Force and motion

A force has both magnitude and direction, so it is a vector quantity. Mass, time and density have magnitude only.

Force is measured with a spring balance, which works by spring extension, in newtons (N).

Frictional force resists movement, so it acts in the direction opposite to the motion, which is to the left.

Weight = mass × 10 N kg⁻¹ = 4.5 × 10 = 45 N.

The weight acts downwards. The buoyant force acts upwards with the same magnitude, so the block floats.

A force from the side changes the direction of motion of a moving object.

Buoyant force = actual weight − apparent weight = 7.5 − 5.0 = 2.5 N.

Density = 60 ÷ 50 = 1.2 g cm⁻³. This is more than water (1.0 g cm⁻³), so the buoyant force is less than the weight and the solid sinks.

In a wheelbarrow the load is between the fulcrum (the wheel) and the effort (the handles). Scissors are first class; a fishing rod and forceps are third class.

600 N × 0.5 m = effort × 1.5 m, so effort = 300 ÷ 1.5 = 200 N.

Moment = force × perpendicular distance = 40 N × 0.25 m = 10 N m. The distance must be in metres.

Pressure = force ÷ area = 60 ÷ 0.03 = 2 000 Pa.

A higher temperature makes the particles move faster, so they collide with the walls more frequently and with greater force. The number of particles stays the same.

A thumbtack goes in because the force is on a small area (high pressure). The straw, vacuum cleaner and plunger all work because the pressure inside is lowered and the higher atmospheric pressure outside pushes.