Flashcards · 9203 · Topic 1

Forces and their effects flashcards

50 cards on Topic 1, subtopics 3.1.1 to 3.1.8. Pick a subtopic or study them all.

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3.1.1 Forces and their interactions

Contact vs non-contact forces, with examples: Contact forces need the objects to touch: friction, air resistance, tension, normal contact force. Non-contact (field) forces act across a gap: gravity, electrostatic, magnetic.
Scalars vs vectors, with examples: Scalar: magnitude only (distance, speed, time). Vector: magnitude and direction (displacement, velocity, acceleration, force, momentum). A vector arrow's length shows magnitude; its head shows direction.
Forces when two objects interact: Each exerts a force on the other. The two forces are equal in size, opposite in direction and act on different objects, so they never cancel. E.g. a boot pushes the ground back; the ground pushes the boot forward.
Weight equation. A 2.0 kg bag where g = 1.6 N/kg: weight?: W = m × g = 2.0 × 1.6 = 3.2 N. Its mass is still 2.0 kg.
Elastic vs inelastic distortion: Elastic: the object returns to its original shape when the forces stop. Inelastic: it stays out of shape afterwards.
A spring of length 20.0 cm is 26.0 cm long under 3.0 N. Find k.: F = k × e. Extension e = 6.0 cm = 0.060 m, so k = 3.0 ÷ 0.060 = 50 N/m.
Force–extension graph for a spring: A straight line through the origin (extension proportional to force) up to the limit of proportionality, then it curves. Gradient = spring constant k; a stiffer spring has a steeper line.
Force and extension practical: two ways to reduce error: Read the rule at eye level with the pointer (avoid parallax), check the zero, use small mass steps and repeat readings. Unload and re-measure to check the distortion was elastic.

3.1.2 Motion

Velocity: Speed in a given direction. v = s ÷ t (m/s), with s the displacement.
A cat walks 6 m north then 2 m south in 8 s. Distance, speed, displacement, velocity?: Distance 8 m, speed 1.0 m/s. Displacement 4 m north, velocity 0.50 m/s north.
Reading a distance–time graph: Flat line: stationary. Straight slope: constant speed (gradient = speed). Curve: changing speed.
How do you find the speed at one instant from a curved distance–time graph?: Draw a tangent to the curve at that point and find its gradient.
A runner covers 100 m in 20 s, rests 10 s, then covers 100 m in 30 s. Average speed?: Total distance ÷ total time, including rests: 200 ÷ 60 = 3.3 m/s.

3.1.3 Resultant forces

Newton's Third Law: features of a force pair: Equal size, opposite direction, same type of force, acting on two different objects.
Are an object's weight and the push of the table on it a Third Law pair?: No. Both act on the same object and are different types of force. The pair to its weight is the object pulling the Earth upwards.
Resultant force: The single force that has the same effect as all the forces acting together. A non-zero resultant force causes acceleration.
Acceleration equation. Velocity rises from 5 m/s to 17 m/s in 4 s:: a = Δv ÷ t = (17 − 5) ÷ 4 = 3.0 m/s². A deceleration is a negative acceleration.
Velocity–time graph: gradient and area: Gradient = acceleration. Area under the line = distance travelled. A horizontal line means constant velocity.
From rest to 9 m/s in 3 s, then 9 m/s for 6 s. Distance travelled?: Area: ½ × 3 × 9 + 6 × 9 = 13.5 + 54 = 67.5 m.
Newton's First Law: If the resultant force is zero, a stationary object stays at rest and a moving object keeps the same speed in the same direction.
Newton's Second Law. A 2000 kg van: driving force 5000 N, resistive forces 1400 N.: F = ma. Resultant = 5000 − 1400 = 3600 N, so a = 3600 ÷ 2000 = 1.8 m/s², in the direction of the resultant.

3.1.4 Momentum

Momentum: p = mv (kg m/s). It has a direction: choose one direction as positive.
Conservation of momentum: In a closed system, the total momentum before an interaction equals the total momentum after.
A 0.40 kg trolley at 2.5 m/s hits a stationary 0.10 kg trolley and they join. Velocity?: Before: 0.40 × 2.5 = 1.0 kg m/s. After: 0.50v. So v = 2.0 m/s.
Two skaters at rest push apart. The 70 kg skater moves right at 1.5 m/s. The 50 kg skater?: Total momentum stays zero. 70 × 1.5 = 105 kg m/s, so the 50 kg skater has 105 kg m/s left: v = 2.1 m/s left.
Force and momentum. A 60 kg passenger at 10 m/s is stopped by a seat belt in 0.25 s.: F = Δp ÷ t = (60 × 10) ÷ 0.25 = 2400 N.
Explain how an air bag or crumple zone reduces injury: It increases the time taken for the momentum to change to zero, so the rate of change of momentum is smaller, so the force on the person is smaller.

3.1.5 Safety in public transport

A vehicle moves at a steady speed in a straight line. What can you say about the forces?: The resultant force is zero: the driving force equals the resistive forces. A larger driving force makes it accelerate; a smaller one makes it decelerate.
Stopping distance, and what thinking and braking distance depend on: Stopping distance = thinking distance + braking distance. Thinking: speed and reaction time. Braking: speed, braking force (road and vehicle condition) and mass.
What makes reaction time longer?: Tiredness, distractions, drugs and alcohol. A longer reaction time increases the thinking distance only.
What happens to the energy when a vehicle brakes?: Friction between brake and wheel does work, reducing the kinetic energy and raising the temperature of the brakes. Braking force × braking distance = kinetic energy lost.
A 900 kg car at 12 m/s; reaction time 0.90 s; braking force 4000 N. Stopping distance?: Thinking: 12 × 0.90 = 10.8 m. KE = ½ × 900 × 12² = 64 800 J, so braking = 64 800 ÷ 4000 = 16.2 m. Stopping distance = 27.0 m.
The speed doubles, with the same braking force. Effect on thinking and braking distance?: Thinking distance doubles. Kinetic energy is proportional to speed², so braking distance becomes four times as long.
How do wet or icy roads and worn tyres or brakes affect braking?: Less friction (or grip), so a smaller braking force and a longer braking distance; worn tyres can skid sooner.

3.1.6 Forces and terminal velocity

Drag: A frictional force on an object moving through a fluid (liquid or gas). It opposes motion and grows as speed grows.
Explain how a falling object reaches terminal velocity: At first drag is zero, so the resultant equals the weight and acceleration is largest. As speed rises, drag rises, so the resultant and acceleration fall. When drag = weight the resultant is zero: constant (terminal) velocity.
A skydiver at terminal velocity opens a parachute. Describe the velocity–time graph.: A sharp fall in speed (drag > weight), then it levels off at a new, lower terminal velocity where drag again equals the (unchanged) weight.
Why does a parachute give a lower terminal velocity?: Its large area gives a large drag at low speed, so the weight is balanced at a lower speed.
How does streamlining help a shark or a car?: A smooth, tapered shape lets the fluid flow smoothly past, reducing drag, so it moves faster (higher maximum speed) using less energy.

3.1.7 Centre of mass

Centre of mass: The point at which the mass of an object may be thought to be concentrated; its weight can be treated as acting there.
Where is the centre of mass of a symmetrical object, and of a freely suspended one?: Symmetrical: on the axis of symmetry. Freely suspended: it comes to rest with its centre of mass directly below the pivot. (It can be outside the object, as in a ring.)
How do you find the centre of mass of an irregular thin lamina?: Hang it from a pin so it swings freely, hang a plumb line from the pin and mark the line. Repeat from two other holes. The lines cross at the centre of mass.
What makes an object stable?: A low centre of mass and a wide base. A high centre of mass and a narrow base make it easier to topple.
When does a tilted object topple?: When the line of action of its weight (through the centre of mass) falls outside its base, giving a resultant moment about the tipping edge.

3.1.8 Moments and levers

Moment of a force: The turning effect of a force: M = F × d, in N m, where d is the perpendicular distance from the pivot to the line of action.
A 450 N force acts 0.20 m from a pivot. Moment?: M = 450 × 0.20 = 90 N m.
Principle of moments: For an object that is not turning, the total clockwise moment equals the total anticlockwise moment about any pivot.
A 240 N weight is 0.75 m left of a pivot. What force 0.50 m right balances it?: F × 0.50 = 240 × 0.75 = 180 N m, so F = 360 N.
How does a lever act as a force multiplier?: With the effort far from the pivot and the load close to it, a small effort overcomes a larger load. The cost: the effort moves further than the load. E.g. crowbar, wheelbarrow, nutcracker.
An object rests in equilibrium on one pivot. What is the support force at the pivot?: It equals the sum of the downward forces.