IB MYP 4-5 Physics- Newton’s laws of motion - Study Notes - New Syllabus
IB MYP 4-5 Physics-Newton’s laws of motion – Study Notes
Key Concepts
- Newton’s laws of motion
Newton’s Laws of Motion
Newton’s First Law of Motion
The first law states:
An object will remain at rest, or move in a straight line at constant velocity, unless acted upon by a net external force.
- This is also called the Law of Inertia.
- Inertia is the tendency of an object to resist changes in its state of motion.
- If no net force acts, the object’s velocity (speed and direction) remains unchanged.
- In everyday life, forces like friction and air resistance usually prevent objects from moving forever without force.
Mathematical Form
When \( F_{\text{net}} = 0 \), acceleration \( a = 0 \), so velocity is constant.
Example:
A hockey puck slides on frictionless ice with a velocity of \( 5 \, \text{m/s} \). What happens if no force acts on it?
▶️ Answer/Explanation
The puck will keep moving at \( 5 \, \text{m/s} \) in the same direction indefinitely, because no net force acts to change its motion.
Example:
A book rests on a table. Why does it remain at rest?
▶️ Answer/Explanation
The book is at rest because the net force is zero: the upward normal force from the table balances the downward gravitational force. No unbalanced force acts to change its motion.
Newton’s Second Law of Motion
The second law states:
Force is equal to the rate of change of momentum. For a constant mass, force equals mass times acceleration.
Formula
\( F_{\text{net}} = m a \)
- \( F_{\text{net}} \) is the vector sum of all forces acting on the object.
- \( m \) is the mass of the object (kg).
- \( a \) is the acceleration (m/s\(^2\)).
- The acceleration is in the same direction as the net force.
Example:
A 10 kg box is pushed with a net force of \( 50 \, \text{N} \). Find its acceleration.
▶️ Answer/Explanation
Using \( a = \dfrac{F_{\text{net}}}{m} \):
\( a = \dfrac{50}{10} = 5 \, \text{m/s}^2 \)
Final Answer: \(\boxed{5 \, \text{m/s}^2}\)
Example:
A 1,200 kg car accelerates at \( 3 \, \text{m/s}^2 \). Find the net force acting on it.
▶️ Answer/Explanation
Using \( F_{\text{net}} = m a \):
\( F_{\text{net}} = 1200 \times 3 = 3600 \, \text{N} \)
Final Answer: \(\boxed{3600 \, \text{N}}\)
Newton’s Third Law of Motion
The third law states:
For every action, there is an equal and opposite reaction.
- Forces always come in pairs: action and reaction.
- The forces are equal in magnitude and opposite in direction.
- They act on different objects, not on the same object.
Example:
When you jump off a boat, the boat moves backward. Why?
▶️ Answer/Explanation
Your feet push the boat backward (action), and the boat pushes you forward (reaction). The two forces are equal and opposite.
Example:
A rocket launches upward. Explain in terms of the third law.
▶️ Answer/Explanation
The rocket expels gases downward at high speed (action), and the gases push the rocket upward with an equal and opposite force (reaction).