Forces and motion flashcards

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What is the difference between distance and displacement?
Distance is the total length of path travelled (a scalar, in m). Displacement is the straight-line distance from start to finish in a stated direction (a vector, in m).
What is the difference between speed and velocity?
Speed is how fast something moves (scalar). Velocity is speed in a given direction (vector), so a car going round a bend at constant speed is still changing velocity.
State the equation for speed and say what each symbol means.
speed = distance ÷ time, v = s ÷ t. v is speed in m/s, s is distance in m, t is time in s.
A car travels 150 m in 12 s at constant speed. What is its speed?
v = s ÷ t = 150 ÷ 12 = 12.5 m/s.
State the equation for acceleration and the meaning of each symbol.
a = (v − u) ÷ t. a is acceleration (m/s²), v is final velocity (m/s), u is initial velocity (m/s), t is time taken (s).
A cyclist speeds up from 4 m/s to 16 m/s in 3 s. Find the acceleration.
a = (v − u) ÷ t = (16 − 4) ÷ 3 = 4 m/s².
What do the gradient and a horizontal section tell you on a distance–time graph?
The gradient is the speed (steeper means faster). A horizontal line means the object is stationary. A curve getting steeper means it is accelerating.
What do the gradient and the area under a velocity–time graph represent?
The gradient is the acceleration. The area between the line and the time axis is the distance travelled (displacement). A horizontal line means constant velocity.
Give the equation linking v, u, a and s for uniform acceleration.
v² = u² + 2as. v is final velocity, u initial velocity (m/s), a acceleration (m/s²), s distance (m).
A ball starts from rest and accelerates at 2.5 m/s² over 20 m. Find its final speed.
v² = u² + 2as = 0 + 2 × 2.5 × 20 = 100, so v = 10 m/s.
What is the acceleration of free fall near the Earth's surface?
About 9.8 m/s² (the same as gravitational field strength, 9.8 N/kg), downwards. Without air resistance all objects fall with the same acceleration.
What is the difference between mass and weight, and how are they linked?
Mass is the amount of matter (kg) and does not change with place. Weight is the gravitational force on it (N). W = m × g, where g is gravitational field strength.
What is the weight of a 6.0 kg object on Earth (g = 9.8 N/kg)?
W = mg = 6.0 × 9.8 = 58.8 N, about 59 N.
What does Newton's first law say?
An object stays at rest or keeps moving at constant velocity unless a resultant force acts on it. Balanced forces mean no change in motion.
State Newton's second law as an equation.
F = ma. The resultant force F (N) equals mass m (kg) × acceleration a (m/s²). The acceleration is in the direction of the resultant force.
A 1200 kg car has a resultant force of 3000 N. What is its acceleration?
a = F ÷ m = 3000 ÷ 1200 = 2.5 m/s².
(Paper 2) State Newton's third law and give an example.
If body A exerts a force on body B, B exerts an equal and opposite force on A. Example: a rocket pushes gas down and the gas pushes the rocket up. The two forces act on different bodies.
Describe the forces that act on a falling skydiver and how they change.
Weight acts down and constant. Air resistance acts up and grows with speed. The resultant (and acceleration) falls to zero when the two are equal, giving terminal velocity.
Explain why a falling object reaches terminal velocity.
As speed rises, drag increases until it equals weight. The resultant force is then zero, so by Newton's first law the object continues at constant velocity.
What happens to a skydiver's speed when the parachute opens?
Drag suddenly exceeds weight, so the resultant force acts upwards and the skydiver decelerates. Speed falls until drag again equals weight at a lower terminal velocity.
What is stopping distance made of?
Stopping distance = thinking distance + braking distance. Thinking distance is travelled during the driver's reaction time; braking distance is travelled once the brakes are applied.
Which factors increase thinking distance and which increase braking distance?
Thinking distance: higher speed, tiredness, alcohol, drugs, distractions. Braking distance: higher speed, larger mass, worn brakes or tyres, wet or icy road.
State Hooke's law and describe the force–extension graph of a spring.
Up to the limit of proportionality, extension is directly proportional to the force applied, so the graph is a straight line through the origin. Beyond that point the line curves and the spring may no longer return to its original length.
A spring extends by 2.0 cm when a 4.0 N force is applied, and obeys Hooke's law. What is the extension for a 6.0 N force?
Extension is proportional to force, so extension = 2.0 × (6.0 ÷ 4.0) = 3.0 cm, provided 6.0 N is still within the limit of proportionality.
(Paper 2) What is the moment of a force, and what is its unit?
The turning effect of a force about a pivot. Moment = force × perpendicular distance from the pivot. The unit is the newton metre (N m).
(Paper 2) A 40 N force acts 0.30 m from a pivot at right angles. Find its moment.
Moment = F × d = 40 × 0.30 = 12 N m.
(Paper 2) State the principle of moments for an object in equilibrium.
For an object in equilibrium, the sum of the clockwise moments about any point equals the sum of the anticlockwise moments, and there is no resultant force.
How can you find the acceleration of a trolley on a slope using light gates?
Use two light gates a known distance apart, or one gate with a card, to get speeds u and v and the time t between them. Then calculate a = (v − u) ÷ t. Repeat and average.
(Paper 2) What is momentum? Give the equation and unit.
Momentum is mass × velocity: p = mv. p is momentum in kg m/s, m is mass in kg, v is velocity in m/s. It is a vector in the direction of the velocity.
(Paper 2) State the principle of conservation of momentum.
In a closed system, total momentum before a collision or explosion equals total momentum after it, provided no external resultant force acts.
(Paper 2) A 2 kg trolley at 3 m/s hits a stationary 1 kg trolley and they stick together. Find their speed.
Momentum before = 2 × 3 = 6 kg m/s. After: (2 + 1) × v = 6, so v = 2 m/s.
(Paper 2) How is force linked to change in momentum? What does a crumple zone do?
F = (mv − mu) ÷ t, so force = change in momentum ÷ time. Crumple zones and seat belts increase the stopping time, which reduces the force on the passengers.

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