Forces & their Interactions Flashcards

All 46 cards in this deck

What is a scalar quantity?

A quantity that has magnitude (size) only.
e.g. mass, speed, distance, energy

What is a vector quantity?

A quantity that has magnitude (size) and an associated direction.
e.g. force, velocity, displacement

How is a vector quantity represented in a diagram?

By an arrow: the length of the arrow represents the magnitude and the direction of the arrow represents the direction.

True or false? Speed is a vector quantity.

False. Speed is a scalar (magnitude only); velocity is the vector version.

True or false? Distance is a scalar but displacement is a vector.

True. Displacement includes a direction, distance does not.

Name three vector quantities met in the Forces topic.

Force, weight, velocity (also acceleration, displacement, momentum).

What is a force?

A push or pull that acts on an object due to its interaction with another object.

What is a contact force?

A force that acts only when two objects are physically touching.
e.g. friction, air resistance, tension, normal contact force

What is a non-contact force? Give examples.

A force that acts when the objects are physically separated: gravitational force, electrostatic force and magnetic force.

True or false? Magnetic force is a contact force.

False. Magnetic force is a non-contact force — it acts without the objects touching.

What name is given to the force acting along a stretched string, rope or towbar?

Tension.

In an interaction between a pair of objects, what can be said about the two forces produced?

Each object exerts a force on the other: the two forces are equal in size, opposite in direction, and act on different objects.
e.g. a magnet and a steel block attract each other equally

Why is force classified as a vector quantity?

Because it has both a magnitude (in newtons) and a direction.

When forces are drawn as arrows, what does the length of each arrow show?

The magnitude (size) of the force — all arrows must be drawn to the same scale.

What labels should be added to each force arrow on a force diagram?

The name of the force and its size in newtons (N).
e.g. "weight = 20 N"

True or false? Two force arrows drawn the same length in one diagram represent forces of the same size.

True. Arrow length is proportional to the magnitude of the force.

(Higher tier) What is meant by resolving a force?

Replacing a single force with two component forces at right angles to each other that together have the same effect.

What is weight?

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

What is mass?

The amount of matter in an object, measured in kilograms (kg); it does not change with location.

What is meant by the 'centre of mass' of an object?

The point from which the weight may be considered to act (the point where the mass appears to be concentrated).

Where is the centre of mass of a uniform sphere or a uniform rectangular sheet?

At its geometric centre.

What instrument is used to measure the weight of an object?

A calibrated spring-balance (a newtonmeter).

True or false? An astronaut's mass is smaller on the Moon than on Earth.

False. Mass stays the same; the weight is smaller because the Moon's gravitational field strength is smaller.

What is the equation linking weight, mass and gravitational field strength?

$W = m\,g$
weight (N) = mass (kg) × gravitational field strength (N/kg)

What is the value of the gravitational field strength close to the Earth's surface?

About $9.8$ N/kg.

What are the steps to calculate the weight of an object?
e.g. a 2 kg bag where $g = 9.8$ N/kg

1. Write the equation: $W = m\,g$.
2. Put in the values: $W = 2 \times 9.8$.
3. Work out the answer with units: $W = 19.6$ N.

What are the steps to find the mass of an object from its weight?
e.g. weight = 50 N, $g = 10$ N/kg

1. Start from $W = m\,g$.
2. Rearrange: $m = W \div g$.
3. Substitute: $m = 50 \div 10 = 5$ kg.

True or false? Weight is measured in kilograms.

False. Weight is a force, so it is measured in newtons (N); mass is measured in kilograms.

What is the relationship between the weight and the mass of an object?

They are directly proportional — doubling the mass doubles the weight (for the same gravitational field strength).

What is a resultant force?

The single force that has the same effect as all the original forces acting on the object together.

What are the steps to find the resultant of two forces acting in a straight line?
e.g. 8 N to the right and 3 N to the left

1. Choose a positive direction (take right as positive).
2. Forces in the same direction add, forces in opposite directions subtract: $8 - 3 = 5$.
3. State the size and direction: 5 N to the right.

True or false? When two forces act on an object in opposite directions, you add them to get the resultant.

False. You subtract them, and the resultant acts in the direction of the larger force.

Why must a resultant force be given with a direction as well as a size?

Because force is a vector quantity.

What is a free body diagram?

A diagram showing all the forces acting on a single isolated object, drawn as labelled arrows.

What are the steps to draw a free body diagram?
e.g. a book resting on a table

1. Draw the object as a simple shape or dot, on its own.
2. Draw an arrow for each force acting on it, pointing the right way with length to scale (weight down, normal contact force up).
3. Label each arrow with the force name and size.

True or false? A free body diagram shows the forces that the object exerts on other objects.

False. It shows only the forces acting on the chosen object.

Which four forces are shown on a free body diagram of a car driving along a level road?

Weight (down), normal contact force (up), driving force (forwards) and friction/air resistance (backwards).

On a free body diagram, from which point are the force arrows usually drawn?

From the object's centre of mass.

What does it mean to say the forces on an object are balanced?

The resultant force on the object is zero.

What is the resultant force on a child standing still on the ground?

Zero — the upward forces are equal to the downward forces.

True or false? If the resultant force on an object is zero, the object must be stationary.

False. It could also be moving at a constant speed in a straight line (constant velocity).

What term describes an object on which all the forces are balanced?

The object is in equilibrium.

What happens to the motion of an object when the forces on it are unbalanced?

There is a resultant force, so its velocity changes (it speeds up, slows down or changes direction) in the direction of the resultant force.

True or false? If the resistive force on a moving object decreases while the driving force stays the same, the resultant force increases.

True. The forces become more unbalanced, so the object speeds up.

(Higher tier) What are the steps to find the resultant of two forces at right angles using a scale vector diagram?
e.g. 4 N to the right and 3 N upwards

1. Choose a scale and draw the two forces tip-to-tail: 4 cm right, then 3 cm up.
2. Draw the resultant arrow from the start to the finish point.
3. Measure it and use the scale: resultant = 5 N at about $37°$ above the horizontal.

(Higher tier) When a resultant force is found from a scale vector diagram, what two things must be given in the answer?

The magnitude in newtons and the direction, e.g. as an angle from the horizontal.