Newton's First Law
Newton's Laws & Momentum - OCR A-Level Physics
Key Definition
Newton's First Law
An object remains at rest or continues to move at constant velocity unless acted upon by a resultant external force.
An object remains at rest or continues to move at constant velocity unless acted upon by a resultant external force.
$$\Delta p = 0 \quad [\text{kg m s}^{-1}]$$
- Derivation of zero change in momentum:
$$\Delta p = p_2 - p_1$$
$$\Delta p = mv - mu$$
- As $v$ is constant, $v = u$ $\therefore$
$$\Delta p = mu - mu = 0$$
- If the direction and magnitude of velocity hasn't changed, there is no change of momentum
- First law defines inertiaThe tendency of an object to resist changes in its state of motion. Greater mass means greater inertia.: the tendency of an object to resist changes in its motion
Key Definition
Newton's Second Law
The resultant force acting on an object is equal to the rate of change of its momentum.
The resultant force acting on an object is equal to the rate of change of its momentum.
$$\Sigma F = F_1 + F_2 + \ldots$$
$$\Sigma F = \frac{\Delta p}{\Delta t}$$
- For a constant mass this becomes:
$$\Sigma F = ma$$
- 1 newton is the force that gives a 1 kg mass an accelerationThe rate of change of velocity. A vector quantity. Measured in m s⁻². of 1 $\text{m s}^{-2}$
- The direction of accelerationThe rate of change of velocity. A vector quantity. Measured in m s⁻². is always in the direction of the resultant force
Examiner Tips and Tricks
- The total sum of forces is equal to $\Sigma F = ma$. Many students forget this point: to find the total acceleration of a system you need the total forces and total mass.
- When a question asks you to 'show that the acceleration is...' using $F = ma$, always show the calculation of the resultant force first. Examiners need to see that you found the net force, not just divided a single force by mass.
Key Definition
Newton's Third Law
Object A exerts a force on Object B. Object B exerts a force back on Object A that is:
Object A exerts a force on Object B. Object B exerts a force back on Object A that is:
- Equal in magnitude
- Opposite in direction
- Same type
- Same line of action
- Acting on 2 different bodies
- Acts instantaneously
Examiner Tips and Tricks
- Many students forget to think about the same type of force, e.g. contact and non-contact. For Newton's Third Law to be true, they must both be contact or both non-contact.
Common Mistake
MEDIUM
Students often: Identify weight and normal contact force as a Newton's Third Law pair.
Instead: They act on the SAME object (the book), so they cannot be a third-law pair. The third-law pair of the book's weight is the gravitational pull of the book on the Earth. The third-law pair of the normal force from the table is the normal force from the book on the table.
Instead: They act on the SAME object (the book), so they cannot be a third-law pair. The third-law pair of the book's weight is the gravitational pull of the book on the Earth. The third-law pair of the normal force from the table is the normal force from the book on the table.