Aim: How do we explain the normal force?

Weight Fg = mg Fg=weight m=mass g=acceleration due to gravity Ex: Calculate the weight of a 12 kg toy cart. What is the difference between weight and mass?

Normal Force The normal force is the force that the ground exerts on an object in a direction perpendicular to its surface.

Ex 1 A 2 kg block of wood is at rest on the table. What is the weight of the block of wood? 19.6 N What is the normal force acting on the wood? 19.6 N

Ex 2 Alice has a mass of 60 kg is standing at rest on a floor. a) What is Alice’s weight? Fg=mg=(60kg)(9.8m/s2) =588N b) What is the normal force acting on Alice? FN=588N

Direction of Normal Force In both of our examples, the direction of the normal force was “Up”. Can you think of a scenario in which the normal force is not directed up? On an inclined plane, the normal force is not directly up

Magnitude of Normal Force In both of our examples, the weight of the object was equal to the normal force. Can you think of any scenarios where weight and normal force will not be equal? On an accelerating elevator, the weight and normal force are not equal.

Elevators

Scale When you step on a scale, what force does the scale read? Your weight or your normal force?

Elevator Problem 1 The weight forces and normal forces acting on Harry Potter(m=60 kg) are shown below. Find the net force acting on Harry. Find the acceleration of Harry in the elevator. Explain how the elevator is moving. Does Harry feel heavier, lighter, or the same as his actual weight?

ON Fnet=ma 0N=60kg(a) a=0m/s2 The elevator is either at rest or moving at constant velocity The scale reads the actual weight of Harry Potter. Harry feels the same as his actual weight.

Elevator Problem 2 The weight forces and normal forces acting on Harry Potter(m=60 kg) are shown below. a) Find the net force acting on Harry. b) Find the acceleration of Harry in the elevator. c) Explain how the elevator is moving. d) Does Harry feel heavier, lighter, or the same as his actual weight?

212 N upwards Fnet=ma 212N=60kga a=3.5m/s2 The elevator is accelerating upwards The scale reads a heavier weight than the actual weight of Harry Potter. Harry feels heavier.

Elevator Problem 3 The weight forces and normal forces acting on Harry Potter(m=60 kg) are shown below. a) Find the net force acting on Harry. b) Find the acceleration of Harry in the elevator. c) Explain how the elevator is moving. d) Does Harry feel heavier, lighter, or the same as his actual weight?

188 N downwards Fnet =ma 188N=60kg(a) a=3.13m/s2 The elevator is accelerating downward The scale reads a lighter weight than the actual weight of Harry Potter. Harry feels lighter.

Elevator Problem 4 The weight forces and normal forces acting on Harry Potter(m=60 kg) are shown below. a) Find the net force acting on Harry. b) Find the acceleration of Harry in the elevator. c) Explain how the elevator is moving. d) Does Harry feel heavier, lighter, or the same as his actual weight?

288 N downwards Fnet=ma 288N=60kg (a) a=4.8m/s2 The elevator is accelerating downwards The scale reads a lighter weight than Harry Potter’s actual weight. Harry feels lighter

Elevator Problem 5 The weight forces and normal forces acting on Harry Potter(m=60 kg) are shown below. a) Find the net force acting on Harry. b)Find the acceleration of Harry in the elevator. c) Explain how the elevator is moving. d) Does Harry feel heavier, lighter, or the same as his actual weight?

112 N upwards Fnet=ma 112N=60kg (a) a=1.87 m/s2 The elevator is accelerating upwards. The scale reads a heavier weight than Harry Potter’s actual weight. Harry feels heavier