Lyzinski Physics Weight and the Normal Force CRHS-South Force due to gravity and the normal force Force due to gravity: A field force (a vector quantity) that always is directed towards the center of the earth. Weight: The magnitude of the Force due to gravity 2 2 Fg W mg gearth = 9.8 m/s gmoon = 1.6 m/s Mass vs. weight Mass: A measure of an objects inertia Weight: Decreases as you move away from the (its tendency to resist a change in center of the earth. NOT an inherent its motion). Inherent property of property of an object an object. Normal Force: A “reactionary” contact force exerted on one object by another in a direction perpendicular () to the surface of contact. 1) Compare the weight of a 60 kg person on the earth with the weight of the same person on the moon. Then, describe a quick (but very costly) way for dieters at NASA to lose weight. 2) A bully is pushing a boy against a locker as shown with a force of 500N. The angle between his arms and the ground is 40o. Draw a free-body diagram of the boy and then find the normal force between the boy and the wall. 3) A 200 kg block, on a VERY rough surface, is being pulled/pushed by two people, one on each end. The first person is pulling with a force of 20 N at an angle of 40o above the ground, while the other person is pushing with a 30 N force at an angle of 50o above the ground. Find the normal force on the block. 4) In terms of the normal force, explain what is necessary for an object to physically “lift” off of the ground. Newton’s 2nd Law with Weight and the Normal Force 5) A 45 kg box is sitting AT REST on the ground. Find… a) the weight (force due to gravity) of the block. b) the normal force on the block. c) the force of friction acting on the block. 6) A 30 kg box is sitting at rest on a frictionless surface. A 100 N force PUSHES down on the the block at an angle of 30o below the horizontal. Find…. a) the normal force acting on the block. b) the acceleration of the block. c) the block’s velocity after being pulled for 5 seconds. 7) A 50 kg block sits on the ground. A force PULLS on the block at an angle of 45o above the horizontal. What is…. a) the weight of the block. b) the normal force on the block just as the block lifts off the ground. c) the pulling force necessary to just barely lift the block off the ground. 8) A 5g bullet is fired into a large oak tree. The bullet leaves the gun at a speed of 300 m/s. If the block of wood slows the bullet down in 5ms (5 milliseconds, or 5 thousandths of a second), then … a) what is the “retarding” force on the bullet. b) how far into the tree does the bullet penetrate? 9) A 2000 kg car is driving down the road at a speed of 30 m/s. It suddenly slams on its brakes and stops in 40 m. Assuming a constant acceleration, find: a) the force applied by the brakes on the car. b) the time required to stop the car. 10) A block of wood is sliding across a frictionless surface at a speed of 10 m/s. Suddenly, it hits a shaggy carpet, which supplies a constant frictional force of 50N that stops the block in 3 seconds. Find the mass of the block. 11) Three forces are applied to a 500 kg box that is sitting at rest on a surface. The first force is 200 Newtons applied downward and to the left (a push) at an angle of 30 o below the horizontal. The 2nd force of 100 Newtons is applied upward and to the right at an angle of 40 o above the horizontal (a pull). The third and final force is 150 Newtons downward and to the right (another push) at an angle of 80o below the horizontal. Find… a) the weight of the block. b) the normal force acting on the block. c) the acceleration of the block. d) the distance that the block has covered when it reaches a velocity of 15 m/s. .