State Newton's first, second and third laws of motion.
Explain what is meant by inertia and how mass is related to it.
State two differences between mass and weight.
A body has a mass of 6.0 kg. Find its weight on Earth (g = 9.81 m s−2).
The same body is taken to a planet where g = 3.7 m s−2. State its mass and find its weight there.
A car of mass 1500 kg has a resultant forward force of 4500 N. Find its acceleration.
A 2.0 kg block is pulled by a force of 12 N to the right while a 4.0 N friction force acts to the left. Find the resultant force and the acceleration.
What resultant force is needed to give a 0.50 kg ball an acceleration of 20 m s−2?
A swimmer pushes backward on the water. State the third-law partner of this force and the body it acts on.
A book rests on a table. Explain why the weight of the book and the upward push of the table on the book are not a third-law pair.
Use Newton's first law to explain why a passenger continues to move forward when a car stops suddenly.
Total: 26 marks. Original work by the TheLucidSTEM team. Written in the style of the papers; no past paper question is reproduced.
Answer key · full worked solutionsclick to reveal
First: an object stays at rest or moves at constant velocity unless acted on by a resultant force. Second: the resultant force equals the rate of change of momentum, and for constant mass F = m a. Third: if body A exerts a force on body B, then B exerts an equal and opposite force on A.
inertia is the reluctance of a body to change its motion. The greater the mass, the greater the inertia, so mass is a measure of inertia.
mass is a scalar measured in kilograms and is the same everywhere; weight is a force (a vector) measured in newtons and varies with g.
W = m g = 6.0 × 9.81 = 58.9 N (about 59 N).
the mass is still 6.0 kg. W = m g = 6.0 × 3.7 = 22.2 N (about 22 N).
a = F / m = 4500 / 1500 = 3.0 m s−2.
resultant force = 12 − 4.0 = 8.0 N to the right; a = F / m = 8.0 / 2.0 = 4.0 m s−2 to the right.
F = m a = 0.50 × 20 = 10 N.
the water pushes the swimmer forward with an equal and opposite force; this partner force acts on the swimmer.
both forces act on the book (the same body), so they are balanced forces, not a third-law pair. A third-law pair acts on two different bodies; the partner of the book's weight is the upward gravitational pull of the book on the Earth.
by the first law, the passenger continues at constant velocity unless a resultant force acts. When the car stops, the seat and floor decelerate with the car, but until a force (from a seatbelt) acts on the passenger, the passenger keeps moving forward at the original velocity.