Questions where students often pick the tempting wrong answer — make sure you know the right one:
In a collision between two objects, what quantity is always conserved (in a closed system)?✗ Kinetic energy is always conserved in collisions because energy is conserved.✓ Total momentum is always conserved; kinetic energy is only conserved in elastic collisions.
When energy is 'lost' as heat due to friction, what has really happened?✗ Energy has been used up and no longer exists.✓ Energy has been transferred to the thermal store of the surroundings — it's not destroyed, just dissipated.
If an object is moving at constant velocity in a straight line, what is the net force on it?✗ There must be a forward force on the object to keep it moving.✓ Zero — by Newton's first law, constant velocity requires zero net force.
When you push a wall, what does Newton's third law say about the wall?✗ The wall only pushes back if your force is strong enough to make it react.✓ The wall exerts an equal and opposite force on you at the same instant.
Practise Mechanics — free games
Test yourself with these quick revision games for this topic:
