An object will remain in uniform motion unless acted on by a resultant force.

Acceleration Use distance time graphs to describe the motion of an object

Recap: Resultant Force

Acceleration A resultant force will cause an object to speed up, slow down, or change direction. In other words, a resultant force will cause an object to accelerate Acceleration is a vector, so the magnitude of the velocity can remain constant and there will still be an acceleration if direction is changing.

e.g. circular motion

Acceleration a = v - u t Initial velocity Final velocity

Newton’s Second Law We know from every day experience that the bigger the force, the more an object will accelerate. Now let’s experimentally verify it.

Plot a graph of F against a. Calculate the gradient. What should the gradient tell us? Why doesn't it?

F = ma Newton’s Second Law kg N m/s2 Force (Resultant Force) Mass Acceleration F = ma

Questions Calculate the force needed to accelerate a car of mass 1500 kg by 5 m/s2 2. Calculate the force needed to accelerate a ball of mass 200 g by 15 m/s2 3.Calculate the acceleration of a train of mass 30 000 kg when driven by a force of 15000 N. 4. Calculate the mass of a toy car if a force of 2 N causes it to accelerate by 10 m/s2. 4. What force is exerted on a ball of mass 100g if it accelerates from rest to 30 m/s in 3 seconds?

6. If a bus accelerates from 10 m/s to 15 m/s in 10 seconds with a force of 3 kN what is its mass? The following car has a mass of 400 kg. Calculate the frictional force if the car accelerated at a rate of 2.5 m/s2 when a driving force of 1500N was applied A tram of 800kg takes 6 seconds to stop after travelling at a constant speed of 18m/s. Calculate the deceleration. 9.The driving force on the train was 1200N how large must be the force applied in order to stop the train? (This is more difficult)

Uses of F = ma Car Safety

List the features of a car that make it safe. Now match them to the method they use. changes shape which absorbs kinetic energy increases the time taken for the car to stop cushions the impact absorbs kinetic energy by stretching a little bit increases the time taken by the passenger to stop a strong metal cage that protects the occupants

Crumple Zones They lengthen the time it takes for car and passengers to come to stop, thereby decreasing the deceleration. If deceleration decreases then by F = ma, force must also be decreased.

Which car has the best crumple zone?

Sports When you catch a ball as the val lands in your hands, you move your hands back, again this decreases acceleration. If deceleration decreases then by F = ma, force must also be decreased.

Force, mass and acceleration Newton’s first law Newton’s second law Equation

Newton I – forces cause changes in motion (accelerations) “An object continues in its current state of motion unless acted on by an external force.” For an object at rest, it stays at rest. For an object that is already moving, it stays at constant speed in a straight line.

Newton II (This puts the mathemtical link between force, mass and acceleration) The acceleration of an object is directly proportional to the resultant (net) force acting on it and inversely proportional to its mass.

In other words… If the force doubles, the acceleration doubles. If the mass doubles, the acceleration ______.

Equation  

Example

Solution a) F=ma 20=30xa a=0.66 m/s2 b) Δv =axt t = 2.25 s

Example

Answer a) a = 0.05 m/s2 b) F = 6 000 000 N

a = v – u t a = F m Acceleration Formulae Worked example: Learning Objectives By the end of this lesson you should… Interpret speed-time graphs to calculate acceleration and distance travelled by objects. Use speed-time graphs and formulas for acceleration to show how crumple zone technologies save lives. Determine how the force acting on an object can be calculated from the its acceleration and mass. Acceleration Formulae Worked example: A car crash results in a car decelerating from 43m/s to 0 in 0.02s. What is the (de)acceleration? a = v – u t a = F m

Match the following statements to your safety features: Learning Objectives By the end of this lesson you should… Interpret speed-time graphs to calculate acceleration and distance travelled by objects. Use speed-time graphs and formulas for acceleration to show how crumple zone technologies save lives. Determine how the force acting on an object can be calculated from the its acceleration and mass. Match the following statements to your safety features: changes shape which absorbs kinetic energy increases the time taken for the car to stop cushions the impact absorbs kinetic energy by stretching a little bit increases the time taken by the passenger to stop a strong metal cage that protects the occupants prevents driver distraction