PHYS 1114: Physics I. Newton s)laws)of)mo.on. Newton s)first)law)of)mo.on. Newton s)first)law)of)mo.on. Newton s)first)law)of)mo.

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PHYS 1114: Physics I Newton s)laws)of)mo.on Lecture 9: Newton s Laws of Motion Now that we have learned how to describe motion, how do we cause the motion that we want? We apply forces on an object!

Newton s,first,law,of,mo3on: an object in motion will, in the absence of forces, tend to remain in motion with neither the speed nor direction changing. This, at first, does,not,seem,obvious.

1 PHYS 1114: Physics I Newton s)laws)of)mo.on Lecture 9: Newton s Laws of Motion Now that we have learned how to describe motion, how do we cause the motion that we want? We apply forces on an object! But what do forces directly affect: Professor Kenny L. Tapp location? velocity? acceleration? Newton answered these questions by postulating three laws of motion. Newton s,first,law,of,mo3on: an object in motion will, in the absence of forces, tend to remain in motion with neither the speed nor direction changing. This, at first, does,not,seem,obvious. Most things on earth tend to slow down and stop. However, when we consider the situation, we see that there are lots of forces tending to slow the objects down such as friction and air resistance. Shayla Lassiem The motion of an object does not change unless it is acted upon by a net force. If v=0, it remains 0 If v,is some value, it stays at that value Another way to say the same thing: No net force velocity is constant acceleration is zero no change of direction of motion Also, when we remove or reduce a lot of the forces on an object, it does tend to keep right on going. Consider a ball rolling on a smooth floor. We don t need forces to keep the motion going! Newton's first law states that if no net force acts on an object, then the velocity of the object remains unchanged. Since at some point during the trip, the velocity is constant, then the total force on the plane must be zero, according to Newton's first law. ΣF= ma = m0 = 0 drag li9 thrust weight

Newton s 1 st Law Examples: blood rushes from your head to your feet when riding on a descending elevator which suddenly stops.

to dislodge ketchup from the bottom of a ketchup bottle, the bottle is often turned upside down, thrust downward at a high speed and then abruptly halted.

while riding a skateboard (or wagon or bicycle), you fly forward off the board when hitting a curb, a rock or another object which abruptly halts the motion

on If we want to change the motion, we push on it (that is, apply forces).

same direction as the resultant force and in inverse proportion to the mass of the object: Σ,F,=,ma. Bountiful, Utah Jan. 21, 2012 YouTube.

2 Newton s 1 st Law Examples: blood rushes from your head to your feet when riding on a descending elevator which suddenly stops. the head of a hammer can be tightened onto the wooden handle by banging the bottom of the handle against a hard surface. to dislodge ketchup from the bottom of a ketchup bottle, the bottle is often turned upside down, thrust downward at a high speed and then abruptly halted. headrests are placed in cars to prevent whiplash injuries during rear-end collisions. while riding a skateboard (or wagon or bicycle), you fly forward off the board when hitting a curb, a rock or another object which abruptly halts the motion of the skateboard. Prof. Tapp demonstrates which of Newton s 1st Law of Motion. Newton s)second)law)of)mo.on If we want to change the motion, we push on it (that is, apply forces). Newton states this in his Second,Law,of,Mo3on: The resultant force (vector sum of the individual forces) on an object causes the object to accelerate in the same direction as the resultant force and in inverse proportion to the mass of the object: Σ,F,=,ma. Bountiful, Utah Jan. 21, 2012 YouTube.com Newton s)second)law)of)mo.on Note that this is a vector equation, and should really be worked in component form: Newton s)second)law)of)mo.on This law tells us how motion changes when a net force is applied. acceleration = (net force)/mass Σ,F x,=,ma x Σ,F y,=,ma y,. We can now see that Newton s First Law of Motion is really just a special case of his Second Law of Motion. Units: [F] = [M] [a] [F] = kg m/s 2 1 Newton (N) 1 kg m/s 2

/M'='200N/10kg'='20'm/s 2 M M=10'kg'''F 1 =200'N''F 2 '='100'N Find'a a'='f net /M'='(200N6100N)/10kg'='10'm/s 2 F 2 There is one further important aspect of motion that Newton identified: the

3 Newton s)second)law)of)mo.on Quick Question 1: F net = ma F 1 M M=10'kg'''F 1 =200'N Find'a Laila Suja, Christina Chau, Jessnie Jose, Josiah Gillespie, Vashti Williams -F g = ma -W = ma -mg = ma -g = a F 1 a'='f net /M'='200N/10kg'='20'm/s 2 M M=10'kg'''F 1 =200'N''F 2 '='100'N Find'a a'='f net /M'='(200N6100N)/10kg'='10'm/s 2 F 2 There is one further important aspect of motion that Newton identified: the distinction between forces that act on an object and forces that act by the object. This leads to his Third,Law,of,Mo3on: For,every,force,by,a,first, object,on,a,second,object,,there,is,a,force,by, the,second,object,on,the,first,object,with,the, same,magnitude,but,in,the,opposite, direc3on. This is sometimes called the law of action and reaction. You can only push on an object and hope that it pushes back. Example: when you walk up a stairs, you use your muscles to push down on the stairs and you trust that the stairs will push back up on you lifting you up the stairs. For every action, there is an equal and opposite reaction. F finger box Finger'pushes'on'box''' 'F finger box '='force'exerted'on'box'by'finger F box finger Box'pushes'on'finger 'F box finger '='force'exerted'on'finger'by'box Third'Law:' 'F box finger '='6'F finger box

4 F A,B = - F B,A. is true for all types of forces F w,m F m,w F m,f F f,m Analyze'the'physics'of'this'athleKc'training'device. Analyze'the'forces'involved'with'returning'the' volleyball'with'a'hit.?

Newton s,laws,of,mo3on Using'your'knowledge'of'physics,'describe'your' expectakon. Dynamics When things do move in response to forces, we have what we call dynamics. Then there is an accelera3on.

Assume for definiteness sake that your mass is 70 kg.

5 Newton s,laws,of,mo3on Using'your'knowledge'of'physics,'describe'your' expectakon. Dynamics When things do move in response to forces, we have what we call dynamics. Then there is an accelera3on. Let s consider a common situation: riding an elevator. Quick Question 2: You are in an elevator at rest on the ground floor. You stand on a scale. What does the scale read? Assume for definiteness sake that your mass is 70 kg. Quick Question 2 SOLN: Standing on a scale in an elevator at rest (or in constant motion), we have a statics problem in one dimension: Σ,F y,,=,,+f scale,g,mg,,=,,0. The scale reads the contact force that the scale is exerting. In this case, it is easy to see that the force of the scale balances your weight, and so the scale reads your weight: Quick Question 2.b: Now the elevator starts moving. When this happens, the scale reads 730,N. Is the elevator moving up, moving down, or can you tell? What is the acceleration of the elevator? F scale,=,mg,=,70,kg,*,9.8,m/s 2,=,686,N.

6 Quick Question 2.b SOLN: Quick Question 2.b SOLN: What we have is still based on Newton s,second,law: in this case, however, although your weight,hasn t,changed, the force,the, scale,exerts,has,increased. This must mean that there is an accelera3on. Since the force of the scale is greater than your weight, the net force is up, and so the acceleration is up. And since you were at rest, you now must be moving up! % Σ,F y,=,+f scale,g,mg,=,ma y, 730 N N = 70 kg * a y or a y = 44 N / 70 kg = +0.63,m/s 2

PHY 1114: Physics I. Newton s)laws)of)mo.on. Newton s)first)law)of)mo.on. Newton s)first)law)of)mo.on. Newton s)first)law)of)mo.on

PHY 1114: Physics I. Newton s)laws)of)mo.on. Newton s)first)law)of)mo.on. Newton s)first)law)of)mo.on. Newton s)first)law)of)mo.on PHY 1114: Physics I Newton s)laws)of)mo.on Lecture 7: Newton s Laws of Motion Fall 2012 ~ Professor Kenny L. Tapp Now that we have learned how to describe motion, how do we cause the motion that we want?