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Flashcards · J249 · Topic 2
Forces flashcards
43 cards on Topic 2, subtopics P2.1 to P2.3. Pick a subtopic or study them all.
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P2.1 Motion
- Scalar and vector pairs for motion
- Distance (scalar) and displacement (vector). Speed (scalar) and velocity (vector). A vector needs a direction.
- Speed equation, and average speed
- speed = distance ÷ time (m/s). Average speed = total distance ÷ total time, including stops.
- A runner covers 4.5 km in 25 minutes. Average speed in m/s?
- 4500 m ÷ 1500 s = 3.0 m/s
- Converting between km/h and m/s
- km/h ÷ 3.6 = m/s m/s × 3.6 = km/h
- Acceleration equation
- a = (v − u) ÷ t a in m/s², u initial velocity and v final velocity in m/s, t in s.
- What does the gradient of a velocity–time graph give?
- The acceleration. A horizontal line means constant velocity; a steeper line means a larger acceleration.
- How do you find the speed at one moment from a curved distance–time graph?
- Draw a tangent to the curve at that point and find its gradient.
- Uniform acceleration equation without time
- v² − u² = 2as
- From rest, an object accelerates at 3.2 m/s² over 40 m. Final speed?
- v² = 2 × 3.2 × 40 = 256, so v = 16 m/s
- Velocity rises from 0 to 12 m/s in 4 s, then stays at 12 m/s for 6 s. Distance?Higher
- ½ × 4 × 12 + 6 × 12 = 24 + 72 = 96 m
- What does the area under a velocity–time graph give?Higher
- The distance travelled. Split the area into rectangles and triangles; for a curved line, count squares to estimate it.
P2.2 Newton's laws
- Contact and non-contact forces: examples
- Contact: normal contact force, friction, drag, tension. Non-contact: gravity, electrostatic, magnetic.
- Resultant force, and equilibrium
- Resultant: the single force that has the same effect as all the forces acting. Equilibrium: the resultant force is zero (at rest or constant velocity).
- Newton’s first law
- If the resultant force is zero, an object stays at rest or keeps moving at the same speed in the same direction. A non-zero resultant changes its velocity.
- Forces of 21 N west and 20 N south act on an object. Size of the resultant?Higher
- R = √(21² + 20²) = √841 = 29 N
- Explain how a falling skydiver reaches terminal velocityHigher
- At first weight > drag, so she accelerates. As speed rises, drag rises and the resultant falls. When drag = weight the resultant is zero and the velocity is constant.
- Newton’s second law, with an example
- Resultant force = mass × acceleration (F = ma). A 1300 kg car with 900 N driving force and 640 N resistance: a = 260 ÷ 1300 = 0.20 m/s².
- InertiaHigher
- A measure of how difficult it is to change the velocity of an object.
- Inertial massHigher
- The ratio of force over acceleration, m = F / a.
- Momentum, and conservation of momentumHigher
- p = m × v (kg m/s), a vector. With no external force, total momentum before a collision = total momentum after.
- A 2.5 kg cart at 4.8 m/s hits a resting 1.5 kg cart and they lock together. Velocity?Higher
- Momentum = 2.5 × 4.8 = 12 kg m/s. v = 12 ÷ 4.0 = 3.0 m/s
- Why does a longer collision time mean a smaller force?Higher
- A longer time gives a smaller acceleration for the same change in velocity, so the force is smaller. In terms of momentum: force = change in momentum ÷ time, so the same change in momentum over a longer time needs a smaller force.
- A 0.30 kg ball moving at 12 m/s is stopped in 0.020 s. Average force?
- a = change in velocity ÷ time = 12 ÷ 0.020 = 600 m/s². F = m × a = 0.30 × 600 = 180 N
- Why does an object moving in a circle at constant speed need a resultant force?Higher
- Its direction keeps changing, so its velocity changes: it is accelerating. A resultant force towards the centre is needed.
- Newton’s third law pair
- Two forces from one interaction: equal in size, opposite in direction, the same type, acting on different objects. They never cancel.
- Work done (by a force)
- Energy transferred when a force moves an object along the line of action of the force. W = F × s (J, N, m).
- Joule (J)
- The work done when a force of 1 N moves an object 1 m along the line of action of the force. So 1 N m = 1 J.
- Power (of a force or device)
- The rate of energy transfer, which is the same as the rate of doing work. P = W ÷ t; 1 W = 1 J per second.
P2.3 Forces in action
- Why does stretching, bending or compressing need more than one force?
- One force on a free object only changes its motion. Changing its shape needs forces in opposite senses on different parts of it.
- Elastic vs plastic deformation
- Elastic: returns to its original shape and size when the forces are removed. Plastic: the change of shape is permanent.
- Force–extension equation for a spring
- F = k × e (N, N/m, m), valid in the linear region (up to the limit of proportionality). k is the gradient of the straight part of the force–extension graph.
- A force of 9.0 N gives a 4.5 cm extension. Spring constant?
- e = 0.045 m, so k = 9.0 ÷ 0.045 = 200 N/m
- Energy stored in a stretched spring
- E = ½ke² in the linear region; in general, the work done = the area under the force–extension graph.
- Gravitational field strength, and the weight equation
- g: the force per kilogram in a gravitational field (10 N/kg at the Earth’s surface). W = m × g (N, kg, N/kg).
- Mass vs weight
- Mass: amount of matter, in kg, the same everywhere, measured with a balance. Weight: a force, in N, that changes with g, measured with a newtonmeter.
- Acceleration in free fall near the Earth
- About 10 m/s² (9.8 m/s² more precisely). All masses fall with the same acceleration when air resistance is absent.
- An object weighs 7.4 N where g = 3.7 N/kg. What does it weigh on Earth (g = 10 N/kg)?
- Mass = 7.4 ÷ 3.7 = 2.0 kg (mass does not change). Weight on Earth = 2.0 × 10 = 20 N.
- Pressure at a surface: equation and units
- p = F ÷ A, where F is the force normal to the surface (Pa, N, m²).
- How does a hydraulic system multiply force?
- The enclosed liquid passes pressure on equally, so F₁ / A₁ = F₂ / A₂. The larger piston gets a larger force but moves a smaller distance.
- Moment of a force
- M = F × d (N m), where d is the perpendicular distance from the pivot to the line of action of the force.
- Principle of moments, with an example
- For balance, total clockwise moment = total anticlockwise moment. 12 N at 0.50 m balances F at 0.20 m on the other side: F = 6.0 ÷ 0.20 = 30 N.
- How does a lever act as a force multiplier?
- effort × effort distance = load × load distance. A longer effort distance needs a smaller effort, but the effort moves further. No energy is gained.
- Gear ratio, and what a small driver turning a large driven gear does
- Gear ratio = teeth on driven gear ÷ teeth on driver gear. Ratio greater than 1: bigger moment, slower turning.