Forces Flashcards

All 36 cards in this deck

State Newton's first law.

An object stays at rest, or keeps moving at a constant velocity, unless a resultant force acts on it.

What is the resultant force on a body moving at a constant velocity?

Zero — the forces on it are balanced.

What happens to the motion of a body when the resultant force on it is not zero?

Its speed and/or its direction changes, i.e. it accelerates in the direction of the resultant force.

True or false? A moving object needs a resultant force acting on it to keep moving at a steady speed in a straight line.

False. With zero resultant force it carries on at constant velocity; a force is only needed to change the motion.

True or false? A resultant force can change the direction of a body without changing its speed.

True. A change of direction is a change of velocity, e.g. an object moving in a circle at constant speed.

What is the name for the state of an object when all the forces on it are balanced?

Equilibrium — the resultant force is zero, so it is at rest or moving at constant velocity.

State Newton's second law as an equation, with units.

force (N) = mass (kg) × acceleration (m/s$^2$), i.e. $F = m \times a$

Which force is used as $F$ in $F = m \times a$?

The resultant (net) force on the object.

What are the steps to find the acceleration of an object from the force on it and its mass?

e.g. a resultant force of $12$ N on a mass of $4$ kg

1. Rearrange $F = m \times a$ to $a = F \div m$
2. Substitute: $a = 12 \div 4$
3. Acceleration $= 3$ m/s$^2$

What are the steps to use $F = m \times a$ when the mass is given in grams?

e.g. a mass of $500$ g accelerating at $2$ m/s$^2$

1. Convert grams to kilograms by dividing by $1000$: $500$ g $= 0.5$ kg
2. Substitute into $F = m \times a$: $F = 0.5 \times 2$
3. Force $= 1$ N

For a constant mass, how does acceleration change if the resultant force is doubled?

The acceleration also doubles — acceleration is directly proportional to resultant force.

True or false? For a constant resultant force, a larger mass gives a larger acceleration.

False. Acceleration is inversely proportional to mass, so a larger mass accelerates less.

In the trolley practical, how is the accelerating force applied to the trolley increased?

Add more weights (masses) to the weight hanger attached to the string.

In the trolley practical, how is the total mass of the moving system kept constant while the force is varied?

Transfer masses from the trolley to the weight hanger (mass removed from trolley = mass added to hanger).

In the trolley practical, how is the runway set up to compensate for the effects of friction?

Raise one end of the runway until the trolley (not attached to the hanger) rolls at constant speed when given a push.

What are the steps to investigate how acceleration depends on force using a trolley?

e.g. starting with $4$ masses on the trolley and none on the hanger

1. Compensate for friction by tilting the runway, then measure the trolley's acceleration with light gates
2. Move one mass from the trolley to the hanger to increase the force while total mass stays constant
3. Repeat and plot acceleration against accelerating force

What shape of graph is expected when acceleration is plotted against accelerating force for a constant total mass?

A straight line through the origin — acceleration is directly proportional to force.

True or false? In the trolley practical, the masses on the hanger are not part of the moving system.

False. The hanger and its masses accelerate too, so they count as part of the total moving mass.

State Newton's third law.

When two objects interact, they exert equal and opposite forces on each other.

Give the features of a Newton's third law force pair.

Equal in size, opposite in direction, acting on different bodies, and of the same type of force.

True or false? The two forces in a Newton's third law pair act on the same object.

False. They always act on two different objects, one on each of the interacting bodies.

True or false? For a book resting on a table, the weight of the book and the upward contact force from the table are a Newton's third law pair.

False. Both act on the book, so they are balanced forces (Newton's first law), not a third law pair.

How does Newton's third law explain conservation of momentum in a collision?

Each object feels an equal and opposite force for the same time, so one gains exactly the momentum the other loses and the total stays the same.

How do the forces compare when a tennis racket hits a ball?

The force of the racket on the ball and the force of the ball on the racket are equal in size and opposite in direction.

Define weight.

The force acting on an object due to gravity (due to the gravitational field it is in), measured in newtons.

State the equation linking weight, mass and gravitational field strength, with units.

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

How is the weight of an object measured?

By hanging it from a calibrated newtonmeter (spring balance / force meter) and reading the force in newtons.

Describe the relationship between the weight of a body and the gravitational field strength at its location.

Weight is directly proportional to gravitational field strength — for a fixed mass, doubling $g$ doubles the weight.

What are the steps to find the mass of an object from its weight?

e.g. weight $50$ N where $g = 10$ N/kg

1. Rearrange $W = m \times g$ to $m = W \div g$
2. Substitute: $m = 50 \div 10$
3. Mass $= 5$ kg

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

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

What is a centripetal force?

The resultant force that acts towards the centre of the circle, keeping an object moving in a circular path.

In what direction does the centripetal force on an object moving in a circle act?

Towards the centre of the circle.

Why does an object moving in a circular orbit at constant speed have a changing velocity?

Its direction of motion is constantly changing, and velocity depends on direction as well as speed.

Which force provides the centripetal force on a satellite orbiting the Earth?

The gravitational force (gravity / the satellite's weight), acting towards the centre of the Earth.

Which force provides the centripetal force on a car going round a bend?

Friction between the tyres and the road, acting towards the centre of the bend.

True or false? An object moving round a circle at constant speed has a resultant force of zero.

False. Its velocity is changing, so there must be a resultant (centripetal) force towards the centre.