Forces (Edexcel GCSE Physics): Flashcards
Exam code: 1PH0
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- FrontNewton's First Law
State Newton's first law of motion.
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Cards in this collection (38)
State Newton's first law of motion.
Newton's first law states that an object will remain at rest, or move with a constant velocity, unless a resultant force acts on it.
What two conditions must be true for an object to have a constant velocity?
No change in speed and no change in direction.
An object's velocity can only change if a ______ force acts on it.
An object's velocity can only change if a resultant force acts on it.
A car travels at a constant velocity. The driving force from the engine is 3 kN.
What is the size of the frictional force acting on the car?
3 kN. Since the car moves at a constant velocity, the resultant force is zero, so the driving and frictional forces are balanced.
True or False?
The Moon orbiting the Earth at a constant speed is an example of Newton's first law.
False.
The Moon's direction is always changing, so its velocity is not constant, even though its speed is, so it is not an example of Newton's first law.
State Newton's second law of motion.
Newton's second law states that the acceleration of an object is directly proportional to the resultant force acting on it and inversely proportional to its mass.
For a constant mass, how does increasing the resultant force on an object affect its acceleration?
The acceleration increases – acceleration is directly proportional to the resultant force.
For a constant force, how does increasing an object's mass affect its acceleration?
The acceleration decreases – acceleration is inversely proportional to mass.
Newton's second law is expressed by the equation ______ = m × a.
Newton's second law is expressed by the equation F = m × a.
A car of mass 900 kg accelerates from 0 to 27 m/s in 3 s.
Calculate the resultant force needed to produce this acceleration.
Acceleration = 27 ÷ 3 = 9 m/s²
Force = m × a = 900 × 9 = 8100 N
What are the SI units of force, mass and acceleration in the equation F = ma?
Force in newtons (N), mass in kilograms (kg), acceleration in metres per second squared (m/s²).
True or False?
If the resultant force on an object doubles while its mass stays the same, its acceleration will halve.
False.
Acceleration is directly proportional to force, so doubling the force doubles the acceleration for a constant mass.
In this investigation, what is the independent variable and what is the control variable in Experiment 1 (varying force)?
Independent variable: force. Control variable: mass (kept constant).
In this investigation, what is the independent variable and what is the control variable in Experiment 2 (varying mass)?
Independent variable: mass. Control variable: force (kept constant).
In Experiment 1, why must any weights removed from the weight hanger be transferred onto the toy car or trolley?
To keep the total mass of the system constant throughout the experiment, avoiding a systematic error.
How is the average speed of the trolley calculated between two measured intervals?
Average speed = distance ÷ time, using the distance between the intervals and the average time taken to travel it.
The toy car or trolley should be ______, not pushed, at the start of the experiment, to avoid introducing an additional force.
The toy car or trolley should be released, not pushed, at the start of the experiment, to avoid introducing an additional force.
True or False?
Timing errors in this experiment are reduced by taking a single measurement for each distance interval.
False.
Timing errors are reduced by taking repeat readings and calculating an average time for each interval.
What safety precaution should be taken when using a weight hanger suspended over the edge of the bench?
Do not stand directly beneath the weight hanger, and place a crash mat underneath it in case the weights fall.
State Newton's third law of motion.
Newton's third law states that whenever two bodies interact, the forces they exert on each other are equal in magnitude and opposite in direction.
State the three rules used to identify a Newton's third law pair.
1) The two forces act on different objects.
2) They are equal in size but opposite in direction.
3) They are the same type of force (e.g. both weight, or both normal contact force).
When a person walks, what is the Newton's third law force pair between their foot and the ground?
The foot pushes the ground backwards, and the ground pushes the foot forwards – equal in magnitude, opposite in direction.
The weight and normal contact force acting on a book resting on a table are ______, not a Newton's third law pair, because both act on the same object.
The weight and normal contact force acting on a book resting on a table are balanced forces, not a Newton's third law pair, because both act on the same object.
True or False?
Newton's third law force pairs act in the same direction.
False.
Third law force pairs are equal in magnitude but act in opposite directions.
A book rests on a table. The Earth's gravitational pull on the book is one force.
What is its Newton's third law pair?
The gravitational pull of the book on the Earth – equal in magnitude and opposite in direction, both being weight forces.
Define weight.
Weight is the force acting on an object due to gravitational attraction, measured in newtons (N).
State the equation linking weight, mass and gravitational field strength.
W = mg, where W is weight (N), m is mass (kg), and g is gravitational field strength (N/kg).
Why is weight a vector quantity but mass a scalar quantity?
Weight is a force, so it has both magnitude and direction (vector). Mass is an amount of matter, so it only has magnitude (scalar).
Name two instruments that can be used to find an object's weight, stating whether each measures weight directly or indirectly.
Top pan balance – measures mass directly, weight found indirectly by calculation (W = mg).
Newton-meter (spring balance) – measures weight directly, in newtons.
If a mass is measured in grams, it must be converted to ______ before being used in the equation W = mg.
If a mass is measured in grams, it must be converted to kilograms before being used in the equation W = mg.
True or False?
An object's mass changes depending on which planet it is on, but its weight stays the same.
False.
An object's mass stays the same everywhere, but its weight changes because the gravitational field strength differs between planets.
A student has a weight of 190 N on Mars, where g = 3.8 N/kg.
Calculate her weight on Earth, where g = 9.8 N/kg.
Mass = W ÷ g = 190 ÷ 3.8 = 50 kg
Weight on Earth = m × g = 50 × 9.8 = 490 N
Define centripetal force.
The resultant force, directed towards the centre of the circle, required to keep an object in uniform circular motion.
An object moves in a circle at a constant speed.
Explain why its velocity is still changing.
Velocity is a vector, so it depends on both speed and direction. Even though the speed is constant, the object's direction is continuously changing, so its velocity keeps changing.
In which direction does the centripetal force act, relative to the object's motion?
Perpendicular to the object's direction of travel, towards the centre of the circle.
Centripetal force is not a separate type of force – it can be provided by any force, such as tension or gravity, depending on the ______.
Centripetal force is not a separate type of force – it can be provided by any force, such as tension or gravity, depending on the situation.
True or False?
An object moving in a circle at a constant speed is in equilibrium.
False.
The object has a resultant force (the centripetal force) acting on it, so it is not in equilibrium.
What is the relationship between the direction of the centripetal force and the direction of the centripetal acceleration?
They act in the same direction – this follows from Newton's second law.
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