Forces, Motion and Energy
Forces, Motion and Energy tests net force, balanced and unbalanced forces, speed and motion graphs, friction, and energy transfers. Exam items often ask you to calculate speed or explain why an object speeds up, slows down, or stays still.
Core rules
- Forces are pushes or pulls with size and direction. Net force determines acceleration; balanced forces mean no change in motion.
- Speed = distance divided by time. On a distance-time graph, slope represents speed; a horizontal line means stationary.
- Friction opposes relative motion and transforms kinetic energy into thermal energy.
- Energy is transferred, not used up. Forms include kinetic, gravitational potential, elastic, thermal, chemical, electrical, light, and sound.
- Weight is a force from gravity on mass, measured in newtons; mass is the amount of matter, measured in kilograms.
Worked example
A runner covers 300 m in 20 s. Average speed = 300 m divided by 20 s = 15 m/s. If she then slows down, the forward force from her muscles is smaller than the opposing friction and air resistance, so the net force is backward. Her speed decreases; her kinetic energy decreases and is transferred to thermal energy in the surroundings and her body.
Traps that cost marks
Adding forces without direction; confusing speed and velocity; thinking a moving object must have a net force; saying energy is destroyed; reading a distance-time graph height as speed; mixing mass and weight units.