How do forces change the motion of an object, and what is the link between force, mass and acceleration?
Newton's laws: balanced and unbalanced forces, Newton's first and second laws including F equals ma, the difference between mass and weight, and friction and free-body force diagrams.
An SQA National 5 Physics answer on Newton's laws, covering balanced and unbalanced forces, Newton's first and second laws including F equals ma, the difference between mass and weight using W equals mg, terminal velocity, and how to find a resultant force from a free-body diagram.
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What this key area is asking
The SQA wants you to explain balanced and unbalanced forces, state and use Newton's first and second laws (including ), find a resultant force from a force diagram, and tell the difference between mass and weight using .
Balanced forces and Newton's first law
This is often tested with terminal velocity. A skydiver speeds up at first because weight is bigger than air resistance, but air resistance grows with speed until it balances weight. At that point the forces are balanced, the resultant is zero, and the skydiver falls at a constant terminal velocity.
Unbalanced forces and Newton's second law
A larger force gives a larger acceleration for the same mass, and a larger mass gives a smaller acceleration for the same force. To use you must first find the resultant force by combining all the forces, taking one direction as positive.
Mass and weight
The gravitational field strength has the same value as the acceleration due to gravity, , because for a freely falling object the only force is weight, so becomes , giving .
Friction and Newton's third law
Friction is a contact force that always acts to oppose the motion (or tendency to move) between two surfaces. It can be useful (grip for tyres and shoes) or a nuisance (wasting energy as heat). The SQA also expects you to know Newton's third law in its simple form: for every action force there is an equal and opposite reaction force. When you push on a wall, the wall pushes back on you with an equal force.
Try this
Q1. State Newton's first law. [1 mark]
- Cue. If the forces on an object are balanced it stays at rest or moves at constant velocity in a straight line.
Q2. A box of mass experiences a resultant force of . Calculate its acceleration. [2 marks]
- Cue. .
Q3. Calculate the weight of a person on Earth, taking . [2 marks]
- Cue. .
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA N5 style3 marksA car of mass 1200 kg accelerates at 2.5 m s per second per second. Calculate the unbalanced force acting on the car.Show worked answer →
Use Newton's second law, which links unbalanced force, mass and acceleration.
Relationship: .
Substitution: .
Markers reward selecting , correct substitution of the mass and acceleration, and a final answer with the unit newton ().
SQA N5 style3 marksAn astronaut has a mass of 80 kg. Calculate the astronaut's weight on Earth (g = 9.8 N per kg) and on the Moon (g = 1.6 N per kg).Show worked answer →
Use the relationship between weight, mass and gravitational field strength.
Relationship: .
On Earth: .
On the Moon: .
The mass stays at everywhere; only the weight changes because is different. Markers reward , both substitutions, and the units in newtons.
Related dot points
- Vectors and scalars: distinguishing the two kinds of quantity, the difference between distance and displacement and between speed and velocity, and combining vectors that act at right angles.
An SQA National 5 Physics answer on vectors and scalars, covering which quantities are scalar and which are vector, the difference between distance and displacement and between speed and velocity, and how to combine two vectors that act at right angles using a scale diagram or Pythagoras.
- Velocity and acceleration: defining and calculating acceleration, and interpreting velocity-time graphs to describe motion and to find acceleration and distance travelled.
An SQA National 5 Physics answer on velocity and acceleration, covering the definition and calculation of acceleration, how it is measured with light gates, and how to read a velocity-time graph to describe the motion, find the acceleration from the gradient and find the distance from the area under the line.
- Energy: work done by a force, gravitational potential energy and kinetic energy, the conservation of energy, and using energy changes to solve motion problems such as a falling or braking object.
An SQA National 5 Physics answer on energy in dynamics, covering work done by a force, gravitational potential energy and kinetic energy, the conservation of energy, and how to combine these to solve problems such as a falling object or a braking car.
- Projectile motion: treating a projectile as separate horizontal (constant velocity) and vertical (constant acceleration) motions, and using these to find the range, time of flight and impact velocity.
An SQA National 5 Physics answer on projectile motion, covering why a projectile is treated as separate horizontal and vertical motions, that the horizontal velocity is constant while the vertical motion accelerates under gravity, and how to find the range, time of flight and impact speed of a horizontally launched projectile.
Sources & how we know this
- SQA National 5 Physics Course Specification — SQA (2019)