on the pedal causes a moment about the pedal axle, which pulls the chain, creating a moment about the rear axle.

What is the unit of a moment?

Newton-metres ($Nm$).

For an object to be in equilibrium, what must be true about the resultant force and the total moments?

The resultant force must be zero and the total clockwise moments must equal the total anticlockwise moments.

What is the definition of a 'fluid'?

A substance that can flow, which includes both liquids and gases.

Why does a flexible bumper on a bumper car reduce the risk of injury?

It increases the time taken for the collision to occur, reducing the rate of change of momentum and the force on the passengers.

If the stopping distance is $53\ m$ and the thinking distance is $15\ m$, what is the braking distance?

$38\ m$.

How is 'inertial mass' calculated?

By dividing the resultant force acting on the object by the acceleration produced ($m = F/a$).

In a vector diagram, what represents the direction of the resultant force?

The direction of the diagonal arrow drawn from the start to the end of the tip-to-tail sequence.

What is the formula for kinetic energy?

$E_{k} = \frac{1}{2} \times m \times v^{2}$

What happens to a spring once it has passed its 'limit of proportionality'?

The extension is no longer directly proportional to the force applied.

In the equation $pV = constant$, what condition must be met regarding temperature?

The temperature of the gas must remain constant.

How is the speed of a wave related to frequency and wavelength?

$v = f \times \lambda$

What is the unit for pressure?

Pascals ($Pa$).