Force & acceleration
- Newton's second law of motion tells us that objects will accelerate if there is a resultant force acting upon them
- This acceleration will be in the same direction as this resultant force
- Where:
- F = force in newtons (N)
- m = mass in kilograms (kg)
- a = acceleration in metres per second squared (m s-2)
Resultant force
- Since force is a vector, every force on a body has a magnitude and direction
- The resultant force is therefore the vector sum of all the forces acting on the body
- The direction of the force is indicated as either positive or negative
Resultant forces on a body
Resultant forces on a body are indicated by a positive or negative value
- The resultant force could also be at an angle, in which case vector addition is used to find the magnitude and direction of the resultant force.
- More details on vector addition can be found in Scalars & Vectors
Acceleration
- Newton’s second law can be used to find the acceleration of an object of a known mass
- Since acceleration is also a vector, it can be either positive or negative depending on the direction of the resultant force
- An object will speed up (positive acceleration) if the resultant force acts in the same direction as the direction of motion
- An object will slow down (negative acceleration) if the resultant force acts in the opposite direction to the direction of motion
- The acceleration will always be in the same direction as the resultant force
Worked example
A rocket produces an upward thrust of 15 MN and has a weight of 8 MN.
Answer:
(a)
Step 1: Sketch a diagram of the forces acting on the rocket
Step 2: Convert all the forces into SI units (newtons)
- Upward acting force (positive direction):
- Thrust = 15 MN =
- Downward acting forces (negative direction):
- Weight = 8 MN =
- Air resistance = 500 kN =
Step 3: Calculate the resultant force acting on the rocket
(b)
Step 1: List the known quantities
- Force, F = 6.5×106 N
- Mass, m = 0.8 × 105 kg
Step 2: State the equation for Newton's Second Law of motion
Step 3: Rearrange to make acceleration the subject
Step 4: Substitute in the known values and calculate the acceleration
Step 5: State the direction of the acceleration
- Remember that acceleration is always in the same direction as the resultant force
Examiner Tip
You can choose which direction is positive as long as you are consistent throughout your calculation. However, the general convention is for the direction of motion to be the positive direction.