StemUp: OCR A A level Physics 3.2: Forces in action

What is the effect of a resultant force acting on a mass? (2)

- A resultant force acting on a mass causes it to accelerate.

- The acceleration is in the same direction as the resultant force.

What is the equation linking force, mass, and acceleration, and what are the units of each quantity? (2)

- The equation is F = ma.

- F is force in newtons (N), m is mass in kilograms (kg), and a is acceleration in metres per second squared (m/s^2).

What is meant by weight? (1)

The gravitational force acting on an object due to its mass, measured in newtons (N).

What is the equation for weight? (1)

W = mg.

Why does an object feel weightless when in free fall? (1)

An object feels weightless in free fall because there is no normal contact force acting on it. Although it still has weight, there is no force pushing back to create the sensation of weight.

What is the difference between mass and weight? (2)

- Mass is a fixed, intrinsic property of an object that does not change with location.

- Weight is the force due to gravity acting on mass and varies depending on the gravitational field strength.

What is the purpose of a free-body diagram? (1)

To show all the individual forces acting on an object.

How are forces represented in a free-body diagram? (2)

- Forces are drawn as arrows pointing in the direction the force acts.

- Each arrow is labelled with the force type and drawn to scale so that larger forces are shown with longer arrows.

What is meant by tension? (1)

Tension is the pulling force exerted by a string, cable, or rope on an object.

What is meant by normal contact force? (1)

The normal contact force is the force a surface exerts perpendicular to itself on an object resting on it.

What is meant by friction? (1)

Friction is the force that opposes relative motion when two surfaces rub against each other.

What is meant by drag? (1)

Drag is the resistive force acting on an object moving through a fluid, such as air or water. It acts in the opposite direction to motion.

Why is the normal contact force acting on an object on a slope less than its weight? (2)

- The normal contact force only opposes the component of the object's weight that acts perpendicular to the slope.

- This component is N = W cos(θ), and since cos(θ) is always less than or equal to 1, the normal force is always less than or equal to the weight.

What factors affect the magnitude of drag on an object? (4)

- Speed

- Cross-sectional area

- Surface texture (smoothness)

- Viscosity/density of the fluid it is moving through.

How does drag scale with speed and with cross-sectional area? (2)

- Drag increases with the square of speed: doubling speed increases drag by a factor of four.

- Drag increases proportionally with cross-sectional area: doubling the area doubles the drag.

What is meant by terminal velocity? (1)

Terminal velocity is the constant speed reached by a falling object when the resistive (drag) forces balance the weight of the object, resulting in no further acceleration.

What are the forces acting on an object at terminal velocity? (2)

- Weight, upthrust and drag.

- At terminal velocity, the downward force of weight is equal in magnitude to the upward resistive forces: resultant force is zero.

How does an object fall in a uniform gravitational field when drag is present? (4)

- At the start of the fall, the only force is weight, so acceleration is equal to g.

- As the object speeds up, drag increases, reducing the net downward force.

- The object continues to accelerate, but at a decreasing rate.

- Eventually, drag equals weight, and the object reaches terminal velocity, moving at constant speed.

How would a velocity-time graph change if an object fell through water instead of air? (2)

- The terminal velocity would be lower in water due to greater drag.

- The object would reach terminal velocity more quickly because drag forces rise faster in denser fluids.

How does the velocity-time graph of a falling object with drag differ from one without drag? (2)

- The velocity-time graph for an object with drag is curved (non-linear), showing decreasing acceleration.

- It would also level off at a lower final velocity, as terminal velocity is reached more slowly than for an object without drag.

What would the acceleration-time graph look like for an object dropped from rest that reaches terminal velocity? (2)

- The graph starts at a = g, as the initial acceleration is g when the object is dropped.

- The graph curves down to zero acceleration as terminal velocity is reached.

What is meant by the moment of a force, and how is it calculated? (2)

- The moment of a force is the turning effect of a force about a point or axis.

- It is calculated using the equation: moment = force × perpendicular distance from the pivot, with units of newton-metres (Nm).

What is meant by a couple? (1)

A couple is a pair of equal and opposite forces acting on an object but not along the same line.

What is meant by the torque of a couple, and how is it calculated? (2)

- The torque of a couple is the turning effect produced by a couple.

- It is calculated using the equation: torque = force × perpendicular separation between the forces, with units of Nm.

What is meant by centre of mass? (1)

The point where an externally applied force causes pure translation without rotation.

What is meant by centre of gravity? (1)

The point through which the object's entire weight appears to act.

Why can an object have a centre of mass without a centre of gravity? (2)

- An object always has a centre of mass regardless of location.

- However, it only has a centre of gravity if it's in a gravitational field.

Why might the centre of gravity and the centre of mass of an object not coincide? (2)

- In a non-uniform gravitational field, different parts of the object experience different gravitational forces.

- This causes the centre of gravity to shift relative to the centre of mass.

How can a plumb-line be used to determine the centre of gravity of an object? (3)

- Suspend the object from a pin and hang a plumb-line in front of it.

- Mark a vertical line downwards along the string.

- Repeat from other suspension points — the intersection of the lines is the centre of gravity.

What happens when an object's centre of gravity falls outside its base? (2)

- The object topples because the weight creates a moment about the edge of the base.

- This moment causes the object to rotate and fall over.

What is the principle of moments? (1)

For an object in rotational equilibrium, the sum of clockwise moments equals the sum of anticlockwise moments.

What are the conditions for equilibrium? (1)

For an object to be in equilibrium, there must be no resultant force and no resultant moment acting on it.

What is meant by coplanar forces? (1)

Coplanar forces are forces that all act in the same plane.

Why do three forces acting on an object in equilibrium form a closed triangle? (2)

- In equilibrium, the vector sum of the forces must be zero.

- This means the forces balance in both the horizontal and vertical directions, so when drawn tip-to-tail, they form a closed triangle.

What is meant by density? (1)

Density is the mass per unit volume of a substance.

How is density calculated and what are its units? (1)

ρ = m/V, where ρ is density in kg/m^3, m is mass in kg, and V is volume in m^3.

How do you determine the density of an irregular solid object? (3)

- Use a balance to measure the object's mass.

- Fully submerge the object in water using a displacement can and collect the displaced water in a measuring cylinder to find the volume.

- Calculate the density using ρ = m/V.

What is meant by pressure? (1)

The force exerted per unit cross-sectional area.

How is pressure calculated and what are its units? (2)

- Pressure is calculated using P = F/A.

- P is pressure in Pa, F is force in N, and A is area in m². 1 Pa = 1 N/m².

What is the equation for pressure in fluids and what are the units of each term? (2)

- The equation is P = hρg.

- Pressure P is in Pa, height h is in m, density ρ is in kg/m³, and g is the gravitational field strength in m/s².

In which direction does pressure act in a fluid? (1)

Pressure in a fluid acts equally in all directions.

Why does pressure increase with depth in fluids? (2)

- As depth increases, the weight of fluid above increases.

- This increased weight leads to a greater downward force per unit area, increasing the pressure.

What is meant by upthrust? (1)

- The upward force exerted by a fluid on a body that is fully or partially submerged in it.

Why does an object submerged in a fluid experience upthrust? (3)

- There is greater pressure on the bottom surface than on the top surface of the object.

- This pressure difference creates a resultant upward force.

- The size of this force is equal to the weight of the displaced fluid.

What does Archimedes' principle state? (1)

The upthrust on an object immersed in a fluid is equal to the weight of the fluid displaced by the object.