Forces & motion flashcards
15 cards on Forces & motion: the key points, worked examples and the slips to watch. Swipe through them, or print them.
Card 1 of 15
Forces & motion
Scalars and vectors
What is the difference between a scalar and a vector, with examples of each?
Answer it in your head, then turn the card over.
Scalars and vectors
Key points
- A scalar has size (magnitude) only: distance, speed, mass, energy, time.
- A vector has size and direction: displacement, velocity, force, acceleration, momentum.
- A vector is drawn as an arrow: its length shows the size, and it points the way.
- Two 5 N forces make 10 N if they push the same way, but 0 N if they push opposite ways.
Watch out
Speed is a scalar but velocity is a vector: a car going round a bend at a steady speed has a changing velocity.
Forces & motion
Resultant force
What is a resultant force, and what does it do to an object's motion?
Answer it in your head, then turn the card over.
Resultant force
Key points
- The resultant force is the single force with the same effect as all the forces acting together.
- Forces in a line: add the ones pointing the same way, subtract the ones pointing opposite ways.
- Resultant zero: the forces are balanced and the motion does not change.
- Resultant not zero: the object accelerates in the direction of the resultant.
Engine 500 N forwards, drag 300 N back
Resultant = 500 − 300 = 200 N forwards
Watch out
Forces pointing opposite ways subtract: adding them to 800 N ignores their directions.
Forces & motion
Weight and mass
What is the difference between weight and mass, and how do you calculate weight?
Answer it in your head, then turn the card over.
Weight and mass
Key points
- Mass is the amount of matter, in kilograms. It is the same anywhere.
- Weight is the force of gravity on that mass, in newtons. It acts at the centre of mass.
- On Earth g is about 9.8 N/kg; on the Moon about 1.6 N/kg, so you weigh less there.
Formula
W = mg: weight (N) = mass (kg) × gravitational field strength (N/kg)
A 65 kg student
On Earth : 65 × 10 = 650 N
On the Moon : 65 × 1.6 = 104 N
Watch out
Weight is a force, in newtons: 65 kg is the student's mass, not their weight.
Forces & motion
Newton's first law
What does Newton's first law say about an object with no resultant force on it?
Answer it in your head, then turn the card over.
Newton's first law
Key points
- With no resultant force, a still object stays still.
- A moving object keeps moving at the same speed in the same direction.
- A car at a steady speed has its driving force exactly balanced by friction and air resistance.
- A resultant force is needed to speed up, slow down or change direction.
Watch out
A moving object does not need a resultant force to keep going, only to change its velocity.
Forces & motion
Newton's second law
How are resultant force, mass and acceleration linked?
Answer it in your head, then turn the card over.
Newton's second law
Key points
- A resultant force makes an object accelerate in the direction of the force.
- Acceleration is proportional to the resultant force, and inversely proportional to the mass.
- Required practical: pull a trolley with different forces and time it with light gates, then change its mass.
Formula
F = ma: resultant force (N) = mass (kg) × acceleration (m/s²)
1200 N on an 800 kg car
a = F ÷ m
a = 1200 ÷ 800 = 1.5 m/s²
Watch out
Use the resultant force: take friction away from the engine's push before you divide.
Forces & motion
Newton's third law
What does Newton's third law say about the forces two objects exert on each other?
Answer it in your head, then turn the card over.
Newton's third law
Key points
- When two objects interact, the forces they exert on each other are equal in size and opposite in direction.
- The two forces act on different objects, so they never cancel out.
- They are always the same type of force: you push the wall, and the wall pushes back on you.
Watch out
A book's weight and the table's push are not a pair: both act on the book. The weight's partner is the book pulling the Earth.
Forces & motion
Terminal velocity
Why does a skydiver reach a steady top speed, and what happens when the parachute opens?
Answer it in your head, then turn the card over.
Terminal velocity
Key points
- At first the weight is bigger than air resistance, so the skydiver accelerates.
- Air resistance grows with speed until it balances the weight.
- Then the resultant force is zero, so the speed stays constant: terminal velocity.
- A parachute raises air resistance above the weight, so they slow to a new, lower terminal velocity.
Watch out
At terminal velocity the skydiver is still falling fast: zero resultant force means no acceleration, not no motion.
Forces & motion
HigherMomentum and inertia
What is momentum, and what happens to it when objects collide?
Answer it in your head, then turn the card over.
Momentum and inertia
Key points
- Momentum = mass × velocity, in kg m/s. It is a vector.
- In a closed system, the total momentum before a collision equals the total momentum after.
- Inertia is the tendency to keep the same velocity unless a resultant force acts: more mass, more inertia.
Formula
p = mv: momentum = mass (kg) × velocity (m/s)
Two trolleys that stick together
A 2 kg trolley at 3 m/s hits a still 1 kg trolley, and they stick.
Before: 2 × 3 + 1 × 0 = 6 kg m/s
After: 6 ÷ (2 + 1) = 2 m/s together
Watch out
Momentum has a direction: two equal masses moving opposite ways at the same speed have zero total momentum.
Forces & motion
Speed and velocity
How do you calculate speed, and how is velocity different?
Answer it in your head, then turn the card over.
Speed and velocity
Key points
- Speed is the distance travelled per second. A journey usually gives the average speed.
- Velocity is speed in a stated direction.
- Typical speeds: walking about 1.5 m/s, cycling about 6 m/s, sound in air about 330 m/s.
Formula
s = vt: distance (m) = speed (m/s) × time (s)
A sprinter runs 100 m in 12.5 s
speed = distance ÷ time
speed = 100 ÷ 12.5 = 8 m/s
Watch out
Put the distance in metres and the time in seconds to get m/s.
Forces & motion
Acceleration
What is acceleration, and how do you calculate it?
Answer it in your head, then turn the card over.
Acceleration
Key points
- Acceleration is the rate of change of velocity, in metres per second squared (m/s²).
- A negative acceleration is a deceleration: the object is slowing down.
- Moving in a circle at a steady speed is accelerating, because the direction, and so the velocity, keeps changing.
Formula
a = Δv ÷ t: acceleration (m/s²) = change in velocity (m/s) ÷ time (s)
A cyclist: 2 m/s to 10 m/s in 4 s
Change in velocity = 10 − 2 = 8 m/s
a = 8 ÷ 4 = 2 m/s²
Watch out
Use the change in velocity, not the final speed: 10 ÷ 4 = 2.5 forgets the cyclist was already moving.
Forces & motion
Uniform acceleration: v² − u² = 2as
How do you link speeds and distance for a steady acceleration, without the time?
Answer it in your head, then turn the card over.
Uniform acceleration: v² − u² = 2as
Key points
- u is the initial velocity, v the final velocity, a the acceleration and s the distance.
- It only works when the acceleration is steady, such as an object falling with no air resistance.
- Near the Earth's surface, a falling object accelerates at about 9.8 m/s².
Formula
v² − u² = 2as
A ball dropped from rest falls 20 m
Take a = 10 m/s² and u = 0
v² = 0 + 2 × 10 × 20 = 400
v = = 20 m/s
Watch out
The last step is a square root: v² = 400 gives v = 20 m/s, not 400 m/s.
Forces & motion
Distance–time graphs
What does the gradient of a distance–time graph tell you, and how do you read its shape?
Answer it in your head, then turn the card over.
Distance–time graphs
Key points
- The gradient is the speed: steeper means faster.
- A straight sloping line is a steady speed; a flat line means stopped.
- A curve means the speed is changing: the gradient of a tangent gives the speed at that moment.
The line rises 120 m in 20 s
Gradient = 120 ÷ 20 = 6 m/s
Watch out
Check the axis label first: only on a distance–time graph does a flat line mean stopped.
Forces & motion
Velocity–time graphs
What do the gradient and the area under a velocity–time graph show?
Answer it in your head, then turn the card over.
Velocity–time graphs
Key points
- The gradient is the acceleration. A line sloping down shows a deceleration.
- A flat line is a steady velocity: the object is still moving.
- The area under the line is the distance travelled: split it into triangles and rectangles.
0 to 12 m/s in 6 s, steadily
Gradient = 12 ÷ 6 = 2 m/s²
Area = ½ × 6 × 12 = 36 m
Watch out
On a velocity–time graph a flat line means steady velocity, not stopped: only a line along the time axis is stopped.
Forces & motion
Stopping distance
What adds up to a car's stopping distance, and what makes each part longer?
Answer it in your head, then turn the card over.
Stopping distance
Key points
- Stopping distance = thinking distance + braking distance.
- Thinking distance is covered at full speed during the reaction time, typically 0.2 s to 0.9 s.
- Tiredness, alcohol, drugs and distractions slow reactions, so the thinking distance grows.
- Wet or icy roads and worn tyres or brakes make the braking distance longer.
20 m/s with a 0.5 s reaction time
Thinking distance = 20 × 0.5 = 10 m
Watch out
Tiredness and alcohol add to the thinking distance; icy roads add to the braking distance. Keep them apart.
Forces & motion
Braking distance and speed
Why does doubling a car's speed make its braking distance about four times longer?
Answer it in your head, then turn the card over.
Braking distance and speed
Key points
- The brakes do work to remove the kinetic energy: braking force × distance = kinetic energy.
- Doubling the speed makes the kinetic energy four times bigger, so the same braking force needs four times the distance.
- Friction at the brakes transfers the kinetic energy to thermal stores, so the brakes heat up.
- Stopping hard from high speed needs a large deceleration, which can overheat the brakes or skid the car.
Watch out
Doubling the speed doubles the thinking distance, but makes the braking distance about four times as long.
Print these cards
Four cards to an A4 sheet, each the size of a standard A6 revision card.
One-sided
Each card prints on one face: the question at the top, the notes under it.
Two-sided
Questions on the front, notes on the back. Print double-sided and flip on the long edge, then cut along the lines.
Put it into practice
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