Forces & Motion
Newton's Laws of Motion
1st Year · 2nd Year · 3rd Year (Junior Cert)
- ✓By the end of this lesson students will be able to state and explain Newton's First Law of Motion (the Law of Inertia).
- ✓By the end of this lesson students will be able to state Newton's Second Law of Motion (F = ma) and apply it to solve simple problems.
- ✓By the end of this lesson students will be able to state and explain Newton's Third Law of Motion (action-reaction).
- ✓By the end of this lesson students will be able to identify action-reaction pairs of forces.
Key concepts
Newton's First Law states that an object will remain at rest or continue to move at a constant velocity (constant speed in a straight line) unless acted upon by an unbalanced external force. This property of objects to resist changes in their state of motion is called inertia. If the forces acting on an object are balanced, its acceleration is zero.
Newton's Second Law states that the acceleration of an object is directly proportional to the net (unbalanced) force acting on it and inversely proportional to its mass. This means a larger force produces a larger acceleration, and a larger mass results in a smaller acceleration for the same force. The direction of the acceleration is the same as the direction of the net force. The unit of force is the Newton (N), where 1 N = 1 kg m/s².
Newton's Third Law states that for every action, there is an equal and opposite reaction. This means that whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object. These forces are always equal in magnitude and opposite in direction, and they always act on different objects.
Key facts to remember
- 1Newton's First Law describes inertia: objects resist changes in their motion.
- 2An object at rest stays at rest, and an object in motion stays in motion with the same speed and in the same direction, unless acted upon by an unbalanced force.
- 3Newton's Second Law is expressed by the formula F = ma.
- 4Force is measured in Newtons (N), mass in kilograms (kg), and acceleration in metres per second squared (m/s²).
- 5Newton's Third Law states that forces always occur in equal and opposite pairs, acting on different objects.
- 6The 'net force' is the overall unbalanced force acting on an object.
Worked examples
Example 1
A net force of 20 N is applied to a trolley with a mass of 5 kg. Calculate the acceleration of the trolley.
Answer
The acceleration of the trolley is 4 m/s².
Remember to include the correct units in your final answer.
Example 2
A car accelerates at 3 m/s² when a net force of 4500 N is applied to it. What is the mass of the car?
Answer
The mass of the car is 1500 kg.
Example 3
Identify the action-reaction pair when a person pushes against a wall.
Answer
Action: Person pushes on the wall. Reaction: Wall pushes on the person.
These two forces act on different objects (the wall and the person).
Common mistakes
- ✗Confusing mass and weight: Mass is a measure of inertia (amount of matter), while weight is the force of gravity acting on an object (W = mg).
- ✗Forgetting to use the correct units (e.g., using grams instead of kilograms for mass in F=ma calculations).
- ✗Incorrectly identifying action-reaction pairs by having them act on the same object.
- ✗Assuming that if an object is moving, there must be an unbalanced force acting on it (Newton's First Law states it can move at constant velocity with balanced forces).
Exam tips
- ★Always show all steps in your calculations, even if they seem simple. This allows for partial marks if your final answer is incorrect.
- ★Pay close attention to units. Ensure all values are in SI units (kg, m, s) before using them in formulae.
- ★Clearly state the formula you are using before substituting values.
- ★For questions involving Newton's Third Law, remember that action and reaction forces always act on different objects.
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