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Transcript

Dr Pusey Learning Outcomes Recall the SI units for force, weight, mass and gravity Define the net or resultant force Draw and calculate the net force by using a free body diagram in one and two dimensions Describe the motion of an object based on the net force. A force is a push or a pull resulting from an interaction between two objects Force is a vector, requires a magnitude and a direction Measured in Newtons Force Description Friction A force opposing the movement of an object Tension A force transmitted through string, rope, wire or cable Normal A support force Air resistance A force acting on an object as it travels through the air (a type of friction) Applied A directional force (such as a push or pull) applied to an object Spring The force exerted by a stretched or compressed spring on any object attached to it Weight The downwards force exerted on the ground by objects due to the pull of gravity Force Description Gravity The attractive force of gravity that exists between two objects with mass. It causes the weight force of an object. The attractive or repulsive force between two charged objects. The attractive or repulsive force between two magnets. Electrical Magnetic Dr Pusey is walking his dog Wally. When Wally wants to say hello to another dog he pulls on the lead with a force of 67.0 N at an angle of 30.0° above the horizontal. Determine the horizontal and vertical components of the force exerted on Dr Pusey by Wally. FH = F.cosθ FH = 67.cos30 ∴ FH = 58.0 N Forwards FV = F.sinθ FV = 67.sin30 ∴ FV = 33.5 N Downwards When all the forces acting on an object are balanced the object is said to be in equilibrium. This means: an object that is not moving stays still an object that is moving continues to move at the same speed and in the same direction When the forces acting on an object are not equal in size they are unbalanced. This means: an object that is not moving starts to move an object that is moving changes speed or direction The overall force is called the net force or the resultant force Example 1 A dinosaur is chasing after some delicious prey. It experiences an applied force from its feet of 500 N and a friction force of 150 N, calculate the net force. Example 1 A dinosaur is chasing after some delicious prey. It experiences an applied force from its feet of 500 N and a friction force of 150 N, calculate the net force. 150 N 500 N Fnet = 500 N – 150 N = 350N forward Example 2 Calculate the net force for these situations: Example 2 Calculate the net force for these situations: When you need to analyse situations involving forces it’s often useful to draw a force diagram Force diagrams (or free-body diagrams) are used to show the magnitude and direction of all forces acting upon an object in a given situation. Drawing free body diagrams 1. 2. 3. 4. 5. 6. Read the problem carefully Draw the object (a box/dot will do) Identify if the object is in equilibrium or not Determine all the forces acting on the object Determine the direction in which each force is acting Draw and label each force as an arrow Example 1 Draw a force diagram for a car accelerating to the right/forwards. Example 1 Draw a force diagram for a car accelerating to the right/forwards. Gravity Example 1 Draw a force diagram for a car accelerating to the right/forwards. Normal Force (Due to the reaction of the road holding the car up) Gravity Example 1 Draw a force diagram for a car accelerating to the right/forwards. Normal Force (Due to the reaction of the road holding the car up) Applied force (car engine) Gravity Example 1 Draw a force diagram for a car accelerating to the right/forwards. Normal Force (Due to the reaction of the road holding the car up) Friction (Opposes motion). Since the car is accelerating, this arrow must be smaller than the applied force (unbalanced) Applied force (car engine) Gravity Your Turn Draw a force diagram for a sky diver who has reached terminal velocity (constant speed, no acceleration). He is also being pushed by his wing suit to the right at a constant speed Draw a force diagram for a sky diver who has reached terminal velocity (constant speed, no acceleration). He is also being pushed by his wing suit to the right at a constant speed Friction (Air resistance) Friction (Drag) Applied force (Thrust) Gravity Since he is not accelerating in any direction, the addition of all these vectors should be ZERO How did you go? Recall the SI units for force, weight, mass and gravity Define the net or resultant force Draw and calculate the net force by using a free body diagram in one and two dimensions Describe the motion of an object based on the net force.