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

The resultant force (or net force) is a single force that has the same effect as all the individual forces acting on an object combined. It determines the direction and magnitude of the object’s overall acceleration.

• Forces in the same direction: add them together

• Forces in opposite directions: subtract the smaller from the larger

• The direction of the resultant is the same as the larger force

True.

When forces cancel each other out completely, the resultant force is zero and the forces are balanced. The object will remain stationary or continue at constant velocity.

A box has 14 N to the left, 4 N to the right, and 8 N to the right. The total force to the right is 12 N. The resultant force is 2 N to the left.

Resultant force = 100 − 80 = 20 N to the right

The forces are unbalanced; there is a net force in the direction of the larger force.

False.

The weight of the book and the normal contact force from the table are equal in magnitude and opposite in direction, so they cancel out. The resultant force is zero and the forces are balanced.

A resultant force must always be stated with both its:

1. Magnitude (the size of the force, in newtons)

2. Direction (e.g. to the left, to the right, upwards, downwards)

A free body diagram models the forces acting on an object. Each force is shown as a vector arrow scaled to its magnitude, pointing in the correct direction, and labelled with the name of the force.

The normal contact force is the force that arises when one object rests against another. It acts at 90° (perpendicular) to the surface of contact. It is labelled N or R in free body diagrams.

1. Weight: downwards

2. Normal contact force: upwards

3. Thrust (driving force): forwards (larger arrow)

4. Friction: backwards (smaller arrow)

The car accelerates, so thrust must be larger than friction.

True.

In a free body diagram, vector arrows representing forces always start at the object and are directed away from it, showing the direction in which each force acts.

Upthrust is the upward buoyancy force acting on an object when it is in a fluid (liquid or gas). It always acts upwards and is labelled U in free body diagrams.

The upthrust = 30 N upwards.

The boat is stationary, so the resultant force must be zero. The upthrust must therefore be equal and opposite to the weight.

In a free body diagram, arrows must be scaled to the magnitude of the force they represent. When forces are balanced, arrows in opposite directions must be the same size.

False.

Friction always acts opposite to the direction of motion. It is a resistive force that acts at the point where two surfaces are in contact.

Resolving a force means splitting a single force into two components that act at right angles to each other (horizontal and vertical). The two components together have the same effect as the original single force.

A force is typically resolved into:

1. A horizontal component

2. A vertical component

These two components, acting together, have the same effect as the original force.

True.

If the force arrows form a closed loop (head to tail), the forces are in equilibrium: there is no resultant force and the object is balanced. An open loop means there is an unbalanced resultant force.

The resultant force (also called net force or unbalanced force) is the single force that has the same effect as all the individual forces acting on an object combined. It is found by adding force vectors head to tail.

1. Draw the first force vector to scale

2. Draw the second force vector head to tail with the first (starting where the first ends)

3. Draw the resultant vector from the tail of the first to the head of the second

4. Measure the resultant’s length and apply the scale to find its magnitude

In the parallelogram method, the two force vectors are drawn tail to tail, the parallelogram is completed, and the diagonal of the parallelogram represents the resultant force.

1. Choose a suitable scale (e.g. 1 cm = 1 kN)

2. Draw the 7.0 kN force (7 cm) and the 5.0 kN force (5 cm) head to tail at right angles

3. Draw the resultant from the start of the first to the end of the second

4. Measure the resultant and apply the scale to find its magnitude (e.g. 8.6 cm → 8.6 kN)

Work done is the energy transferred when a force causes an object to move through a distance in the direction of the force. It is measured in joules (J).

Work done is measured in joules (J). One joule is the work done when a force of one newton moves an object a distance of one metre in the direction of the force.

False.

Work is only done when a force causes an object to move through a distance. If the object does not move, no work is done.

The equation for calculating work done is:

work done = force x distance

A joule (J) is the unit of work done and energy. One joule is the work done when a force of 1 N moves an object a distance of 1 m in the direction of the force.

Using W = F x s:

W = 200 x 5 = 1000 J

False.

Work done is measured in joules (J). Newtons (N) are the unit of force, not work done.

The person applies an upward force on the box and moves it through a distance (the height lifted). Since the force causes a displacement, work is done on the box against gravity.

Weight is the force acting on an object due to gravitational attraction. It is measured in newtons (N) and is a vector quantity.

Mass is the amount of matter in an object, measured in kilograms (kg), and does not change with location. Weight is the gravitational force on an object, measured in newtons (N), and changes depending on the gravitational field strength.

False.

Weight is a vector quantity because it has both magnitude and direction. The direction of weight is always towards the centre of the gravitational field (downwards on Earth).

The equation for calculating weight is:

weight = mass x gravitational field strength

Gravitational field strength (g) is the force per unit mass exerted on an object in a gravitational field, measured in N/kg. On Earth, g is approximately 9.8 N/kg.

Weight is measured directly using a calibrated spring balance, also known as a newton-meter. It measures force in newtons by stretching a spring.

False.

Mass is a measure of the amount of matter in an object and stays the same everywhere. Only weight changes because the Moon has a weaker gravitational field strength (about 1.6 N/kg compared to 9.8 N/kg on Earth).

Using W = m x g:

W = 60 x 9.8 = 588 N

Gravitational potential energy is the energy an object has due to its height in a gravitational field. The higher an object is raised, the more energy is stored in its gravitational potential energy store.

When an object is lifted, energy is transferred to its gravitational potential energy store. The amount of energy transferred depends on the object's mass, the gravitational field strength, and the height it is raised.

False.

Gravitational potential energy is measured in joules (J). Watts (W) are the unit of power, not energy.

The equation for gravitational potential energy is:

E_p = mass x g x h

Gravitational field strength (g) is the force per unit mass in a gravitational field, measured in N/kg. On Earth, g = 9.8 N/kg. On the Moon, g is approximately 1.6 N/kg.

Using E_p = m x g x h:

E_p = 5 x 9.8 x 3 = 147 J

True.

The gravitational field strength on the Moon (about 1.6 N/kg) is much less than on Earth (9.8 N/kg). This means the weight of any mass is lower on the Moon, so less force is needed to lift it.

When an object falls, energy is transferred away from its gravitational potential energy store to its kinetic energy store (and some to the thermal energy store of the surroundings due to air resistance).

Elastic deformation occurs when an object returns to its original shape and size after the deforming force is removed. Examples include rubber bands, steel springs, and fabrics.

Inelastic deformation occurs when an object does not return to its original shape after the deforming force is removed — it remains permanently distorted. Examples include plastic and clay.

False.

"Elastic" means the object returns to its original shape when the deforming force is removed. It does not mean the object can stretch to a large extension.

If only one force acts on a stationary object, it would accelerate rather than change shape. A second force in the opposite direction is needed to prevent overall movement and allow the object to be stretched, bent, or compressed.

When a spring is compressed, the two forces act towards each other. When a spring is stretched, the two forces act away from each other.

Elastic deformation: rubber band (or steel spring / fabric)

Inelastic deformation: plastic (or clay / glass)

False.

Bending requires two forces that act at different points on the object (or at an angle to each other), not at the same point. This causes the object to distort and bend.

In elastic deformation, the object returns completely to its original shape when the force is removed. In inelastic deformation, the object remains permanently distorted even after the force is removed.

Hooke's Law states that the extension of an elastic object is directly proportional to the force applied, provided the limit of proportionality is not exceeded.

The spring constant (k) is a measure of the stiffness of a spring. A higher spring constant means a stiffer spring that requires more force per unit extension. The unit is newtons per metre (N/m).

F = k × e

• F = force in newtons (N)

• k = spring constant in newtons per metre (N/m)

• e = extension (or compression) in metres (m)

Extension = final length − original length.

True.

When force (F) is on the y-axis, the spring constant is the gradient. A steeper line means a larger spring constant, indicating a stiffer spring.

1. Extension e = 40 − 33 = 7 cm = 0.07 m

2. F = k × e = 4900 × 0.07 = 343 N

The limit of proportionality is the point beyond which the relationship between force and extension is no longer directly proportional. Above this point, Hooke's Law no longer applies.

The spring constant equals the gradient of the straight-line (Hooke's law) region of the graph:

k = ΔF ÷ Δe

Only the linear region below the limit of proportionality should be used.

A pointer (also called a fiducial marker) is used in the Hooke's Law experiment to give a precise, consistent reference point for reading the ruler, reducing random errors in the readings.

• Independent variable: force, F (varied by changing the number of masses)

• Dependent variable: extension, e (measured from the ruler)

• Control variable: spring constant, k (same spring used throughout)

Extension = final length − original length

The original length is the spring length with no masses; the final length is measured from the bottom of the spring once masses are added.

In the spring experiment, the force applied to the spring is the weight of the masses, calculated using the equation W = mg.

True.

Taking measurements at eye level ensures the line of sight is perpendicular to the ruler, preventing parallax error — a systematic error that would cause all readings to be consistently too high or too low.

A linear region (straight line) shows that force is directly proportional to extension, meaning the spring is obeying Hooke's Law for those values of force and extension.

Any two from:

1. Wear goggles in case the spring snaps

2. Stand up so feet are not under the masses

3. Place a mat below the masses in case they fall

4. Use a G clamp to secure the clamp stand to the desk

False.

Extension is always measured as the total increase from the original length (no masses) — not the incremental change between successive readings.

Elastic potential energy is the energy stored in an elastic object when work is done on it by stretching or compressing it. It is stored in the elastic potential energy store and measured in joules (J).

E_e = ½ × k × e²

• E_e = elastic potential energy in joules (J)

• k = spring constant in newtons per metre (N/m)

• e = extension in metres (m)

False.

Because elastic potential energy depends on e², doubling the extension quadruples the energy stored (2² = 4). The relationship is non-linear.

1. Extension e = 11.4 − 10.0 = 1.4 cm = 0.014 m

2. E_e = ½ × 250 × (0.014)² = 0.0245 J ≈ 0.025 J

When a spring is stretched, work is transferred to the spring's elastic potential energy store. When the spring is released, energy is transferred away from its elastic potential energy store.

True.

As long as the spring is not inelastically deformed (i.e. has not exceeded its limit of proportionality), all the work done on the spring is stored as elastic potential energy.

1. e = 4.5 cm = 0.045 m

2. Rearrange E_e = ½ke²: k = 2E_e ÷ e²

3. k = (2 × 0.2) ÷ (0.045)² = 198 N/m