Download Newton`s 2nd Law Problems

HONORS PHYSICS
Example Problems – F=ma & Four Equations
1. A car (m=1000 kg) accelerates from rest to a speed of 55 miles/hr in 9s. Find
the force exerted by the car.
2. A person (m=90 kg) on a bike (m =18 kg) exerted a force of 230 N over a
distance of 30 m to increase her speed up to 12 m/s what was her initial
speed?
3. A bowling ball moving at 8 m/s hits the pins and slows down to 1.8 m/s in
0.85s. The force that the pins exerted on the bowling ball was 30 N. Find the
mass of the bowling ball.
4. A motorist in a car moving at 26 m/s spots fluffy in the middle of the road 60
m ahead. The driver’s reaction time is 0.7s. He slams on the brakes and the
1200 kg car skids to a stop with an average friction force of 8000 N.
Assuming the cat was not smart enough to move, did the car hit the cat?
5. A snow sled experiences a frictional force of 45 N as it glides on the snow
regardless of whether a person is on the sled or not. First your friend pushes
the empty sled with a force of 60 N causing the sled to accelerate at 1.2 m/s2.
Your friend now pushes you on the sled with a fore of 240 N causing an initial
acceleration of 2.2 m/s2. What is your mass?
Newton’s 2nd Law Problems
1. A 1kg toy is moving across a smooth floor with a velocity of 5.0 m/s. An
unbalanced force of 2.0 N acts on the car for 4.0 s. Determine the velocity of
the car at the end of the interval for each of the following cases. (a) force acts
in the direction of the motion of the car (b) force acts in the opposite direction
of motion of the car. (13 m/s, -3.0 m/s)
2. An electron has a mass of 9.1 x 10-31kg. Between the electrodes of a cathoderay tube (CRT), it moves a distance of 4.0 mm, accelerated by a net electrical
force of 5.6 x 10-15N. Assuming it started from rest, find its acceleration and
final velocity. (6.2 x 1015 m/s2, 7 x 106 m/s)
3.
A bullet of mass 20 g strikes a fixed block of wood at a speed of 320 m/s.
The bullet embeds itself in the block of wood penetrating to a depth of 6.0
cm. Calculate the average net force acting on the bullet while it is being
brought to rest. (1.7 x 104 N)
4. A 0.5 kg skateboard is at rest on a rough, level floor on which two lines have
been drawn 1.0 m apart. A constant horizontal force is applied to the
skateboard at the beginning of the interval and is removed at the end. The
skateboard takes 8.5 s to travel the 1.0 m distance, and it then coasts for
another 1.25 m before coming to rest. Calculate the force applied to the
skateboard, and the constant frictional force apposing its motion.
(2.6 x 10-2N, 1.2 x 10-2N)
5. A 40 kg sprinter starts from rest and 2.0 s later is running at a speed of 8.0
m/s. what is the average net horizontal force acting on her? What exerts this
force? In which direction? (1.6 x 102N)
6. An 8 g bullet traveling at 400 m/s passes through a heavy block of wood in 4
x 10-4s. Emerging with a velocity of 100 m/s. Ignore any motion of the wood.
(a) What is the average force the wood applied to the bullet? (b) How thick is
the block of wood? (-6000 N, .1 m)
7. A 0.22 caliber rifle shoots a bullet of mass 1.8 g with a muzzle velocity of 500
m/s. If the barrel is 25 cm long, what is the average force exerted on the
bullet while it is in the barrel? (900 N)
8. A motorist has a reaction time of 0.60 s. While driving at 72 km/h, he sees
Fluffy the cat run suddenly onto the road, 40 m in front of his car. If the mass
of the car is 1000 kg and the average horizontal force supplied during braking
is 8000 N, will he be able to stop in time to avoid hitting Fluffy? If so, how
close will they be? (yes, 3 m)
9. A child’s wagon experiences a frictional force of 73 N whenever it is in motion,
regardless of the load it is carrying. An applied horizontal force of 128 N
causes the wagon to accelerate at 5 m/s2. The same applied force, with a
child on the wagon, causes it to accelerate at 1 m/s2. What is the mass of the
child? (44 kg)
10. A sled of 6.0 kg is moving along a smooth, horizontal ice surface with a
velocity of vo. A force of 36 N is applied to the sled in its direction of motion,
increasing its velocity 2 vo while it moves 10 m. Find (a) the sled’s original
velocity, vo and (b) the length of time that the force acted. (6.3 m/s, 1.1 s)