SP2: Motion and Forces

What is a scalar quantity?

A quantity that has only magnitude (size) and no direction

Give examples of scalar quantities

Speed, Distance, Mass, Energy, Temperature, Time

Give examples of vector quantities

Force, Velocity, Displacement, Weight, Acceleration, Momentum

What is a vector quantity?

A quantity that has both magnitude (size) and direction

What is speed?

The distance an object travels per unit of time

What is velocity?

The speed of an object in a specific direction

What is distance?

How far an object moves

What is displacement?

The change in position of an object from a point of reference

What is the equation for the average speed?

Total distance / Total time

What is acceleration?

The rate at which velocity changes

What is the equation for acceleration?

Change in Velocity (m/s^2) / Change in Time (s)

What is the equation for uniform acceleration?

Final Velocity^2 - Initial Velocity^2 \= 2 x Acceleration x Distance

On a distance/time graph, what does the gradient represent?

The speed of the object (Steeper line mines the object is travelling faster).

On a distance/time graph, what does the curve represent?

The acceleration of the object (Steeper curve means the object is speeding up)

How do you calculate the speed at a given time from a distance/time graph for an accelerating object (curve)?

Draw a tangent to the curve at the required time; Calculate the gradient of the tangent.

On a velocity/time graph, what does the gradient represent?

Acceleration (Steeper line means a greater acceleration).

On a velocity/time graph, what does the flat line represent?

Steady speed

On a velocity/time graph, what does the curve represent?

Changing acceleration

How can the distance travelled by an object be calculated from a velocity/time graph?

It is equal to the area under the graph

What is the speed of sound?

330 m/s in air

What is the average human walking speed?

1.5 m/s

What is the average for human running speed?

3 m/s

What is the average human cycling speed?

6 m/s

What is Newton's First Law?

If the resultant force on a stationary object is zero, the object will remain stationary. If the resultant force on a moving object is zero, it'll just carry on moving at the same velocity.

What is Newton's Second Law?

The force of an object and its acceleration are directly proportional, and is inversely proportional to its mass. (F \= ma)

What is the Resultant force?

The overall force acting on an object

What is the equation for force, linking mass and accleration?

Force (N) \= Mass (Kg) x Acceleration (m/s^2)

What is weight?

The force that acts on an object due to gravity and the object's mass.

What quantities does weight depend on?

The object's mass and the gravitational field strength at the given position in the field.

What is the equation for weight?

Weight (N) \= Mass (Kg) x Gravitational Field Strength (N/Kg)

What piece of equipment can be used to measure an object's weight?

A calibrated spring-balance of newton-meter

Why does an object not have a constant velocity when it is travelling at a constant speed in a circle?

The direction of the object is changing constantly, so the velocity is not constant, but the speed is as the object is still travelling at the same speed, just in a different direction.

What is the centripetal force?

A force that acts on an object moving in a circular path and is directed toward the centre around which the object is moving.

Explain the light gate practical

Measure the mass of the trolley and place it at the top of the ramp and set up the light gates in two positions along the ramp. Each light gate will record the time when the trolley passes through it and the speed of the trolley at that time. Attach a string to the trolley and pass it over a pulley at the end of the bench. Attach a fixed mass to the end of the string and release it so the trolley accelerates. Repeat with varying masses attached to the trolley

What is inertial mass?

A measure of how difficult it is to change a given object's velocity; The ratio of force over acceleration

What is Newton's Third Law?

For every action there is an equal and opposite reaction

What is momentum?

The property that all moving objects have; the product of the mass and velocity of an object.

What is the equation for momentum?

Momentum (kg m/s) \= Mass (Kg) x Velocity (m/s)

What is the conservation of momentum?

In a closed system, the total momentum before the collision is the same after the collision.

What is the force of an object in terms of change in momentum and time?

Force (N) \= Change in Momentum (Kg m/s) / Time (s)

What is the stopping distance?

The distance covered in the time between the driver first spotting a hazard and the car coming to a complete stop.

What is the calculation for stopping distance?

Stopping distance \= thinking distance + braking distance

What is the thinking distance?

The distance the vehicle travels during the drivers reaction time

What is the reaction time?

The time between the driver spotting the hazard and taking action

What factors affect the reaction time?

Tiredness, drugs, alcohol, distractions

What is the breaking distance?

The time it takes for your car to stop after applying the breaks.

What factors affect the braking distance?

Speed, friction between tyres and road, condition of brakes, road conditions, mass of car

How do you measure human reaction times?

Person A and B hold each end of a ruler with the 0cm mark at the bottom. Person A drops the ruler without telling person B. Person B catches it. The distance travelled corresponds to their reaction time. Control variables like keeping the ruler the same in repeated experiments and have the same person dropping it

Why is important during the ruler test that the 0cm mark is at the bottom?

So you can obtain the distance directly without having to calculate it; otherwise, a zero error would need to be accounted for.

What is the typical range of values for human reaction time.

0.2-0.9 seconds

Describe the energy transfers that take place when a car applies its breaks.

Work is done by the friction force between the breaks and wheel. Kinetic energy of the wheel is transferred to heat and is dissipated to the surroundings through the brake disks.

What is the word done by the brakes equal to?

Work Done by Brakes (Force x Distance) \= Energy in the car's kinetic energy stores (1/2 x Mass x Velocity^2)

What are two consequences of a vehicle undergoing very large deceleartions?

Kinetic energy converted to heat energy is very high causing brakes to overheat; Loss of control of the vehicle.

What safety features increase the collision time which reduces the force?

Stretchy seat belts and airbags; Crumple zones which are areas at the front and back of a vehicle which crumple up easily in a collision, increasing the time taken to stop.