Forces Flashcards
All 22 cards in this deck
State Newton's first law.
An object stays at rest, or keeps moving at constant velocity, unless a resultant force acts on it.
True or false? An object moving at a constant velocity must have a resultant force acting on it.
False. Constant velocity means the forces are balanced, so the resultant force is zero.
What happens to a body when the resultant force acting on it is not zero?
Its speed and/or its direction changes (it accelerates).
How do you find the resultant of two forces acting in opposite directions along the same line?
Subtract the smaller force from the larger one. The resultant acts in the direction of the larger force.
State Newton's second law as an equation, with units.
force (N) = mass (kg) × acceleration (m/s²), i.e. F = m × a
A rocket's engines produce a constant force at take-off. What happens to its acceleration during the first few seconds, and why?
Acceleration increases, because mass decreases as fuel is burned and F = ma.
In the force and acceleration core practical, how is the trolley's acceleration measured?
Using light gates and an interrupter card on the trolley, as weights on a hanger over a pulley accelerate it.
In the force and acceleration core practical, what is transferred between the trolley and the weight hanger, and why?
Mass (weights) — the mass removed from the trolley is added to the hanger, so the total mass of the moving system stays constant.
True or false? Sloping the runway in the force and acceleration core practical removes friction from the experiment.
False. It compensates for the effects of friction, so the trolley rolls at constant speed with no weights attached.
Define weight.
The force acting on an object due to gravity (due to the gravitational field it is in), measured in newtons.
State the equation linking weight, mass and gravitational field strength, with units.
weight (N) = mass (kg) × gravitational field strength (N/kg), i.e. W = m × g
How is the weight of an object measured?
With a newtonmeter (a calibrated spring balance).
True or false? An astronaut's mass is smaller on the Moon than on Earth.
False. Mass is the same everywhere; only the weight is smaller, because weight is proportional to gravitational field strength, which is smaller on the Moon.
An object moves in a circular orbit at constant speed. What is happening to its velocity?
Its velocity is continuously changing, because its direction is continuously changing.
What is the centripetal force?
The resultant force that keeps an object moving in a circular path; it acts towards the centre of the circle.
What provides the centripetal force on a ball being whirled in a horizontal circle on a string?
The tension in the string, acting towards the centre of the circle.
State Newton's third law.
When two objects interact, they exert equal and opposite forces on each other.
Give the four features of a Newton's third law pair of forces.
Equal in size, opposite in direction, the same type of force, and acting on different bodies.
True or false? The weight of a book resting on a table and the normal contact force of the table on the book are a Newton's third law pair.
False. They act on the same body (the book), so they are balanced forces, not a third law pair.
A book rests on a table. Name the Newton's third law partner of the force the table exerts on the book.
The force the book exerts on the table — equal in size, opposite in direction and the same type of force.
For an object of fixed mass, how is its acceleration related to the resultant force on it?
Acceleration is directly proportional to the resultant force, so doubling the force doubles the acceleration.
For a fixed resultant force, how is an object's acceleration related to its mass?
Acceleration is inversely proportional to mass, so a larger mass gives a smaller acceleration.