P4 Explaining Motion

GCSE Revision Guide

Table of Contents How can we describe motion What are forces What is the connection between forces an

d motion?

Speed, distance and time What are forces? Resultant force

Distance-time graphs One force causes another Movement with balanced and unbalanced forces

Velocity-time graphs Friction Reducing forces in car crashes

Interpreting velocity-time graphs Force and momentum change

Distance-time graphs (Higher Tier)

How can we describe motion-Intro

The speed of a moving object can be calculated if the distance travelled and the time taken are known. The faster an object moves, the steeper is the line representing it on a distance-time graph.The velocity of an object is its speed in a particular direction. In velocity-time graphs sloping lines represent steadily increasing or decreasing velocities. Horizontal lines represent movement at constant velocities.

Speed, distance and time

You should recall from your Key Stage 3 studies how to calculate the speed of an object from the distance travelled and the time taken.The equationWhen an object moves in a straight line at a steady speed, you can calculate its average speed if you know how far it travels and how long it takes. This equation shows the relationship between speed, distance travelled and time taken:Speed is distance divided by time taken.For example, a car travels 300 m in 20 s.Its speed is 300 ÷ 20 = 15 m/s.The instantaneous speed of an object is the speed of an object at a particular instant. In practice it is the average speed over a very short period of time.

Distance-time graphsYou should be able to draw and explain distance-time graphs for objects moving at steady speeds or standing still.

Background informationThe vertical axis of a distance-time graph is the distance travelled from the start, and the horizontal axis is the time taken from the start.

Features of the graphsWhen an object is stationary, the line on the graph is horizontal. When an object is moving at a steady speed, the line on the graph is straight, but sloped.The diagram shows some typical lines on a distance-time graph.time (s) on x axis, distance (m) on y axisNote that the steeper the line, the greater the speed of the object. The blue line is steeper than the red line because it represents an object moving faster than the object represented by the red line.The red lines on the graph represent a typical journey where an object returns to the start again. Notice that the line representing the return journey slopes downwards.Changes in distances in one direction are positive, and negative in the other direction. If you walk 10 m away from me, that can be written as +10 m; if you walk 3 m towards me, that can be written as –3 m.

Velocity-time graphs

You should be able to explain velocity-time graphs for objects moving with a constant velocity or a changing velocity.Background informationThe velocity of an object is its speed in a particular direction. This means that two cars travelling at the same speed, but in opposite directions, have different velocities. One velocity will be positive, and the velocity in the other direction will be negative.The vertical axis of a velocity-time graph is the velocity of the object and the horizontal axis is the time taken from the start.Features of the graphsWhen an object is moving with a constant velocity, the line on the graph is horizontal. When an object is moving with a steadily increasing velocity, or a steadily decreasing velocity, the line on the graph is straight, but sloped. The diagram shows some typical lines on a velocity-time graph.time (s) on x axis, velocity (m/s) on y axisSpeed - time graphThe steeper the line, the more rapidly the velocity of the object is changing. The blue line is steeper than the red line because it represents an object that is increasing in velocity much more quickly than the one represented by the red line.Notice that the part of the red line between 7 and 10 seconds is a line sloping downwards (with a negative gradient). This represents an object that is steadily slowing down.

Interpreting velocity-time graphs

You should be able to draw and interpret the shape of a velocity-time graph for an object that is stationary, for an object moving in a straight line with constant speed and for an object moving in a straight line with steadily increasing or decreasing speed.Velocity-time graph. Velocity in metres/second is on the vertical (y) axis and time in seconds in on the horizontal (x) axis. The line goes straight from 0 to 3 seconds on the x axis. The object speeds up and it's velocity increases from 0 to 8 m/s in 2 seconds. The speed remains constant at 8 m/s for 3 seconds, before decreasing back to 0 in 2 seconds.In the graph, the object is stationary for the first 3 seconds, then has a steadily increasing speed for 2 seconds. For the next 3 seconds it has a constant speed, and for the last 2 seconds it has a steadily decreasing speed.You can see that the speeds are changing steadily between 3 and 5 seconds and between 8 and 10 seconds, because the lines are not just going up and down, but are also straight.One example of a velocity-time graph is a lorry tachograph. Tachograph records are circular disks recording the speed of the vehicle. This shows whether the lorry driver has been keeping to the speed limit and taking regular rest breaks.A tachograph records the speed of the vehicle. This section of a tachograph disk shows that the driver started driving shortly after 9:30, and was travelling at 60 miles/hour until 10:50, when he took a ten-minute break. At 11:10 he speeded up again, and took a 10-minute break about 12:00.A tachograph records the speed of the vehicleThis section of a tachograph disk shows that the driver started driving shortly after 9:30, and was travelling at 60 miles/hour until 10:50, when he took a 20-minute break. At 11:10 he speeded up again, and took a 10-minute break at about 12:00.

Distance-time graphs (Higher Tier)

You should be able to calculate gradients on distance-time graphs, and to draw and interpret graphs where the speed is increasing or decreasing.Background informationTo calculate the gradient of the line on a graph, you need to divide the change in the vertical axis by the change in the horizontal axis.Distance-time graphsThe gradient of a line on a distance-time graph represents the speed of the object. Study this distance-time graph.

What are forces-Intro

Forces occur when there is an interaction between two objects. These forces always happen in pairs – when one object exerts a force on another, it always experiences a force in return.

What are forcesThe green arrow shows the force on the weights as the weightlifter pushes upwards.

The red arrow shows the downwards force on the weightlifter's arm muscles.

These two forces are an interaction pair. They are equal in size, and opposite in direction.

You only have an interaction pair if the forces are caused by the interaction. In this case, the compression in the weightlifter's muscles is caused by the weight pushing down, and the upwards force on the weight is caused by the weightlifter's muscles

One force causes another

Sometimes a force is produced as a response to another force – these are not the same as interaction pairs.

A book on a table has a downwards force (its weight) due to gravity.

This downwards force, pushing on the table, produces an upwards force called reaction.

Friction

Another common force is friction.

When two surfaces slide past each other, the interaction between them produces a force of friction.

In this diagram, the book is moving to the right across the table as shown by the red arrow.

What is the connection between forces and motion?

The momentum of an object is its mass multiplied by its velocity. The larger the mass and velocity the larger the momentum.

Forces change momentum - the larger the force the more quickly the momentum changes.

The resultant force is the overall result of all forces acting on an object.

Resultant Forces Sometimes several forces act on the same object. Look at this diagram of a moving car:There are several forces acting on the car, shown by the arrows.Gravity pulls down on the carThe reaction force from the road pushes up on the wheelsThe driving force from the engine pushes the car alongThere is friction between the road and the tyresAir resistance acts on the front of the carResultant forceThe resultant force is the sum of all the different forces acting on the car.You have to take account of the directions – the reaction forces on the wheels (blue arrows) add up to the same as the weight (green arrow), so these cancel out.The driving force from the engine (yellow arrow) is in the opposite direction to the counter forces of friction (red arrows) and air resistance (purple arrow).When the car is increasing its speed then all these forces add to give a single resultant force forwards.

Slide under construction

500 m/s

20 m/s

0.05 m/s

A car travels 100 m in 5 s. What is its average speed?

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What do objects increasing in speed not have

Momentum

Mass

Balanced Forces

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M=mv

P=mv

M=p/v

How do you calculate momentum

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