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Newton's Second Law
Newton's Second Law
5.1
5.1
State Newton's Second Law of Motion.
1. View videos Khan Academy:
5.2
5.2
Distinguish between mass and weight and their units.
Mass and weight are often used interchangeably in everyday language. However, in science, these terms are distinctly different from one another. Mass is a measure of how much matter is in an object. The typical measure of mass is the kilogram. Weight, on the other hand, is a measure of the force of gravity acting on an object. Weight is equal to the mass of an object (m) multiplied by the acceleration due to gravity (g). Like any other force, weight is measured in terms of newtons for science, but kg in usual practice.
Assuming the mass of an object is kept intact, it will remain the same, regardless of its location. However, because weight depends on the acceleration due to gravity, the weight of an object can change when the object enters into a region with stronger or weaker gravity. For example, the acceleration due to gravity on the Moon is 1.67m/s2 (which is much less than the acceleration due to gravity on Earth, 9.80m/s2). If you measured your weight on Earth and then measured your weight on the Moon, you would find that you “weigh” much less (approximately 1/6th), even though you do not look any different. This is because the force of gravity is weaker on the Moon.
When you stand on a bathroom scale, you depress springs that compress in proportion to your weight. The springs provide a measure of your weight (for an object which is not accelerating). This is a force in newtons. The measurement is divided by 9.80 to give a reading in mass units of kilograms. The scale measures weight but is calibrated to provide information about mass.
Task 5.2a
Task 5.2a
Answer the following questions:
What is mass?
What is weight?
How are they related?
What would the weight of an 80kg object approximate to on the moon?
5.3
5.3
Describe qualitatively the relationship between force, mass and acceleration.
Task 5.3a
Task 5.3a
Explain in words the relationship between net force, mass and acceleration for a truck going down a hill.
5.4
5.4
Calculate the net force, mass and acceleration of objects using the equation: (sigma)F=ma or Fnet = m x a
Task 5.4a
Task 5.4a
Complete the worksheet below.
Task 5.4b
Task 5.4b
Q1
Suppose that the net external force (push minus friction) exerted on a lawn mower is 51 N parallel to the ground. The mass of the mower is 24 kg. What is its acceleration?
Figure 3. The net force on a lawn mower is 51 N to the right. At what rate does the lawn mower accelerate to the right?.
Discussion
The direction of the acceleration is the same direction as that of the net force, which is parallel to the ground. There is no information given in this example about the individual external forces acting on the system, but we can say something about their relative magnitudes. For example, the force exerted by the person pushing the mower must be greater than the friction opposing the motion (since we know the mower moves forward), and the vertical forces must cancel if there is to be no acceleration in the vertical direction (the mower is moving only horizontally). The acceleration found is small enough to be reasonable for a person pushing a mower. Such an effort would not last too long because the person’s top speed would soon be reached.
Q2
Prior to manned space flights, rocket sleds were used to test aircraft, missile equipment, and physiological effects on human subjects at high speeds. They consisted of a platform that was mounted on one or two rails and propelled by several rockets.
Calculate the magnitude of force exerted by each rocket, called its thrust T, for the four-rocket propulsion system shown in Figure 4. The sled’s initial acceleration is 49m/s2, the mass of the system is 2100 kg, and the force of friction opposing the motion is known to be 650 N.
5.5
5.5
Apply Newton’s 2nd law to everyday situations (e.g. vehicles and rockets).
The reactive force, reaction force, normal force (N) are names given to the push of the Earth upwards on an object in contact with it.
All forces acting on the car, although we usually only consider
Normal force of earth pushing up on the car (N)
Earth's gravity pulling the car down (g)
Thrust of engine pushing the car forward (T)
Friction of the road acting on tyre against movement (f)
and sometimes air resistance, or drag, as another friction force.
N and g will cancel (equal and opposite), T > f means the car moves forward. If the car brakes, then friction acting on the wheels adds to the friction from the road and at the same time the thrust is reduced by the driver moving off the accelerator. The car's forward motion will slow and stop.
5.6
5.6
Give examples of how knowing this law has enabled designers of safety equipment to save lives.
Task 5.6A for yellow and green
Task 5.6A for yellow and green
A. View slideshare at right
B. Answer the questions:
1. How does a car act like a cardboard box in an accident?
2. As the stopping distance decreases, what happens to the stopping force?
3. Force x Distance equals what?
4. What is designed to happen to the occupant capsule of a modern car in an accident?
5. In a crash which of the following do you think would be safest for the occupants, a large car, a small car or a van?
6. In a crash which of these would be most unsafe for the occupants?
7. In car versus pedestrian accidents, which is safer for the pedestrian, a car fitted with a bull-bar or one without? Why?
8. According to Newton a moving body will keep moving unless...?
9. To stop the driver hitting the steering wheel modern cars are fitted with...?
10. When the stopping distance increases what happens to the forces upon the occupants?
11. The faster the car goes the more _____________ ______________ it has.
12. If 2 cars collide at 80 km/h the work done is ________________ so it is the same as/greater/less than if a single car collides with a barrier at 80 km/h (choose 1 option).
13. KE =
14. If you double the stopping speed, the stopping distance...?
15. If you triple the stopping speed, the stopping distance...?
16. If you quadruple the stopping speed, the stopping distance...?
17. If you have an accident at 50 km/h it has the same force on you as if the car you are in was dropped from a ________ storey building.
If you have an accident at 100 km/h it has the same force on you as if the car you are in was dropped from a ________ storey building.
Crumple zones of cars are designed to absorb energy by crumpling during a crash. In scientific terms, the crumpling is work being done. Work is done whenever things are moved or rearranged by a force. The amount of work done depends on the size of the force and the distance over which the force acts. The larger the force acting, the greater the work done. The longer the distance over which the force acts, the greater the work done.
W = Fs = ΔE
Work and energy are scalar quantities, so no direction is needed. If you apply a force to an object and it does not move, then no work is done. When a force is applied to an object and it moves, the object will gain energy. The change in the amount of kinetic energy is equal to the amount of work done. (Zhang, et al., 2015)
Task 5.6b (for yellow and green)
Task 5.6b (for yellow and green)
Research the websites below and at right:
http://www.hk-phy.org/contextual/mechanics/mom/impul04_e.html Figure 1. Physics Of Crumple Zones
You should be able to explain how seatbelts, crumple zones and airbags reduce the effect of a collision in terms of work (Work = Force x distance; W = F x s) and Newton’s laws.
http://youtu.be/glcDR3cNQlQ Youtube Video "Plastics in Automotive- The Crumple Zone" at time 1:24
Task 5.6c (optional) for yellow and green
Task 5.6c (optional) for yellow and green
Complete simulation at right:
https://www.explorelearning.com/index.cfm?method=cResource.dspDetail&resourceID=1078
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