What's covered
Key facts
When forces are balanced (resultant = 0 N), a stationary object stays still and a moving object continues at constant speed in a straight line.
The elastic limit is the point beyond which a spring is permanently deformed and will no longer return to its original length when the force is removed.
An object is accelerating whenever its speed or its direction of motion changes.
From F = ma: acceleration = F ÷ m; the greater the force for a fixed mass, the greater the acceleration; the greater the mass for a fixed force, the smaller the acceleration.
The atmosphere exerts pressure on all surfaces due to the weight of air above; at sea level this is approximately 101,000 Pa (101 kPa).
Contact forces require physical contact between objects; examples include friction, air resistance, normal (reaction) force, and tension.
The elastic potential energy stored in a stretched spring is E = ½ke² (half the spring constant times the extension squared).
Acceleration = change in velocity ÷ time taken.
A falling object accelerates due to gravity (10 m/s²) until air resistance equals its weight; then it falls at constant terminal velocity with zero acceleration.
An object floats when the upthrust acting on it is equal to (or greater than) its weight; it sinks when its weight is greater than the upthrust.
Sample questions
A taste of the 52 questions in this topic, answers marked. Sign up to practise the full set with spaced repetition.
What is a force?
- ✓A push or pull on an object
- •A type of energy store
- •Speed of an object
- •The mass of an object
What is the equation for Hooke's Law?
- •F = k/e
- ✓F = ke
- •F = ke²
- •F = ma
What does Newton's first law say about an object with no resultant force on it?
- •It always slows down and eventually stops
- •It changes direction but keeps the same speed
- •It speeds up steadily in the direction it faces
- ✓It stays still or keeps moving at constant velocity
What is the equation for pressure?
- •P = A/F
- •P = F × A
- ✓P = F/A
- •P = F/d
What is the equation for speed?
- ✓speed = distance ÷ time
- •speed = distance × time
- •speed = force ÷ mass
- •speed = time ÷ distance
Which is a contact force?
- •Electric force
- ✓Friction force
- •Gravity force
- •Magnetic force
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