A body will remain at rest or move with constant velocity unless acted on by a resultant force
If there are no external forces acting on the car, other than friction, and it is moving at a constant velocity, what is the value of the frictional force F?
Step 1: Identify the prompt (or 'clue') word in the question to choose which rules to apply
Step 2: Calculate the missing force
ΣF = 0
F = D
Step 3: Write the answer including units
F = 6 k N
The acceleration of an object with constant mass is directly proportional to the resultant force on it
ΣF = ma
An object with a mass of 750 g accelerates in a straight line at 11 m s−2.
Determine the resultant force acting on the object.
Step 1: Write down the known quantities
Step 2: Identify the equation needed and substitute in the values
ΣF = ma = 0.75 × 11 = 8.25
Step 3: Write the final answer using the correct significant figures and units
ΣF = 8.3 N
Suggest two ways in which a designer could increase the maximum velocity of a car.
Step 1: Identify exactly what the question is asking for
Step 2: Write down the forces affecting the motion
Step 3: Suggest changes the designer could make
The direction you consider positive is your choice, as long as the signs of the numbers (positive or negative) are consistent throughout your answer. A clear, labelled diagram will tell both you and the Examiner which direction is positive, so nothing gets mixed up.
Having said that, there is a general rule to consider the direction the object is initially travelling in as positive. Therefore all vectors in the direction of motion will be positive and opposing vectors, such as drag forces, will be negative.
If you do get an acceleration in your answer with a negative sign, you have found that the object is slowing down. This is the Super Power of Physics equations!
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