forces, motion and momentum

physics AQA GCSE

What is a force?

A push or pull that acts on an object due to its interaction with another object (vector)

What is the x-axis on a graph?

The dependent variable

What is the y-axis on a graph?

The independent variable

What is force measured in?

Newtons (N)

What is normal contact force?

The equal and opposite force exerted by an object to support the object resting on it

What is a scalar?

A quantity which has only magnitude

What is a vector?

A quantity with both magnitude and direction

Define acceleration and give its units

Acceleration is the rate of change of velocity, measured in m/s²

What is the main equation for acceleration?

a = (v - u)/t

What is the equation for force?

F = ma

What is the suvat equation with squares?

2as = v² - u²

What is the suvat equation for distance?

s = ((v - u)/2) x t

What is the suvat equation for final velocity?

V = u + at

What is the acceleration of any item dropped downwards on earth?

9.8m/s² downwards

Equation for weight

W = mg

What causes air resistance?

Collisions between air particles and the moving object

What affects air resistance?

Velocity and surface area

How does velocity affect air resistance?

The higher the velocity the higher the air resistance as more air particles are hitting the object

How does surface area affect air resistance?

Increased surface area increases air resistance as there is more space for air particles to hit

What is terminal velocity?

When a moving object has reached a constant velocity

What is the gradient of a distance time graph?

Speed

What is the gradient of a displacement time graph?

Velocity

What is the gradient of a velocity time graph?

Acceleration

What is the area under a velocity time graph?

Distance travelled

What do the directions of arrows in a free body diagram show?

The direction of the force

What does the length of arrows on a free body diagram show?

The size/magnitude of the force

What is a resultant force?

The overall force acting on an object (sum of all forces and direction)

How is an object with a resultant force of 0N described?

In equilibrium

How do you use scale drawings to find the resultant force on an object?

By using a scale to draw the two forces we have perpendicular (tip to tail) to each other, forming a triangle and measuring the hypotenuse, then using the scale to find the magnitude of the force. Then using a protractor to find the bearing, which is the direction of the force

How do you resolve a vector (split it up to find its horizontal and vertical components)?

Using a scale, draw a line representing the force then, using a protractor, use the angle of the direction of the force to draw a horizontal or vertical line and form a triangle, or just draw two perpendicular lines to form a triangle. Measure these lines and use the scale to find the magnitude.

Define spring constant

How difficult it is/how much energy is needed to stretch or compress an object

What is friction?

Friction is a force which works in opposition to the motion of an object and causes energy to be transferred by heating, raising the temperature of the object and its surroundings

What is elastic deformation?

When an object is able to return to its original shape after being deformed

What is inelastic deformation?

When an object is permanently deformed and unable to return to its original shape

What is extension?

The increase in the length of a spring when it’s stretched

What’s the relationship between force and extension?

Force is directly proportional to extension

Equation for force in relation to extension

F = ke (k is the spring constant in N/m, e is extension in m)

What is Hooke’s law?

The extension of a spring is directly proportional to the force applied up until the limit of proportionality (the force at which the spring’s deformation becomes inelastic)

Equation for elastic potential energy

E_p = ½ x ke²

What are some of the features of a force extension graph?

The line will be straight passing through the origin before reaching the limit of proportionality, where it will begin to curve. Before the curve, the gradient will be the spring constant and the area under the line the energy transferred

What is Newton’s first law?

An object at rest will remain at rest, and an object in motion will remain in motion at a constant speed unless acted on by an external force

What is Newton’s second law?

The acceleration of an object is directly proportional to its force and indirectly proportional to its mass (F = ma)

What is Newton’s third law?

When two objects interact they exert an equal and opposite force on each other

What is inertia?

The tendency of the motion of an object to remain unchanged

What is inertial mass?

A measure of how hard it is to change an object’s velocity

Equation for work done

W = F x s

Units for work done

Joules (J)

What is stopping distance?

The minimum distance required to stop a vehicle in an emergency

How do you calculate stopping distance?

Stopping distance = thinking distance + braking distance

What is thinking distance?

How far the vehicle travels from when the driver sees the hazard to when they react/brake the vehicle (their reaction time)

What is the equation for thinking distance?

Thinking distance = speed x (reaction) time

What factors affect thinking distance?

The speed of the vehicle (faster = further distance) and the reaction time of the driver (longer = further distance, can be influenced by drugs, fatigue, distractions, alcohol etc)

What is braking distance?

How far the vehicle travels before stopping once the brakes have been applied

Equation with braking force and distance

Braking force x braking distance = ½ x m x v²

What factors affect braking distance?

The velocity (!!!) and the mass, both of which affect the kinetic energy of the car, which needs to reach 0J to stop. Condition/quality of brakes and tires - worse condition leads to longer distance. Road/weather conditions - icy/wet/snowy etc road leads to less friction between car and road, causes car to skid, increases distance

Definition of momentum

A vector quantity related to how hard it is to change the motion of an object

Equation for momentum

P = mass x velocity

Units for momentum

kgm/s

Equation for change in momentum

Δp = F x Δt

What is the law of conservation of momentum?

The total momentum before an event equals the total momentum after an event in a closed system (one where no external forces, e.g. friction, act)

How are seatbelts used to protect people in a car crash?

Seatbelts give slightly, therefore increasing the time taken for the person to slow down from the speed of the car to stationary, meaning that the resultant force is smaller. Otherwise there would be a large change in momentum in a very short period of time which would lead to a very large resultant force.