Presentation on theme: "Newton’s 3 rd Law For every force there is an equal and opposite force."— Presentation transcript:
Newton’s 3 rd Law For every force there is an equal and opposite force
Identify at least six pairs of action- reaction force pairs in the following diagram.
Rockets are unable to accelerate in space because... a. there is no air in space for the rockets to push off of. b. there is no gravity in space. c. there is no air resistance in space. d.... nonsense! Rockets do accelerate in space.
It is a common misconception that rockets are unable to accelerate in space. The fact is that rockets do accelerate. Rockets are able to accelerate due to the fact that they burn fuel and push the exhaust gases in a direction opposite the direction which they wish to accelerate
While driving down the road, an unfortunate bug strikes the windshield of a bus. Quite obviously, this is a case of Newton's third law of motion. The bug hit the bus and the bus hit the bug. Which of the two forces is greater: the force on the bug or the force on the bus?
Trick Question! Each force is the same size. For every action, there is an equal... (equal!). The fact that the bug splatters only means that with its smaller mass, it is less able to withstand the larger acceleration resulting from the interaction.
A gun recoils when it is fired. The recoil is the result of action-reaction force pairs. As the gases from the gunpowder explosion expand, the gun pushes the bullet forwards and the bullet pushes the gun backwards. The acceleration of the recoiling gun is... a. greater than the acceleration of the bullet. b. smaller than the acceleration of the bullet. c. the same size as the acceleration of the bullet
The force on the gun equals the force on the bullet. Yet, acceleration depends on both force and mass. The bullet has a greater acceleration due to the fact that it has a smaller mass. Remember: acceleration and mass are inversely proportional
If a Mack truck and a Volkswagen have a head-on collision, which vehicle will experience the greater impact force? a) the Mack truck b) the Volkswagen c) both the same d)... it depends on other factors
Does the scale read 100N, 200N, or zero?
For every force there exists an equal and opposite force. Consider action and reaction forces in the case of a rock falling under the influence of gravity. If action is considered to be that of the earth pulling down on the rock, can you clearly identify the reaction force?
End of Chapter exercise questions For each of the following interactions, identify action and reaction forces. (a) A hammer hits a nail. (b) Earth gravity pulls down on you. (c) A helicopter blade pushes air downward. When you drop a rubber ball on the floor it bounces almost to its original height. What causes the ball to bounce? Suppose two carts, one twice as massive as the other, fly apart when the compressed spring that joins them is released. How fast does the heavier cart roll compared with the lighter cart?
If a massive truck and small sports car have a head-on collision, upon which vehicle is the impact force greater? Which vehicle experiences the greater acceleration? Explain your answers. Which team wins in a tug-of-war: the team that pulls harder on the rope, or the team that pushes harder against the ground? Explain. A stone is shown at rest on the ground. (a) The vector shows the weight of the stone. Complete the vector diagram showing another vector that results in zero net force on the stone. (b) What is the conventional name of the vector you have drawn?
Here a stone is suspended at rest by a string. (a) Draw force vectors for all the forces that act on the stone. (b) Should your vectors have a zero resultant? (c) Why, or why not? Here the same stone is being accelerated vertically upward. (a) Draw force vectors to some suitable scale showing relative forces acting on the stone. (b) Which is the longer vector, and why?
Problems If you apply a net force of 5 N on a cart with a mass 5 kg, what is the acceleration? If you increase the speed of a 2.0-kg air puck by 3.0 m/s in 4.0 s, what force do you exert on it? A boxer punches a sheet of paper in midair, and brings it from rest up to a speed of 25 m/s in 0.05 s. If the mass of the paper is kg, what force does the boxer exert on it? If you stand next to a wall on a frictionless skateboard and push the wall with a force of 30 N, how hard does the wall push on you? If your mass is 60 kg, what's your acceleration?