Question 1

What is the difference between speed and velocity?

Accepted Answer

Speed is a scalar quantity that measures how fast an object is moving, regardless of direction. Velocity is a vector quantity that includes both speed and direction. For example, a car travelling at 30 m/s north has a velocity of 30 m/s north; if it turns and goes south at the same speed, its velocity changes even though its speed remains constant. In calculations, you use speed for distance-time graphs and velocity for displacement-time graphs.

Question 2

How do I calculate resultant force when forces are at angles?

Accepted Answer

When forces act at angles, you can find the resultant force by drawing a scale vector diagram (tip-to-tail method) or by resolving each force into horizontal and vertical components. For the diagram method, draw each force arrow to scale, placing the tail of the next at the tip of the previous. The resultant is the arrow from the tail of the first to the tip of the last. Measure its length and angle. For components, use trigonometry: horizontal component = F cos θ, vertical component = F sin θ. Then add all horizontal components and all vertical components separately, and use Pythagoras to find the resultant magnitude: R = √(ΣFx² + ΣFy²).

Question 3

What is the conservation of momentum and how do I apply it?

Accepted Answer

The principle of conservation of momentum states that in a closed system (no external forces), the total momentum before an event (like a collision or explosion) equals the total momentum after. To apply it, write: total momentum before = total momentum after. For two objects colliding, this is m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂, where u is initial velocity and v is final velocity. Remember that momentum is a vector, so direction matters – assign positive and negative signs. For explosions, the total momentum before is zero, so after the explosion, the momenta of the fragments must sum to zero (e.g., a gun recoiling backwards as the bullet moves forward).

Question 4

How do I find the spring constant from a force-extension graph?

Accepted Answer

The spring constant k is the gradient of the straight-line portion of the force-extension graph (where Hooke's law applies). Choose two points on the line, calculate change in force (ΔF) divided by change in extension (Δx): k = ΔF/Δx. The units are N/m. If the graph is a curve beyond the limit of proportionality, the spring constant is not constant. Also note that the gradient gives the stiffness – a steeper line means a larger spring constant (stiffer spring).

Question 5

What is Newton's third law and can you give an example?

Accepted Answer

Newton's third law states that whenever two objects interact, they exert equal and opposite forces on each other. These forces are the same type (e.g., both gravitational or both contact) and act simultaneously. For example, when you sit on a chair, your body exerts a downward force on the chair (due to weight), and the chair exerts an equal upward force on you (the normal reaction). These forces act on different objects: the downward force acts on the chair, the upward force acts on you. They do not cancel because they are on different bodies.

Question 6

How do I calculate acceleration from a velocity-time graph?

Accepted Answer

Acceleration is the gradient (slope) of a velocity-time graph. To calculate it, choose two points on the straight-line segment, find the change in velocity (Δv) and the change in time (Δt), then use a = Δv/Δt. If the line is horizontal, acceleration is zero (constant velocity). If the line slopes upward, acceleration is positive; if downward, negative (deceleration). For curved sections, you would need to draw a tangent to find instantaneous acceleration, but at GCSE you usually deal with straight lines.

Topic 1 – Key concepts of physics

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Related Topics in EDEXCEL GCSE Physics

Topic 10 – Electricity and circuits

Topic 11 – Static electricity

Topic 12 – Magnetism and the motor effect

Topic 13 – Electromagnetic induction

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