Forces in Action (OCR GCSE Physics A (Gateway)): Flashcards

The weight of an object equals its mass multiplied by the gravitational field strength of the gravitational field.

True.

Weight (W) equals mass (m) multiplied by gravitational field strength (g): W = mg. Since g is constant in a given location, weight and mass are directly proportional.

An astronaut's weight is less on the Moon because the Moon has a smaller gravitational field strength than Earth. Since W = mg, a lower value of g gives a smaller weight even though the astronaut's mass stays the same.

Free fall is when an object falls solely under the influence of gravity, with no other forces (such as air resistance) acting on it. All objects in free fall near Earth accelerate at the same rate.

When an object is lifted upwards, its gravitational potential energy increases. This is because GPE = mgh — as height (h) increases, the energy stored in the object's gravitational field increases.

False.

A falling object loses gravitational potential energy as its height decreases. The GPE is converted into kinetic energy as the object speeds up during its fall.

Complete the table for the GPE equation: GPE = m × g × h

A moment is the turning effect of a force about a pivot. It is calculated as force multiplied by the perpendicular distance from the pivot. The unit is newton metres (Nm).

The door handle is placed far from the hinge (pivot) to maximise the perpendicular distance. Since moment = force × distance, a larger distance creates a larger moment for the same applied force, making the door easier to open.

True.

This is the principle of moments. For a balanced object, the sum of all clockwise moments about a pivot equals the sum of all anticlockwise moments about the same pivot.

A moment is calculated by multiplying the force by the perpendicular distance from the pivot to the line of action of the force.

A couple is a pair of equal and opposite forces that do not act through the same point. Even though the forces are balanced, a couple causes the object to rotate because the forces create moments in the same direction around the pivot.

A pivot is a fixed point around which an object can rotate. Forces acting at a distance from a pivot create a moment (turning effect) about that point.

True.

Moment = force × perpendicular distance. If the distance increases while the force stays the same, the moment increases. This is why spanners have long handles.

For the see-saw to balance, the principle of moments must be satisfied: the clockwise moment must equal the anticlockwise moment. The heavier adult must sit closer to the pivot so that the lighter child at a greater distance produces an equal moment.

Complete the table for the moment equation: M = F × d

A lever is a simple machine that increases the size of a force acting on an object. The applied force must act further from the pivot than the force to be overcome, creating a larger moment.

Applying force further from the pivot increases the perpendicular distance, which increases the moment. A larger moment means the lever can overcome a greater load force at a smaller distance, making the lever act as a force multiplier.

False.

When a small gear drives a larger gear, the larger gear rotates slower but produces a greater moment. A smaller gear driving a larger one is like a low gear — more turning force, less speed.

A lever acts as a force multiplier because the applied force acts at a greater distance from the pivot than the load force.

Two meshed gears rotate in opposite directions. Where the teeth of the gears meet, both sets of teeth move in the same direction, which causes one gear to turn clockwise and the other to turn anticlockwise.

A gear is a toothed wheel that rotates on an axle or shaft. Gears transmit and amplify the rotational effect of forces using moments. The size of a gear determines the moment and speed of rotation it produces.

True.

Both levers and gears transmit and amplify forces using moments. The distance from the pivot or axle determines how the turning force is multiplied or transferred between parts.

When a larger gear is driven by a smaller gear, the larger gear rotates slower but produces a greater moment. This is because the turning force on the larger gear acts further from its axle (pivot), increasing the moment.

Pressure is the force per unit area acting on a surface. It is calculated as force divided by area. The unit of pressure is the pascal (Pa), equivalent to one newton per square metre.

Pressure equals force divided by the cross-sectional area over which the force acts.

True.

As depth increases in a fluid, the weight of fluid above a point increases. This greater weight creates a higher pressure on objects at that depth. Pressure also increases with the density of the fluid.

The sharp point of the drawing pin has a very small area, which concentrates the force to create a large pressure on the surface. Your finger pushes on the flat head, which has a larger area, creating a much smaller pressure — so the pin goes into the surface instead.

The pascal (Pa) is the SI unit of pressure. One pascal equals one newton per square metre (1 Pa = 1 N/m²). It is named after the scientist Blaise Pascal.

False.

Atmospheric pressure decreases as altitude increases. At higher altitudes there are fewer air molecules above a surface, reducing the weight of air and therefore the pressure it exerts.

A denser liquid has a greater weight per unit volume. This means the column of liquid above a point exerts a greater downward force, increasing the pressure at that depth. Pressure in a fluid is given by p = hρg, so higher density (ρ) means higher pressure.

Complete the table for the pressure equation: p = F ÷ A

An object submerged in water experiences pressure from both the water and the atmosphere above it. The water pressure acts evenly across the whole surface of the object in all directions, at right angles to each surface.