MOTION GRAPHS Distance – time graphs The motion of the objects can be shown with the help of distance - time and speed – time graphs. Distance – time graphs Object at rest (stationary object)

Object moving with constant speed Object increasing speed non - uniformly (accelerating)

Object decreasing speed constantly Object decreasing speed non - uniformly (decelerating non - uniformly)

Note: The gradient of the distance-time graph represents the speed of the object.

Example Question: The graph shown below represents the distance traveled by a car plotted against time.

(a) Describe the motion of the car between point A and E. Between A and B the speed of the car increasing, then between B and C car moves with constant speed, after that between C and D the speed of the car decreasing and between D and E car is at rest. (b) How far has the car travelled at the end of 30seconds? 60 m (c) Calculate the speed of the car between point B and C? (e) What is the distance travelled between A and D? (f)What is the average speed between A and D?

Speed – time graphs Object at rest (stationary object) Object moving with constant speed

Object moving with uniform acceleration Object moving with non - uniformly acceleration

Object decelerating uniformly Object decelerating non - uniformly

Note: the gradient of the speed-time graph is equal to the acceleration of the object.

the area under the speed-time graph represents the distance travelled by the object.

The speed – time graph below shows the motion of a lorry. Example question: The speed – time graph below shows the motion of a lorry. (a) Describe the motion of the lorry. First the lorry accelerates non – uniformly, then it moves with uniform acceleration. After some time speed of lorry decreases and moves with constant speed, finally the lorry decelerates uniformly to the rest.

(b) Calculate the acceleration of the lorry between 2s and 4s. a = 𝑣 −𝑢 𝑡 = 30 −10 2 = 10 m/s² (c) Calculate distance travelled by the lorry between 5s and 7s. A = l x b = 40 x 2 = 80m

Free fall When an object is released near to the surface of the Earth, it falls under the influence of gravity. This is known as free fall. The acceleration of free fall or acceleration due to gravity is denoted by g. The acceleration due to gravity is 10 m/s2. This is true in the absence of air resistance. The experiment below shows, all the air has been removed from the tube. Both feather and lead ball dropped from same height and reaches bottom of the tube at the same time. This is because all the objects in the absence of air resistance, accelerates towards the earth at same rate that is10m/s2.

terminal velocity When any object falls through air, due to the gravitational force or weight the object will exerts a resistive force. This resistive force increases as the object moves faster. Resistive force acts in the opposite direction to which the object is moving. And after some time the resistive force and weight of the object becomes equals, so the object moves with constant speed, this is known as terminal velocity. Consider a parachutist jumping from aircraft.

Consider a parachutist jumping from aircraft. When the parachutist jumps from the aircraft, his speed increase downward due to weight of his body or gravitational force As his speed increases his air resistance will also increases in the opposite direction of fall. After sometime air resistance will be big enough to balance the parachutist’s weight. At this point the forces are balanced so his speed becomes uniform - this is called terminal velocity. 17 November 2018

When he opens his parachute the air resistance suddenly increases and the speed decreases. After some time again air resistance decreases until it balances his weight. The parachutist has now reached a new, lower terminal velocity. 17 November 2018

Speed-time graph of a parachutist is shown below… Speed decreasing Parachute opens – diver slows down speed Speed increases… Terminal velocity reached… Diver hits the ground New, lower terminal velocity reached Time 17 November 2018