Motion, Forces and Energy
Unit 1 of Cambridge IGCSE Physics 0625. Speed and acceleration, forces, momentum, moments, work, energy and power. It is the biggest unit on the paper and the one that decides most grades.
What you need to know
- Distance is a scalar, displacement is a vector. Speed is a scalar, velocity is a vector. Examiners take marks for using the wrong one.
- On a distance-time graph the gradient is speed. On a speed-time graph the gradient is acceleration and the area under the line is distance travelled.
- An object falling through air reaches terminal velocity when the drag force equals the weight, so the resultant force is zero and the acceleration is zero. It does not mean the object stops.
- Weight is a force in newtons. Mass is in kilograms. W = mg.
- Moments: a beam balances when the sum of clockwise moments equals the sum of anticlockwise moments about the same pivot.
- Energy is conserved. Efficiency is useful energy out divided by total energy in.
Equations
- speed = distance / time
- acceleration = change in velocity / time taken
- F = ma
- W = mg
- p = mv (momentum)
- moment = force x perpendicular distance from the pivot
- work done = force x distance moved in the direction of the force
- kinetic energy = 1/2 mv^2
- gravitational potential energy = mgh
- power = work done / time taken
- density = mass / volume
- pressure = force / area
Motion, Forces and Energy questions students ask
What is the difference between speed and velocity?
Speed is a scalar, it only has size. Velocity is a vector, it has size and direction. Two cars travelling at 30 m/s in opposite directions have the same speed and different velocities.
How do you find acceleration from a speed time graph?
Acceleration is the gradient of the line. Take the change in speed on the vertical axis and divide by the time taken on the horizontal axis.
What does the area under a speed time graph tell you?
The area under the line is the distance travelled. On a distance time graph the area means nothing, there you want the gradient.
Why does a falling object reach terminal velocity?
As it speeds up the drag force grows. When drag equals weight the resultant force is zero, so the acceleration is zero and the speed stays constant. It does not stop, it stops speeding up.
What is the difference between mass and weight?
Mass is the amount of matter in kilograms and it does not change. Weight is the force of gravity on that mass in newtons, and it changes with gravitational field strength. W equals mg.
How do you calculate the resultant force?
Add the forces along one direction and subtract the ones acting the other way. If the resultant is zero the object is either still or moving at constant velocity.
Why does a crumple zone reduce the force in a crash?
It increases the time the collision takes. The change in momentum is the same, and force equals change in momentum divided by time, so a longer time gives a smaller force.
How do you calculate efficiency?
Efficiency is useful energy output divided by total energy input, times 100 for a percentage. It can never be more than 100 percent.
What is the difference between kinetic and gravitational potential energy?
Kinetic energy is energy due to movement, half m v squared. Gravitational potential energy is energy due to height in a gravitational field, mgh.
Why is energy never lost?
Energy is conserved. It is transferred to the surroundings, usually as thermal energy, and it spreads out so it becomes less useful. Write transferred, not lost, or you lose the mark.
Mistakes that cost marks
- Writing "the object slows down because there is no force" instead of naming the resultant force.
- Forgetting to convert grams to kilograms or centimetres to metres before substituting.
- Using the whole distance instead of the perpendicular distance when calculating a moment.
- Saying "energy is lost" when the mark scheme wants "energy is transferred to the surroundings as thermal energy".
- Reading the area under a distance-time graph. The area means nothing there. It is the gradient you need.
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