Forces Flashcards
All 26 cards in this deck
State Newton's first law.
A body stays at rest or moves at constant velocity unless acted on by a resultant force.
What is the resultant force on a body moving at constant velocity?
Zero — the forces on it are balanced.
What happens to a body when the resultant force on it is not zero?
Its speed and/or direction changes (it accelerates).
True or false? A box at rest on a floor has its weight balanced by an equal upward force from the floor.
True. The two forces are equal in size and opposite in direction, so the resultant force is zero.
State Newton's second law as an equation, with units.
force (N) = mass (kg) × acceleration (m/s²),
What is the unit of force, and how is it written?
The newton, N. ()
True or false? In the acceleration must be squared before multiplying.
False. Only the unit has the square (m/s²); you multiply mass by acceleration as given.
In the core practical on force and acceleration, what is varied to change the accelerating force on the trolley?
The number of masses hanging from the string over the pulley.
What equipment is used to measure the acceleration of the trolley in the force and acceleration core practical?
Light gates connected to a data logger (with a card on the trolley).
In the force and acceleration core practical, what must be kept the same when investigating how force affects acceleration?
The total mass being accelerated (and the same trolley and runway).
What relationship between force and acceleration should the core practical results show, for a constant mass?
Acceleration is directly proportional to the force (double the force, double the acceleration).
Define weight.
The force acting on an object due to gravity (mass × gravitational field strength), measured in newtons.
In which direction does the weight of an object act?
Vertically downwards.
How is weight measured?
With a calibrated spring balance (newtonmeter), which reads the force in newtons.
What is the equation for weight?
weight (N) = mass (kg) × gravitational field strength (N/kg)
True or false? An astronaut's mass changes when they travel to the Moon.
False. Mass stays the same; only the weight changes because the gravitational field strength is different.
What is the relationship between the weight of a body and the gravitational field strength?
Weight is directly proportional to the gravitational field strength (for a fixed mass).
State Newton's third law.
For every action there is an equal and opposite reaction — the forces act on two different objects.
In a Newton's third law force pair, how do the two forces compare?
Equal in size, opposite in direction, the same type of force, and acting on two different objects.
A book rests on a table. What is the Newton's third law partner of the force the table exerts on the book?
The force the book exerts downwards on the table, equal in size and opposite in direction.
What is mass, and what unit is it measured in?
The amount of matter in an object, measured in kilograms (kg).
A book rests on a table. Are the book's weight and the normal contact force from the table a Newton's third law pair?
No. Both forces act on the book, and one is gravitational while the other is a contact force. A third-law pair acts on two different objects and is the same type of force.
What does it mean for an object to be in equilibrium?
There is no resultant force on it, so it stays at rest or keeps moving at a constant velocity.
In the force and acceleration core practical, what happens to the acceleration when the total mass is increased but the force is kept the same?
The acceleration decreases.
What is a resultant force?
One force that could replace all the forces acting on an object and change its motion in exactly the same way.
How do you find the resultant of forces acting along a straight line?
Choose one direction as positive, count forces in the opposite direction as negative, then add them all together.