Mechanics: Student misconceptions - PowerPoint Presentation

Mechanics: Student misconceptions Coffee and pi Wednesday 13 June 2018 Lesley Swarbrick AMSP Associate

You’ve heard of Elf on a Shelf?

Which cells go together? Which is the odd one out? Momentum Interaction pair Weight Mass Average speed DisplacementPowerWork Done VelocityResultant forceFriction Gravitational potential energyAccelerationDistance Kinetic energyImpulseTension ThrustUpthrustResistance Gravity Normal reaction Elastic potential energy Driving force Time Can you think of 5 more “mechanics” words? How could you relate these words to a student's experience?

Drawing Force Diagrams (you need a mini-whiteboard and a pen)

Draw a diagram to show… the forces which act on a block which is: a) at rest on a smooth horizontal surface 1. W N

Draw a diagram to show… the forces which act on a block which is: c) at rest on a rough surface inclined at an angle 20 ˚ to horizontal. W N 20 ° F

Draw a diagram to show… the forces which act on a block which is: d) sliding down a smooth surface inclined at an angle 30 ˚ to horizontal. W N 30 ° a v

Draw a diagram to show… the forces which act on a block which is: e) sliding down a rough surface inclined at an angle 30 ˚ to horizontal. W N 30 ° F a v

Draw a diagram to show… the forces which act on a block which is: f) p ulled down a smooth surface inclined at an angle 10 ˚ to horizontal by a string parallel to the plane. W N 10 ° T a v

Draw a diagram to show… the forces which act on a block which is: g) pulled down a rough surface inclined at an angle 20 ˚ to horizontal by a string parallel to the plane. W N 20 ° T a F v

Draw a diagram to show… the forces which act on a block which is: h) pulled up a rough surface inclined at an angle 20 ˚ to horizontal by a string inclined at 40˚ to horizontal. W N 20 ° F a T 20 ° v

Draw a diagram to show… the forces acting on a ladder which is leaning with one end against a smooth vertical wall and the other end standing on a rough horizontal ground. W F 1 N 1 N 2

Draw a diagram to show… the forces acting on a particle which is suspended from a fixed point by a string when: a) it is hanging at rest W T

Draw a diagram to show… the forces acting on a car whose engine is creating a driving force to pull a trailer over a rough road so that they are accelerating. T T D a W t W c N t N c R t R c

Draw a diagram to show… the forces acting on two particles connected by a light inextensible string passing over a smooth fixed pulley after the system is released from rest. T T a a W 1 W 2

Draw a diagram to show… the forces acting on two particles connected by a light inextensible string passing over a smooth pulley where one is on a rough horizontal plane and the second hangs freely a T T a W 1 W 2 F N

Draw diagrams to show… the forces acting on a person in a lift, suspended by a light inextensible cable, that is accelerating upwards The forces on a lift, suspended by a light inextensible cable, that is carrying a person and accelerating upwards a W p N p a W p N p W l T N p

7. Two bricks, one on top of the other, rest on a horizontal surface. Draw diagrams to show (a) the forces acting on the bottom brick, (b) the forces acting on the top brick. N W N N W´

Impulse – positive or negative? Two particles, P and Q, have masses 3m and m respectively. They are moving in opposite directions towards each other along the same straight line on a smooth horizontal plane and collide directly. The speeds of P and Q immediately before the collision are 2u and 4u respectively. The magnitude of the impulse received by each particle in the collision is 21/4 mu . (a) Find the speed of P after the collision. (3) (b) Find the speed of Q after the collision. (3)

Lift problems: what forces are included? A lift of mass 250 kg is being raised by a vertical cable attached to the top of the lift. A woman of mass 60 kg stands on the horizontal floor inside the lift. The lift ascends vertically with constant acceleration 2 m/s². There is a constant downwards resistance of magnitude 100 N on the lift. By modelling the woman as a particle, find the magnitude of the normal reaction exerted by the floor of the lift on the woman. (3)

Force diagrams: what is thrust? A vertical light rod PQ has a particle of mass 0.5 kg attached to it at P and a particle of mass 0.75 kg attached to it at Q, to form a system, as shown in Figure 2. The system is accelerated vertically upwards by a vertical force of magnitude 15 N applied to the particle at Q. Find the thrust in the rod.

Suvat or not suvat? A particle P moves on the x-axis. At time t seconds, t ≥0, the acceleration of P is ( 2t – 3) m/s² in the positive x direction. At time t seconds, the velocity of P is v m/s in the positive x direction. When t = 3, v = 2, find v in terms of t.

Friction: which direction is it acting? A particle P of mass 2 kg is held at rest in equilibrium on a rough plane by a constant force of magnitude 40 N. The direction of the force is inclined to the plane at an angle of 30°. The plane is inclined to the horizontal at an angle of 20°, as shown in Figure 2. The line of action of the force lies in the vertical plane containing P and a line of greatest slope of the plane. The coefficient of friction between P and the plane is μ. Given that P is on the point of sliding up the plane, find the value of μ.

Suvat :modelling the correct part of the motion A cyclist is moving along a straight horizontal road and passes a point A. Five seconds later, at the instant when she is moving with speed 10 m/s, she passes the point B. She moves with constant acceleration from A to B. Given that AB = 40m, find (a) the acceleration of the cyclist as she moves from A to B, (4) (b) the time it takes her to travel from A to the midpoint of AB (5)