Science You're Expected to Know · Lesson 4

Motion, force, energy and simple circuits

Speed = distance ÷ time. If the passage gives a distance and a time, divide distance by time and keep the units: meters ÷ seconds gives m/s.

Without air resistance, heavy and light objects dropped from the same height fall together: gravity speeds them up at the same rate. In real air, a light, spread-out object such as a crumpled sheet of paper is slowed more by air resistance than a compact one.

Energy changes form. An object high up has gravitational potential energy; as it moves down, that energy becomes kinetic energy, the energy of motion, so it speeds up. Starting higher means more energy to convert and a faster speed at the bottom.

In a series circuit, the parts are connected in one loop, so the current is the same at every point in the loop. Adding more identical bulbs in series with the same battery makes the current smaller. Exactly how much smaller is something the passage’s data will show you.

Three Physics Stations

A physics class worked through three lab stations in one period.

Station 1: Students dropped four balls, one at a time, from a height of 1.8 m and timed each fall with a sensor. Three balls were solid steel; the fourth was a loosely crumpled ball of paper. Each ball was dropped five times, and Table 1 gives the average fall times.

Station 2: Students released the same 0.50 kg cart from rest at four heights on a smooth ramp. At the bottom, the cart rolled across a flat 0.50 m section, and a timer recorded how long it took to cross (Table 2).

Station 3: Students connected 1, 2 or 3 identical bulbs in series with the same 6.0 V battery. They measured the current at Point X, next to the battery, and at Point Y, just after the first bulb (Table 3).

Table 1
BallMass (g)Average fall time (s)
Steel ball 1100.61
Steel ball 2500.61
Steel ball 32000.61
Paper ball50.78
Average fall time from 1.8 m. Steel balls of 10 g, 50 g and 200 g each took 0.61 s. The 5 g paper ball took 0.78 s, the longest time.
Table 2
Release height (m)Time to cross 0.50 m (s)
0.100.36
0.200.25
0.400.18
0.800.125
Time for the cart to cross the flat 0.50 m section after release from 0.10, 0.20, 0.40 and 0.80 m: 0.36, 0.25, 0.18 and 0.125 s. Higher release heights give shorter crossing times.
Table 3
Number of bulbsCurrent at Point X (A)Current at Point Y (A)
10.300.30
20.150.15
30.100.10
Current with 1, 2 and 3 bulbs in series: 0.30, 0.15 and 0.10 A at Point X, and the same 0.30, 0.15 and 0.10 A at Point Y. More bulbs, less current; X and Y always match.

Worked example Easy

Show the whole passage

Three Physics Stations

A physics class worked through three lab stations in one period.

Station 1: Students dropped four balls, one at a time, from a height of 1.8 m and timed each fall with a sensor. Three balls were solid steel; the fourth was a loosely crumpled ball of paper. Each ball was dropped five times, and Table 1 gives the average fall times.

Station 2: Students released the same 0.50 kg cart from rest at four heights on a smooth ramp. At the bottom, the cart rolled across a flat 0.50 m section, and a timer recorded how long it took to cross (Table 2).

Station 3: Students connected 1, 2 or 3 identical bulbs in series with the same 6.0 V battery. They measured the current at Point X, next to the battery, and at Point Y, just after the first bulb (Table 3).

Table 1
BallMass (g)Average fall time (s)
Steel ball 1100.61
Steel ball 2500.61
Steel ball 32000.61
Paper ball50.78
Average fall time from 1.8 m. Steel balls of 10 g, 50 g and 200 g each took 0.61 s. The 5 g paper ball took 0.78 s, the longest time.
Table 2
Release height (m)Time to cross 0.50 m (s)
0.100.36
0.200.25
0.400.18
0.800.125
Time for the cart to cross the flat 0.50 m section after release from 0.10, 0.20, 0.40 and 0.80 m: 0.36, 0.25, 0.18 and 0.125 s. Higher release heights give shorter crossing times.
Table 3
Number of bulbsCurrent at Point X (A)Current at Point Y (A)
10.300.30
20.150.15
30.100.10
Current with 1, 2 and 3 bulbs in series: 0.30, 0.15 and 0.10 A at Point X, and the same 0.30, 0.15 and 0.10 A at Point Y. More bulbs, less current; X and Y always match.

Based on Table 2, what was the cart’s average speed across the flat section after it was released from 0.20 m?

  1. 0.125 m/s
  2. 0.50 m/s
  3. 2.0 m/sAnswer
  4. 4.0 m/s

How to solve it

  1. From the passage: distance 0.50 m; time at the 0.20 m release, 0.25 s.
  2. From background: speed = distance ÷ time.
  3. 0.50 ÷ 0.25 = 2.0, so 2.0 m/s.

Why each choice is right or wrong

  • A. Incorrect. 0.125 multiplies distance by time (0.50 × 0.25). Speed is distance divided by time.
  • B. Incorrect. 0.50 divides time by distance (0.25 ÷ 0.50), which is upside down.
  • C. Correct. The passage gives a 0.50 m section, and Table 2 gives 0.25 s for the 0.20 m release. From background, speed = distance ÷ time = 0.50 m ÷ 0.25 s = 2.0 m/s.
  • D. Incorrect. 4.0 m/s uses 0.125 s, the time for the 0.80 m release, from the bottom row of Table 2.

Worked example Medium

Show the whole passage

Three Physics Stations

A physics class worked through three lab stations in one period.

Station 1: Students dropped four balls, one at a time, from a height of 1.8 m and timed each fall with a sensor. Three balls were solid steel; the fourth was a loosely crumpled ball of paper. Each ball was dropped five times, and Table 1 gives the average fall times.

Station 2: Students released the same 0.50 kg cart from rest at four heights on a smooth ramp. At the bottom, the cart rolled across a flat 0.50 m section, and a timer recorded how long it took to cross (Table 2).

Station 3: Students connected 1, 2 or 3 identical bulbs in series with the same 6.0 V battery. They measured the current at Point X, next to the battery, and at Point Y, just after the first bulb (Table 3).

Table 1
BallMass (g)Average fall time (s)
Steel ball 1100.61
Steel ball 2500.61
Steel ball 32000.61
Paper ball50.78
Average fall time from 1.8 m. Steel balls of 10 g, 50 g and 200 g each took 0.61 s. The 5 g paper ball took 0.78 s, the longest time.
Table 2
Release height (m)Time to cross 0.50 m (s)
0.100.36
0.200.25
0.400.18
0.800.125
Time for the cart to cross the flat 0.50 m section after release from 0.10, 0.20, 0.40 and 0.80 m: 0.36, 0.25, 0.18 and 0.125 s. Higher release heights give shorter crossing times.
Table 3
Number of bulbsCurrent at Point X (A)Current at Point Y (A)
10.300.30
20.150.15
30.100.10
Current with 1, 2 and 3 bulbs in series: 0.30, 0.15 and 0.10 A at Point X, and the same 0.30, 0.15 and 0.10 A at Point Y. More bulbs, less current; X and Y always match.

Which statement best explains why the cart crossed the flat section faster after being released from a greater height?

  1. It started with more potential energy, which became kinetic energy.Answer
  2. It had more mass when released higher, so it gained more energy.
  3. It gained potential energy and lost kinetic energy as it rolled down.
  4. It spent longer on the ramp, so it kept speeding up on the flat part.

How to solve it

  1. From the passage: higher release, shorter crossing time (0.36, 0.25, 0.18, 0.125 s), so a faster cart; the mass stayed 0.50 kg.
  2. From background: going down, potential energy becomes kinetic energy; higher start, more energy.
  3. Only the first choice matches both the data and the energy trade.

Why each choice is right or wrong

  • A. Correct. Table 2 shows crossing times falling from 0.36 s to 0.125 s as release height rose, so the cart got faster (passage). From background, a higher start gives more gravitational potential energy, which turns into kinetic energy, the energy of motion, on the way down.
  • B. Incorrect. The passage says the same 0.50 kg cart was used every time, so its mass never changed.
  • C. Incorrect. This reverses the trade. Moving down, the cart loses potential energy and gains kinetic energy.
  • D. Incorrect. Nothing in the passage says the cart sped up on the flat part. This invents a fact to explain the data.

Trap Your everyday sense versus the table

It feels as if heavy things should fall faster. Table 1 says otherwise: steel balls of 10 g, 50 g and 200 g all took 0.61 s. When intuition and the data disagree, answer from the data. And don’t over-read a single row: the slow paper ball differs from the steel balls in shape and material as well as mass, so it can’t prove that mass matters.