Forces & Motion (AQA GCSE Combined Science: Synergy: Physical Sciences): Flashcards

A train travels at a constant speed of 50 m/s for 200 s. The distance it travels is 10 000 m.

(s = v × t = 50 × 200 = 10 000 m)

True.

The equation v = s/t applies to objects moving at constant speed. For non-uniform motion, you must use the average speed equation with total distance and total time taken.

average speed = distance ÷ time

average speed = 100 ÷ 10.5

average speed ≈ 9.52 m/s

Speed is the distance an object travels per unit of time. It is a scalar quantity, measured in metres per second (m/s). Speed does not involve direction.

False.

Average speed is calculated using the total distance travelled divided by the total time taken for the whole journey, not just the initial and final speeds.

A distance-time graph shows how the distance of an object from a starting position changes over time. The gradient (slope) of the line gives the speed of the object.

A horizontal line on a distance-time graph means the distance is not changing — the object is stationary (not moving).

True.

A steeper slope on a distance-time graph represents a greater speed. The gradient of the line equals the speed of the object.

On a distance-time graph, the speed of an object is equal to the gradient of the line. If the object is accelerating, the line will be a curve.

The instantaneous speed of an accelerating object at a particular moment can be found from a distance-time graph by calculating the gradient of a tangent drawn to the curve at that point.

The gradient is calculated using:

gradient = Δy ÷ Δx

where Δy = change in distance and Δx = change in time. A large gradient triangle should be used for accuracy.

False.

A curved line with an increasing slope represents an accelerating object (speeding up). A decreasing slope on the curve represents a decelerating object (slowing down).

distance = speed × time

distance = 20 × 5

total distance = 100 m

Circular motion is motion along a circular path. An object in circular motion moves at constant speed but has a continuously changing direction, so its velocity is always changing.

No, its velocity is not constant. Although the speed is constant, the direction of motion is continuously changing. Since velocity is a vector quantity (it has direction), a change in direction means a change in velocity.

False.

An object moving in a circle at constant speed is accelerating because its direction (and therefore its velocity) is continuously changing. Acceleration is a change in velocity, which includes a change in direction.

Velocity is a vector quantity. Motion in a circle involves constant speed but changing velocity.

One example of circular motion is the International Space Station orbiting the Earth. It travels at a constant speed (~7660 m/s) but constantly changes direction, so its velocity is always changing.

False.

Newton's first law requires constant velocity (constant speed AND constant direction). The Moon moves at constant speed but changes direction continuously, so it is not an example of Newton's first law.

Acceleration is the rate of change of velocity. It is calculated using:

a = Δv ÷ t

where a = acceleration (m/s²), Δv = change in velocity (m/s), and t = time taken (s).

a = Δv ÷ t

• a = acceleration in m/s²

• Δv = change in velocity in m/s

• t = time in s

Change in velocity = final velocity − initial velocity

False.

An object that slows down is decelerating and has a negative acceleration. An object that speeds up has a positive acceleration.

Change in velocity = final velocity − initial velocity. A negative change in velocity means the object is decelerating.

Deceleration is a negative acceleration — it occurs when an object slows down. The change in velocity is negative because the final velocity is less than the initial velocity.

Δv = 8 − 20 = −12 m/s

a = Δv ÷ t = −12 ÷ 4

a = −3 m/s²

(The negative sign confirms the car is decelerating.)

True.

A typical family car accelerates at approximately 2–3 m/s². By comparison, a falling object accelerates at about 10 m/s² and a rocket at about 30 m/s².

A falling object near the Earth's surface accelerates at approximately 10 m/s² due to gravity.

A velocity-time graph shows how the velocity of an object changes with time. The gradient (slope) of the line gives the acceleration, and the area under the graph gives the distance (displacement) travelled.

A flat horizontal line on a velocity-time graph means the velocity is constant — the acceleration is zero and the object is moving at a constant velocity.

False.

The area under a velocity-time graph represents the distance (displacement) travelled by the object. The gradient of the line represents the acceleration.

On a velocity-time graph, a steep slope represents large acceleration, a gentle slope represents small acceleration, and a flat line represents constant velocity.

The area under a velocity-time graph represents the displacement (or distance travelled) by the object. If the area forms a triangle, use ½ × base × height; if a rectangle, use base × height.

acceleration = gradient = Δv ÷ Δt

acceleration = (20 − 0) ÷ 10

acceleration = 2 m/s²

False.

A curve on a velocity-time graph indicates changing acceleration. Constant acceleration is shown by a straight line on the velocity-time graph.

distance = area under graph = base × height

distance = 8 × 15

distance = 120 m

Uniform acceleration is constant acceleration — the velocity of an object changes by the same amount every second. For uniformly accelerating objects, the equation v² = u² + 2as applies.

v² = u² + 2as

• v = final speed (m/s)

• u = initial speed (m/s)

• a = acceleration (m/s²)

• s = distance travelled (m)

True.

The equation v² = u² + 2as is specifically used when the time taken is not known. It links final speed, initial speed, acceleration, and distance without requiring time.

A car starts from rest (u = 0) and accelerates uniformly at 3 m/s² over a distance of 24 m. Its final speed is 12 m/s.

(v² = 0 + 2 × 3 × 24 = 144; v = 12 m/s)

v² = u² + 2as

16² = 0² + 2 × 2.5 × s

256 = 5s

s = 51.2 m

False.

The equation v² = u² + 2as only applies to objects moving with uniform (constant) acceleration. If the acceleration is changing, this equation cannot be used.

v² = u² + 2as

0 = 10² + 2 × a × 5

0 = 100 + 10a

a = −10 m/s²

Deceleration = 10 m/s²

Terminal velocity is the constant speed reached by a falling object when the upward air resistance (drag) equals the downward weight force. At this point the resultant force is zero and acceleration is zero.

Initially, the weight force is much greater than the air resistance (which is very small at low speeds). There is a large resultant force downwards, so the skydiver accelerates.

True.

At terminal velocity, air resistance equals weight, so the resultant force is zero. With no resultant force, acceleration is zero and the object falls at a constant speed.

The acceleration due to gravity (g) is approximately 9.8 m/s² near the Earth's surface. In the absence of air resistance, all objects fall with this same acceleration regardless of their mass.

As a falling object speeds up, the air resistance force increases. When it equals the weight force, the resultant force is zero and the object reaches terminal velocity.

The acceleration due to gravity near the Earth's surface is approximately 9.8 m/s². This means a freely falling object gains 9.8 m/s of speed every second.

False.

In the absence of air resistance, all objects near the Earth's surface fall with the same acceleration of approximately 9.8 m/s², regardless of their mass.

The correct term is air resistance (or drag). It is caused by friction between the object and air particles. Air pressure is a different concept and does not describe this force.

Newton's First Law states: if the resultant force on an object is zero, and the object is stationary, it remains stationary; if the object is moving, it continues at the same speed and in the same direction (constant velocity).

The forces on the object must be balanced — the resultant force is zero. When a vehicle travels at a steady speed, the resistive forces balance the driving force.

False.

According to Newton's First Law, an object can continue moving at constant velocity with no resultant force acting on it. A force is only needed to change the velocity.

A car travelling at a steady speed on a straight road has balanced resistive forces and balanced driving force. The resultant force is zero.

The resultant force is the single force that has the same effect as all the individual forces acting on an object combined. If the resultant force is zero, the object remains at rest or continues at constant velocity.

The rock will continue to drift at the same speed in the same direction indefinitely. With no resultant force, its velocity does not change — this is Newton's First Law.

False.

The Moon moves at constant speed but is always changing direction. Newton's First Law requires constant velocity (constant speed AND direction). The Moon is not an example of Newton's First Law.

Newton's Second Law states that the acceleration of an object is proportional to the resultant force acting on it and inversely proportional to its mass.

F = ma

F = ma

• F = resultant force in newtons (N)

• m = mass in kilograms (kg)

• a = acceleration in m/s²

False.

For a given force, a greater mass results in a smaller acceleration. Acceleration is inversely proportional to mass (F = ma, so a = F/m).

A 500 kg car experiences a resultant force of 2000 N. Its acceleration is 4 m/s².

(a = F/m = 2000/500 = 4 m/s²)

Inertial mass describes how difficult it is to change an object's velocity. It is defined as the ratio of force to acceleration (m = F/a). Objects with larger inertial mass require a greater force to produce the same acceleration.

a = F ÷ m

a = 300 ÷ 60

a = 5 m/s²

False.

Resultant force is a vector quantity. It has both magnitude and direction. A negative value for resultant force indicates the force acts in the opposite direction to the object's motion.

The trolley with the largest mass has the smallest acceleration. Since F = ma, for the same force, acceleration is inversely proportional to mass.

Required Practical 14 investigates: (1) the effect of varying force on the acceleration of an object of constant mass, and (2) the effect of varying mass on the acceleration produced by a constant force.

The mass of the trolley/car system must be kept constant. Any masses removed from the weight hanger must be transferred onto the trolley to keep the total mass constant.

False.

When investigating the effect of mass on acceleration (Experiment 2), the independent variable is mass and the force is the control variable (kept constant).

In RP14, a bench pulley and slotted masses is used to provide the force, and a stopwatch is used to time the trolley between measured intervals.

Pushing the trolley would give it an initial velocity, which would affect the measurement of acceleration. The trolley must be released from rest so that all acceleration comes from the weight of the hanging masses.

Repeat readings are taken for each timing measurement and an average time is calculated. This reduces the effect of random errors, particularly human reaction time errors when using the stopwatch.

True.

Using small masses means the trolley accelerates more slowly, giving more time to press the stopwatch at each interval. This makes the time measurements more accurate.

Newton's Third Law states: whenever two objects interact, the forces they exert on each other are equal in magnitude and opposite in direction. These force pairs are always the same type of force.

1. The two forces act on different objects

2. The forces are equal in size but act in opposite directions

3. The forces are the same type (e.g. both gravitational, both contact forces)

False.

This is an example of Newton's First Law (balanced forces on one object). For Newton's Third Law, the equal and opposite forces must act on two different objects and be the same type.

When a foot pushes the ground backwards, the ground pushes the foot forwards with an equal and opposite force. This is Newton's Third law.

By Newton's Third Law, the wall exerts a force of 50 N to the left on the person. The forces are equal in magnitude and opposite in direction.

False.

Newton's Third Law force pairs are always the same type of force. If one force is gravitational, the paired force is also gravitational. Weight and normal contact force are different types and are not a Newton's Third Law pair.

Newton's First Law describes forces acting on a single object that are balanced. Newton's Third Law describes forces acting on two different interacting objects that are equal and opposite.

Momentum (p) is defined by

where m = mass (kg) and v = velocity (m/s). Momentum is a vector quantity measured in kg m/s. An object at rest has zero momentum.

The principle of conservation of momentum states that in a closed system, the total momentum before an event equals the total momentum after the event. This applies to collisions and explosions.

p = 70 × 4

p = 280 kg m/s

False.

Momentum is a vector quantity because it depends on velocity, which has both magnitude and direction. An object moving in the opposite direction has negative momentum compared to one moving in the original direction.

Before a collision, a 990 kg car moves at 10 m/s and a 4200 kg van is at rest. The total momentum before is 9900 kg m/s.

(p = 990 × 10 + 0 = 9900 kg m/s)

A closed system means there are no external forces acting (e.g. no friction) and the energy within the system is constant.

True.

Since momentum is a vector, two objects of equal mass moving at the same speed in opposite directions have momenta that are equal and opposite, giving a total momentum of zero.

Kinetic energy (E_k) is the energy an object has due to its mass and speed. It is calculated using:

E_k = ½ mv²

where m = mass (kg) and v = speed (m/s). The unit is joules (J).

E_k = ½ mv²

• E_k = kinetic energy in joules (J)

• m = mass in kilograms (kg)

• v = speed in metres per second (m/s)

False.

Kinetic energy depends on speed squared (E_k = ½mv²). If speed doubles, kinetic energy increases by a factor of four (2² = 4).

A 1200 kg car moves at 27 m/s. Its kinetic energy is 437 400 J.

(E_k = ½ × 1200 × 27² = 0.5 × 1200 × 729 = 437 400 J)

When an object slows down, energy is transferred away from its kinetic store. The kinetic energy decreases as the speed decreases.

False.

An object at rest has a speed of zero, so its kinetic energy is zero (E_k = ½mv² = 0). Kinetic energy only exists when an object is moving.

E_k = ½mv²

100 = ½ × 2 × v²

100 = v²

v = 10 m/s

Stopping distance is the total distance a vehicle travels from when the driver reacts to an emergency to when the vehicle stops completely.

Stopping distance = thinking distance + braking distance

Thinking distance is the distance a vehicle travels during the driver's reaction time — from when the driver sees the hazard to when they apply the brakes. It equals speed × reaction time.

Three factors that increase thinking distance:

1. Tiredness (increases reaction time)

2. Distractions (e.g. using a mobile phone)

3. Intoxication (alcohol or drugs)

True.

For a given braking force, the greater the speed, the greater the stopping distance. Both thinking distance and braking distance increase with speed.

A car has a stopping distance of 40 m and a thinking distance of 14 m. The braking distance is 26 m.

(braking distance = 40 − 14 = 26 m)

Three factors that increase braking distance:

1. Adverse road conditions (wet or icy roads reduce friction)

2. Worn tyres (less grip, reduced friction)

3. Poor brake condition (reduced braking force)

True.

Thinking distance is directly proportional to speed (thinking distance = speed × reaction time). If speed doubles, thinking distance doubles. This produces a straight line through the origin on a thinking distance vs speed graph.

Braking distance is the distance a vehicle travels from when the brakes are applied until it stops. It is affected by speed, road conditions, tyre condition, and brake condition.

The braking force is the friction force applied by the brakes to a vehicle's wheels. It transfers energy from the vehicle's kinetic store to thermal stores (heating the brakes), reducing the vehicle's speed.

The temperature of the brakes increases because the braking force does work against friction. Energy is transferred from the vehicle's kinetic store to the thermal store of the brakes.

False.

The greater the speed of a vehicle, the greater the braking force required to stop it within a given distance. This is because the vehicle has greater kinetic energy that must be transferred.

Large decelerations can be dangerous because:

1. The brakes may overheat, making them less effective

2. The driver may lose control of the vehicle

The work done by the brakes equals the kinetic energy of the car, so: braking force × braking distance = ½ × mass × velocity².

braking force = ½mv² ÷ braking distance

= (0.5 × 1500 × 18²) ÷ 24

= 243 000 ÷ 24

≈ 10 000 N

False.

Braking distance is proportional to speed squared (from braking force × distance = ½mv²). If speed doubles, braking distance increases four times.